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a ad7709 information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and registered trademarks are the property of their respective companies. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700 www.analog.com fax: 781/326-8703 ?2003 analog devices, inc. all rights reserved. rev. a 16-bit - adc with switchable current sources functional block diagram v dd iexc1 8i iexc2 8i iout1 iout2 iexc3 i oscillator and pll xtal2 refin1(? refin2(? refin1(+) refin2(+) xtal1 dout reset rdy cs sclk din serial interface and control logic buf pga mux i/o port v dd 16-bit - adc ain1 ain2 ain3/p3 ain4/p4 aincom ad7709 v dd gnd pwrgnd p1/sw1 p2/sw2 i = 25 a features 16-bit - adc programmable gain front end simultaneous 50 hz and 60 hz rejection at 20 hz update rate vref select allows absolute and ratiometric measurement capability isource select 16-bit no missing codes 13-bit p-p resolution @ 20 hz, 20 mv range 16-bit p-p resolution @ 20 hz, 2.56 v range interface 3-wire serial spi , qspi, microwire, and dsp compatible schmitt trigger on sclk power specified for single 3 v and 5 v operation normal: 1.25 ma typ @ 3 v power-down: 7 a (32.768 khz crystal running) on-chip functions rail-to-rail input buffer and pga selectable reference inputs 3 switchable, ratioed current sources for v be measurements 4-bit digital i/o port low-side power switches applications sensor measurement temperature measurement pressure measurements weigh scales portable instrumentation 4?0 ma loops general description the ad7709 is a complete analog front end for low frequency measurement applications. it contains a 16-bit - adc, selectable reference inputs, three switchable matched excitation current sources, low-side power switches, and a digital i/o port. the 16-bit channel with pga accepts fully differential, unipolar, and bipolar input signal ranges from 1.024 refin/128 to 1.024 refin. it can be configured as two fully differential input channels or four pseudo-differential input channels. signals can be converted directly from a transducer without the need for signal conditioning. the device operates from a 32.768 khz crystal with an on-chip pll generating the required internal operating frequency. the output data rate from the part is software programmable. the p-p resolution from the part varies with the programmed gain and output data rate. the part operates from a single 3 v or 5 v supply. when operating from 3 v supplies, the power dissipation for the part is 3.75 mw. the ad7709 is housed in a 24-lead tssop package.
rev. a ad7709 e2e table of contents features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 general description . . . . . . . . . . . . . . . . . . . . . . . . . 1 functional block diagram . . . . . . . . . . . . . . . . . 1 specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 timing characteristics . . . . . . . . . . . . . . . . . . . . . 6 absolute maximum ratings . . . . . . . . . . . . . . . . . 8 ordering guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 pin function descriptions . . . . . . . . . . . . . . . . . . 9 typical performance characteristics . . . . 10 adc circuit information . . . . . . . . . . . . . . . . . . . 11 overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 s - d adc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 noise performance . . . . . . . . . . . . . . . . . . . . . . . . . 13 on-chip registers . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 communications register . . . . . . . . . . . . . . . . . . . . . . . . 14 status register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 configuration register . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 filter register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 adc data result register . . . . . . . . . . . . . . . . . . . . . . . . 18 configuring the ad7709 . . . . . . . . . . . . . . . . . . . . . 19 digital interface . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 microcomputer/microprocessor interfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 ad7709-to-68hc11 interface . . . . . . . . . . . . . . . . . . . . . 21 ad7709-to-8051 interface . . . . . . . . . . . . . . . . . . . . . . . . 21 ad7709-to-adsp-2103/adsp-2105 interface . . . . . . . . 21 circuit description . . . . . . . . . . . . . . . . . . . . . . . . 22 analog input channels . . . . . . . . . . . . . . . . . . . . . . . . . . 22 programmable gain amplifier . . . . . . . . . . . . . . . . . . . . . 23 bipolar/unipolar configuration . . . . . . . . . . . . . . . . . . . . 23 data output coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 excitation currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 crystal oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 reference input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 reset input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 power-down mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 grounding and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 temperature measurement . . . . . . . . . . . . . . . . . . . . . . . 26 3-wire rtd configurations . . . . . . . . . . . . . . . . . . . . . . 27 smart transmitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 outline dimensions . . . . . . . . . . . . . . . . . . . . . . . . . 29 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
rev. a e3e ad7709 specifications 1 (v dd = 2.7 v to 3.6 v or 4.75 v to 5.25 v, refin(+) = 2.5 v; refin(e) = gnd; gnd = 0 v; xtal1/xtal2 = 32.768 khz crystal; all specifications t min to t max , unless otherwise noted.) parameter ad7709a, ad7709b unit test conditions adc channel specification output update rate 5.4 hz min 0.732 ms increments 105 hz max adc channel no missing codes 2 16 bits min 20 hz update rate resolution 13 bits p-p 20 mv range, 20 hz update rate 16 bits p-p 2.56 v range, 20 hz update rate output noise and update rates see tables ii to v integral nonlinearity 2 30 ppm of fsr max typically 2 ppm fsr refin gain = 21 024 . offset error 3 m v typ offset error drift vs. temperature 10 nv/ 0.75 lsb typ b grade, v dd = 4 v 0.2 % of fs typ a grade gain drift vs. temperature 0.5 ppm/ 2.56 v 100 db typ on 20 mv range analog inputs differential input voltage ranges 1 024 . refin gain v nom refin = refin(+) e refin(e) gain = 1 to 128 adc range matching 2 m v typ input voltage = 19 mv on all ranges absolute ain1eain4 voltage limits 2 gnd + 100 mv v min v dd e 100 mv v max ain1eain4 analog input current dc input current 2 1 na max dc input current drift 5 pa / 125 na/v typ input current varies with input range dc input current drift 2 pa/v/ 1h z, 16.65 hz update rate, sf = 82 @ 60 hz 100 db min 60 hz 1 hz, 20 hz update rate, sf = 68 common-mode rejection @ dc 100 db typ input range = 2.56 v, ain = 1 v 110 db typ on 20 mv range @ 50 hz 2 100 db min 50 hz 1 hz, range = 2.56 v, ain = 1 v @ 60 hz 2 100 db min 60 hz 1 hz, range = 2.56 v, ain = 1 v reference inputs (refin1 and refin2) refin voltage 2.5 v nom refin = refin(+) e refin(e) refin voltage range 2 1v min v dd v max absolute refin voltage limits 2 gnd e 30 mv v min v dd + 30 mv v max average reference input current 0.5 m a/v typ average reference input current drift 0.01 na/v/ 1 hz, sf = 82 @ 60 hz 100 db min 60 hz 1 hz, sf = 68 common-mode rejection @ dc 110 db typ input range = 2.56 v, ain = 1 v @ 50 hz 110 db typ 50 hz 1 hz, range = 2.56 v, ain = 1 v @ 60 hz 110 db typ 60 hz 1 hz, range = 2.56 v, ain = 1 v see notes on page 5.
rev. a e4e ad7709 parameter ad7709a, ad7709b unit test conditions excitation current sources (iexc1, iexc2, and iexc3) output current iexc1, iexc2 200 m a nom iexc3 25 m a nom initial tolerance at 25 10 % typ drift 200 ppm/ 2.5 % max b grade, no load (between iexc1 and iexc2) 2.5 % typ a grade, no load drift matching (between iexc1 and iexc2) 20 ppm/ 5% max b grade, no load (between 8 iexc3 and iexc1/iexc2) 5% typ a grade, no load drift matching (between 8 iexc3 and iexc1/iexc2) 20 ppm/ 5% iexc1, iexc2 1.25 m a/v typ a, b grades 2.6 m a/v max b grade iexc3 1 m a/v max b grade 1 m a/v typ a grade load regulation 300 na/v typ output compliance v dd e 0.6 v max gnd e30 mv v min low-side power switches (sw1 and sw2) r on 3 w typ v dd = 5 v, a and b grade 5 w max b grade 4.5 w typ v dd = 3 v, a and b grade 7 w max b grade allowable current 2 20 ma max continuous current per switch logic inputs all inputs except sclk and xtal1 2 v inl , input low voltage 0.8 v max v dd = 5 v 0.4 v max v dd = 3 v v inh , input high voltage 2.0 v min v dd = 3 v or 5 v sclk only (schmitt-triggered input) 2 v t(+) 1.4/2 v min/v max v dd = 5 v v t(e) 0.8/1.4 v min/v max v dd = 5 v v t(+) e v t(e) 0.3/0.85 v min/v max v dd = 5 v v t(+) 0.95/2 v min/v max v dd = 3 v v t(e) 0.4/1.1 v min/v max v dd = 3 v v t(+) e v t(e) 0.3/0.85 v min/v max v dd = 3 v xtal1 only 2 v inl , input low voltage 0.8 v max v dd = 5 v v inh , input high voltage 3.5 v min v dd = 5 v v inl , input low voltage 0.4 v max v dd = 3 v v inh , input high voltage 2.5 v min v dd = 3 v input currents (except xtal) 2 m a max v in = v dd e70 m a max v in = gnd, typically e40 m a @ 5 v and e20 m a at 3 v; weak pull-ups on the logic inputs input capacitance 10 pf typ all digital inputs (continued) specifications
rev. a parameter ad7709a, ad7709 bu nit test conditions logic outputs (excluding xtal2) v oh , output high voltage 2 v dd e 0.6 v min v dd = 3 v, i source = 100 m a v ol , output low voltage 2 0.4 v max v dd = 3 v, i sink = 100 m a v oh , output high voltage 2 4v min v dd = 5 v, i source = 200 m a v ol , output low voltage 2 0.4 v max v dd = 5 v, i sink = 1.6 ma floating-state leakage current 10 m a max floating-state output capacitance 10 pf typ data output coding binary unipolar mode offset binary bipolar mode i/o port v inl , input low voltage 2 0.8 v max v dd = 5 v 0.4 v max v dd = 3 v v inh , input high voltage 2 2.0 v min v dd = 3 v or 5 v input currents 2 m a max v in = v dd e70 m a max v in = gnd, typically e40 m a @ v dd = 5 v and e20 m a at v dd = 3 v; weak pull-ups on the logic inputs input capacitance 10 pf typ all digital inputs v oh , output high voltage 2 v dd e 0.6 v min v dd = 3 v, i source = 100 m a v ol , output low voltage 2 0.4 v max v dd = 3 v, i sink = 100 m a v oh , output high voltage 2 4v min v dd = 5 v, i source = 200 m a v ol , output low voltage 2 0.4 v max v dd = 5 v, i sink = 1.6 ma floating-state output leakage current 10 m a max floating-state output capacitance 10 pf typ start-up time from power-on 300 ms typ from standby mode 1 ms typ oscpd = 0 from power-down mode 300 ms typ oscpd = 1 power requirements power supply voltage v dd e gnd 2.7/3.6 v min/max v dd = 3 v nom 4.75/5.25 v min/max v dd = 5 v nom power supply currents i dd current 1.5 ma max v dd = 3 v, 1.25 ma typ 1.75 ma max v dd = 5 v, 1.45 ma typ i dd (low power mode) 7 m a max b grade, v dd = 3 v, standby mode 7 m a typ a grade, v dd = 3 v, standby mode 1.5 m a max b grade, v dd = 3 v, power-down mode 1.5 m a typ a grade, v dd = 3 v, power-down mode 26 m a max b grade, v dd = 5 v, standby mode 26 m a typ a grade, v dd = 5 v, standby mode 6.5 m a max b grade, v dd = 5 v, power-down mode 6.5 m a typ a grade, v dd = 5 v, power-down mode i dd for one conversion second 107 5 m a typ v dd = 3 v, standby mode 134 5 m a typ v dd = 5 v, standby mode notes 1 temperature range e40
rev. a ad7709 e6e timing characteristics 1, 2 limit at t min , t max parameter (a, b version) unit conditions/comments t 1 30.5176 m s typ crystal oscillator period t 2 50 ns min reset r rdy cs st cs escaest scaedd dd dd a cs edd dd dd sc sc cs rescet rtsce scae rdy w cs escaest dscest dscet sc sc cs rescet ntes sa r dd s scsc t t cs scds t t ttc rdy adct rdy dd dd nd ta dd
rev. a ad7709 e7e i sink (1.6ma with v dd = 5v 100 a with v dd = 3v) 1.6v i source (200 a with v dd = 5v 100 a with v dd = 3v) to output pin 50pf figure 1. load circuit for timing characterization t 12 t 13 t 14 t 15 t 11 t 16 msb lsb cs sclk din figure 2. write cycle timing diagram t 5 t 5a t 4 t 6 t 3 t 9 msb lsb cs sclk t 8 t 10 t 7 dout rdy figure 3. read cycle timing diagram
rev. a ad7709 e8e warning! esd sensitive device pin configuration top view (not to scale) 1 ad7709 gnd v dd xtal2 xtal1 rdy dout din reset sclk cs p1/sw1 pwrgnd iout1 iout2 refin1(+) refin1(e) ain1 ain2 ain3/p3 ain4/p4 aincom p2/sw2 refin2(+) refin2(e) 2 3 4 5 6 7 8 9 10 11 12 24 23 22 21 20 19 18 17 16 15 14 13 absolute maximum ratings * (t a = 25 q ja thermal impedance . . . . . . . . . . . . . . . . . . . 97.9 q jc thermal impedance . . . . . . . . . . . . . . . . . . . . . 14
rev. a ad7709 e9e pin function descriptions pin no. mnemonic function 1 iout1 output for internal excitation current source. either current source iexc1, iexc2, iexc3, or a combina- tion of the current sources, can be switched to this output. 2 iout2 output for internal excitation current source. either current source iexc1, iexc2, iexc3, or a combina- tion of the current sources, can be switched to this output. 3 refin1(+) positive reference input. refin1(+) can lie anywhere between v dd and gnd + 1 v. the nominal refer- ence voltage (refin1(+) e refin1(e)) is 2.5 v, but the part is functional with a reference range from 1 v to v dd . 4 refin1(e) negative reference input. this reference input can lie anywhere between gnd and v dd e 1 v. 5 ain1 analo g input. programmable gain input that can be used as a pseudo-differential input when used with aincom or as the positive input of a fully differential input pair when used with ain2. 6 ain2 analo g input. programmable gain input that can be used as a pseudo-differential input when used with aincom or as the negative input of a fully differential input pair when used with ain1. 7 ain3/p3 analog input/digital port bit. programmable gain input that can be used as a pseudo-differential input when used with aincom or as the positive input of a fully differential input pair when used with ain4. this pin can also be programmed as a general-purpose digital input bit. 8 ain4/p4 analog input/digital port bit. programmable gain input that can be used as a pseudo-differential input when used with aincom or as the negative input of a fully-differential input pair when used with ain3. this pin can also be programmed as a general-purpose digital input bit. 9 aincom all analog inputs are referenced to this input when configured in pseudo-differential input mode. 10 refin2(+) positive reference input. refin2(+) can lie anywhere between v dd and gnd + 1 v. the nominal refer ence voltage (refin2(+) e refin2(e)) is 2.5 v, but the part is functional with a reference range from 1 v to v dd . 11 refin2(e) negative reference input. this reference input can lie anywhere between gnd and v dd e 1 v. 12 p2/sw2 dual-purpose pin. it can act as a general-purpose output (p2) bit or as a low-side power switch (sw2) to pwrgnd. 13 pwrgnd ground point for the low-side power switches sw2 and sw1. pwrgnd must be tied to gnd. 14 p1/sw1 dual-purpose pin. it can act as a general-purpose output (p1) bit or as a low-side power switch (sw1) to pwrgnd. 15 reset dradcrst dd sc scdt adctsc s t a ada dd sc cs cstad cs ad cs ad scdndta dd cs rdy rdy sad rdy adc t rdy dt sdasradt d n sdasradd adc a dd dn nd rad dd sn ta c ta c
rev. a ad7709etypical performance characteristics e10e 200 32767 32766 32768 32770 32769 600 500 400 300 700 code occurrence 32771 100 0 tpc 3. noise histogram oscillator v dd = 5v t a = 25 c time base = 100ms/div trace 1 = trace 2 = 2v/div v dd tpc 4. typical oscillator power-up 32767 100 0 200 400 300 32771 32770 32769 32768 32772 reading number code read 500 32766 32765 32764 600 700 800 900 1000 v dd = 5v input range = 20mv update rate = 19.79hz v ref = 2.5v t a = 25 c tpc 1. typical noise plot on 20 mv input range 2.5 0 1.0 3.0 2.5 2.0 1.5 3.5 5.0 4.5 4.0 2.0 1.5 1.0 0.5 3.0 v ref e v rms noise e v 20mv range 2.56v range v dd = 5v v ref = 2.5v input range = 2.56v update rate = 19.79hz t a = 25 c tpc 2. rms noise vs. reference input
rev. a ad7709 ?1 adc circuit information overview the ad7709 incorporates a - adc channel with on-chip digital filtering intended for the measurement of wide dynamic range, low frequency signals such as those in weigh-scale, strain-gauge, pressure transducer, or temperature measurement applications. - adc th is ch a nne l can be programmed to have one of eight input voltage ranges from 20 mv to 2.56 v. this channel can be configured as either two fully differential inputs (ain1/ain2 and ain3/ain4) or four pseudo-differential input channels (a in 1/ aincom, ain2/aincom, ain3/aincom, and ai n4/ ai ncom). buffering the input channel means that the part can accommodate significant source impedances on the analog input and that r, c filtering (for noise rejection or rfi reduction) can be placed on the analog inputs if required. the adc employs a - conversion technique to realize up to 16 bits of no-missing-codes performance. the - modulator converts the sampled input signal into a digital pulse train whose duty cycle contains the digital information. a sinc 3 programm able low-pass filter is then employed to decimate the modulator output data stream to give a valid data conversion result at programm able output rates from 5.35 hz (186.77 ms) to 105.03 hz (9.52 ms). a chopping scheme is also employed to minimize adc channel offset errors. a block diagram of the adc input channel is shown in figure 4. the sampling frequency of the modulator loop is many times higher than the bandwidth of the input signal. the integrator in the modulator shapes the quantization noise (which results from the analog-to-digital conversion) so that the noise is pushed toward one-half of the modulator frequency. the output of the - modulator feeds directly into the digital filter. the digital filter then band-limits the response to a frequency significantly lower than one-half of the modulator frequency. in this manner, the 1-bit output of the comparator is translated into a band- limited, low noise output from the ad7709 adc. the ad7709 filter is a low-pass, sinc 3 , or (sin(x)/x) 3 filter whose primary function is to remove the quantization noise introduced at the modulator. the cutoff frequency and decimated output data rate of the filter are programmable via the sf word loaded to the filter register. a chopping scheme is employed where the complete signal chain is chopped, resulting in excellent dc offset and offset drift speci- fications, and is extremely beneficial in applications where drift, noise rejection, and optimum emi rejection are important fac- tors. with chopping, the adc repeatedly reverses its inputs. the decimated digital output words from the sinc 3 filters there- fore have a positive offset and negative offset term included. as a result, a final summing stage is included so that each output word from the filter is summed and averaged with the previous filter output to produce a new valid output result to be written to the adc data register. the input chopping is incorporated into the input multiplexer while the output chopping is accomplished by an xor gate at the output of the modulator. the chopped modulator bit stream is applied to a sinc 3 filter. the programming of the sinc 3 deci- mation factor is restricted to an 8-bit register sf, the actual decimation factor is the register value 8. the decimated out- put rate from the sinc 3 filter (and the adc conversion rate) will therefore be: f sf f adc mod = ? ? ? ? ? ? 1 3 1 8 where: f adc is the adc update rate. sf is the decimal equivalent of the word loaded to the filter register. f mod is the modulator sampling rate of 32.768 khz. programming the filter register determines the update rate for the adc. the chop rate of the channel is half the output data rate. the frequency response of the filter h( f ) is as follows: 1 8 8 1 2 2 3 sf sf f / f f/f f/f f/f mod mod out out ? ? ? ? ? ? ? ? ? ? ? ? sin ( ) sin ( ) sin ( ) sin ( ) where: f mod = 32,768 hz. sf = value programmed into filter register. f out = f mod /(sf 8 3) the following shows plots of the filter frequency response for the sf words shown in table i. the overall frequency response is the product of a sinc 3 and a sinc response. there are sinc 3 notches at integer multiples of 3 f adc , and there are sinc notches at odd integer multiples of f adc /2. the 3 db frequency for all values of sf obeys the following equation: fdb f adc 3024 () = . the signal chain is chopped as shown in figure 4. the chop frequency is: f f chop adc = ? ? ? ? ? ? 2 sinc 3 filter mux buf pga - mod xor analog input digital output 1 8 sf 3 ( (8 sf ) 3 1 2 a in + v os a in ?v os f chop f in f mod f chop f adc ) figure 4. adc channel block diagram
rev. a ad7709 e12e as shown in the block diagram, the sinc 3 filter outputs alternately contain +v os and ev os , where v os is the respective channel offset. this offset is removed by performing a running average of 2, w hich means that the settling time to any change in programming of the adc will be twice the normal conversion time, while an asynchronous step change on the analog input will not be fully reflected until the third subsequent output. t f t settle adc adc = ? ? ? = 2 2 the allowable range for sf is 13 to 255, with a default of 69 (45h). the corresponding conversion rates, conversion times, and settling times are shown in table i. note that the conver- sion time increases by 0.732 ms for each increment in sf. frequency e hz 0 e140 e200 0 650 50 100 150 200 250 300 350 400 450 500 550 600 700 a ttenuation e db e20 e120 e160 e180 e60 e100 e40 e80 sf = 13 output data rate = 105hz input bandwidth = 25.2hz first notch = 52.5hz 50hz rejection = e23.6db, 50hz 1hz rejection = e20.5db 60hz rejection = e14.6db, 60hz 1hz rejection = e13.6db figure 5. filter profile with sf = 13 frequency e hz 0 e80 e160 0 100 10 a ttenuation e db 20 30 40 50 60 70 80 90 e20 e40 e120 e140 e60 e100 sf = 82 output data rate = 16.65hz input bandwidth = 4hz 50hz rejection = e171db, 50hz 1hz rejection = e100db 60hz rejection = e58db, 60hz 1hz rejection = e53db figure 6. filter profile with sf = 82 table i. adc conversion and settling times for various sf words data update rate settling time sf word f adc (hz) t settle (ms) 13 105.3 19.04 69 (default) 19.79 101.07 255 5.35 373.54 normal mode rejection is the major function of the digital filter on the ad7709. the normal mode 50 1 hz rejection with an sf word of 82 is typically e100 db. the 60 1 hz rejection with sf = 68 is typically e100 db. simultaneous 50 hz and 60 hz rejection of better than 60 db is achieved with an sf of 69. choosing an sf word of 69 places notches at both 50 hz and 60 hz. figures 5 to 8 show the filter rejection for a selection of sf words. frequency e hz 0 e80 e160 0 100 10 a ttenuation e db 20 30 40 50 60 70 80 90 e20 e40 e120 e140 e60 e100 sf = 69 output data rate = 19.8hz input bandwidth = 4.74hz first notch = 9.9hz 50hz rejection = e66db, 50hz 1hz rejection = e60db 60hz rejection = e117db, 60hz 1hz rejection = e94db figure 7. filter profile with default sf = 69 giving filter notches at both 50 hz and 60 hz frequency e hz 0 e80 e160 0 100 10 a ttenuation e db 20 30 40 50 60 70 80 90 e20 e40 e120 e140 e60 e100 sf = 255 output data rate = 5.35hz input bandwidth = 1.28hz 50hz rejection = e93db, 50hz 1hz rejection = e93db 60hz rejection = e74db, 60hz 1hz rejection = e68db figure 8. filter profile with sf = 255
rev. a ad7709 e13e noise performance tables ii and iii show the output rms noise and output peak-to- peak resolution in bits (rounded to the nearest 0.5 lsb) for a selection of output update rates. the numbers are typical and generated at a differential input voltage of 0 v. the output up date rate is selected via the sf7esf0 bits in the filter register. it is important to note that the peak-to-peak resolution figures repre sent the resolution for which there will be no code flicker within a six-sigma limit. the output noise comes from two sources. the first is the electrical noise in the semiconductor devices (device noise) used in the implementation of the modulator. second, when the analog input is converted into the digital domain, quantization noise is added. the device noise is at a low din adc status register (8 bits) configuration register (24 bits) filter register (8 bits) adc data register (16 bits) register select decoder dout dout dout dout dout din din wen r/ w stby oscpd 0 0 a1 a0 figure 9. on-chip registers level and is independent of frequency. the quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. the numbers in the tables are given for the bipolar input ranges. for the unipolar ranges, the rms noise num bers will be the same as the bipolar range, but the peak-to-peak resolution is now based on half the signal range, which effectively means losing 1 bit of resolution. on-chip registers the ad7709 is controlled and configured via a number of on-chip registers, as shown in figure 9 and described in more detail in the following pages. in the following descriptions, set implies a logic 1 state and cleared implies a logic 0 state, unless otherwise stated. table ii. typical output rms noise vs. input range and update rate for the ad7709 (output rms noise in v) sf data update input range word rate (hz) 20 mv 40 mv 80 mv 160 mv 320 mv 640 mv 1.28 v 2.56 v 13 105.3 1.50 1.50 1.60 1.75 3.50 4.50 6.70 11.75 69 19.79 0.60 0.65 0.65 0.65 0.65 0.95 1.40 2.30 255 5.35 0.35 0.35 0.37 0.37 0.37 0.51 0.82 1.25 t able iii. peak-to-peak resolution vs. input range and update r ate for the ad7709 (peak-to-p eak resolution in bits) sf data update input range word rate (hz) 20 mv 40 mv 80 mv 160 mv 320 mv 640 mv 1.28 v 2.56 v 13 105.3 12 13 14 15 15 15.5 16 16 69 19.79 13 14 15 16 16 16 16 16 255 5.35 14 15 16 16 16 16 16 16
rev. a ad7709 e14e communications register (a1, a0 = 0, 0) the communications register is an 8-bit write-only register. all communications to the part must start with a write operation t o the communications register. the data written to the communications register determines whether the next operation is a read or write operation, and to which register this operation takes place. for read or write operations, once the subsequent read or wr ite operation to the selected register is complete, the interface returns to where it expects a write operation to the communicatio ns register. this is the default state of the interface, and on power-up or after a reset ad cr dnad t crcrcrcrcr cr r c r c r c r c r c r c r c r c n e w w n wen we acr wen cr cr r w a a cr sty s aad aad cr scd d ad t adc adcw adc crcr t crcr aa ratad t rst a a r c rw srr c r r adcdr
rev. a ad7709 ?5 status register (a1, a0 = 0, 0; power-on-reset = 00h) the adc status register is an 8-bit read-only register. to access the adc status register, the user must write to the communica - tions register, selecting the next operation to be a read and load bits a1?0 with 0, 0. table vi outlines the bit designations for the status register. sr0 to sr7 indicate the bit location, sr denoting the bits are in the status register. sr7 denotes the first b it of the data stream. the number in brackets indicates the power-on-reset default status of that bit. table vi. status register bit designations bit bit location name description sr7 rdy ready bit for adc. set when data is written to the adc data register. the rdy bit is cleared automatically after the adc data register has been read or a period of time before the data register is updated with a new conversion result. sr6 0t his bit is autom atically cleared. sr5 0t his bit is autom atically cleared. sr4 0t his bit is autom atically cleared. sr3 er r adc error bit. this bit is set at the same time as the rdy bit. set to indicate that the result written to the adc data register has been clamped to all zeros or all ones. error sources include overrange, underrange. c leared by a write to the mode bits to initiate a conversion. sr2 0t his bit is autom atically cleared. sr1 stby standby bit indication. when this bit is set, the ad7709 is in power-down mode. this bit is cleared when the adc is powered up. sr0 lock pll lock status bit. set if the pll has locked onto the 32 .768 khz crystal oscillator clock. if the user is w orried about exact sampling frequencies, etc., the lock bit should be interrogated and the re sult dis carded if the lock bit is 0. 7 r s6 r s5 r s4 r s3 r s2 r s1 r s0 r s ) 0 ( y d r) 0 ( 0) 0 ( 0) 0 ( 0) 0 ( r r e) 0 ( 0) 0 ( y b t s) 0 ( k c o l
rev. a ad7709 ?6 table vii. configuration register bit designations bit bit location name description config23 psw2 power switch 2 control bit. set by user to enable power switch sw2/p2 to pwrgnd. cleared by user to enable use as a standard i/o pin. w hen the adc is in standby mode, the power switches are open. config22 psw1 power switch 1 control bit. set by user to enable power switch sw1/p1 to pwrgnd. cleared by user to enable use as a standard i/o pin. when the adc is in standby mode, the power switches are open. config21 i3en1 iexc3 current source enable bit config20 i3en0 iexc3 current source enable bit config19 i2en1 iexc2 current source enable bit config18 i2en0 iexc2 current source enable bit config17 i1en1 iexc1 current source enable bit configuration register (a1, a0 = 0, 1; power-on-reset = 000007h) the configuration register is a 24-bit register from which data can either be read or to which data can be written. this regist er is used to select the input channel and configure the input range, excitation current sources, and i/o port. table vii outlines the bit de signations for this register. config23 to config0 indicate the bit location, config denoting the bits are in the configuration reg ister. config23 denotes the first bit of the data stream. the number in brackets indicates the power-on-reset default status of that b it. a write to the configuration register has immediate effect and do es not reset the adc. therefore, if a current s ource is sw itched while the adc is converting, the user will have to wait for the full settling time of the sinc 3 filter before obtaining a fully settled output. this equates to three outputs. 7 g i f n o c7 g i f n o c 7 g i f n o c 7 g i f n o c7 g i f n o c6 g i f n o c6 g i f n o c 6 g i f n o c 6 g i f n o c6 g i f n o c5 g i f n o c5 g i f n o c 5 g i f n o c 5 g i f n o c5 g i f n o c4 g i f n o c4 g i f n o c 4 g i f n o c 4 g i f n o c4 g i f n o c3 g i f n o c3 g i f n o c 3 g i f n o c 3 g i f n o c3 g i f n o c2 g i f n o c2 g i f n o c 2 g i f n o c 2 g i f n o c2 g i f n o c1 g i f n o c1 g i f n o c 1 g i f n o c 1 g i f n o c1 g i f n o c0 g i f n o c0 g i f n o c 0 g i f n o c 0 g i f n o c0 g i f n o c ) 0 ( l e s f e r) 0 ( l e s f e r ) 0 ( l e s f e r ) 0 ( l e s f e r) 0 ( l e s f e r) 0 ( 2 h c) 0 ( 2 h c ) 0 ( 2 h c ) 0 ( 2 h c) 0 ( 2 h c) 0 ( 1 h c) 0 ( 1 h c ) 0 ( 1 h c ) 0 ( 1 h c) 0 ( 1 h c) 0 ( 0 h c) 0 ( 0 h c ) 0 ( 0 h c ) 0 ( 0 h c) 0 ( 0 h c) 0 ( i n u) 1 ( 2 n r) 1 ( 1 n r) 1 ( 0 n r 3 2 g i f n o c3 2 g i f n o c 3 2 g i f n o c 3 2 g i f n o c3 2 g i f n o c2 2 g i f n o c2 2 g i f n o c 2 2 g i f n o c 2 2 g i f n o c2 2 g i f n o c1 2 g i f n o c1 2 g i f n o c 1 2 g i f n o c 1 2 g i f n o c1 2 g i f n o c0 2 g i f n o c0 2 g i f n o c 0 2 g i f n o c 0 2 g i f n o c0 2 g i f n o c9 1 g i f n o c9 1 g i f n o c 9 1 g i f n o c 9 1 g i f n o c9 1 g i f n o c8 1 g i f n o c8 1 g i f n o c 8 1 g i f n o c 8 1 g i f n o c8 1 g i f n o c7 1 g i f n o c7 1 g i f n o c 7 1 g i f n o c 7 1 g i f n o c7 1 g i f n o c6 1 g i f n o c6 1 g i f n o c 6 1 g i f n o c 6 1 g i f n o c6 1 g i f n o c ) 0 ( 2 w s p) 0 ( 2 w s p ) 0 ( 2 w s p ) 0 ( 2 w s p) 0 ( 2 w s p) 0 ( 1 w s p) 0 ( 1 w s p ) 0 ( 1 w s p ) 0 ( 1 w s p) 0 ( 1 w s p) 0 ( 1 n e 3 i) 0 ( 1 n e 3 i ) 0 ( 1 n e 3 i ) 0 ( 1 n e 3 i) 0 ( 1 n e 3 i) 0 ( 0 n e 3 i) 0 ( 0 n e 3 i ) 0 ( 0 n e 3 i ) 0 ( 0 n e 3 i) 0 ( 0 n e 3 i) 0 ( 1 n e 2 i) 0 ( 1 n e 2 i ) 0 ( 1 n e 2 i ) 0 ( 1 n e 2 i) 0 ( 1 n e 2 i) 0 ( 0 n e 2 i) 0 ( 0 n e 2 i ) 0 ( 0 n e 2 i ) 0 ( 0 n e 2 i) 0 ( 0 n e 2 i) 0 ( 1 n e 1 i) 0 ( 1 n e 1 i ) 0 ( 1 n e 1 i ) 0 ( 1 n e 1 i) 0 ( 1 n e 1 i) 0 ( 0 n e 1 i) 0 ( 0 n e 1 i ) 0 ( 0 n e 1 i ) 0 ( 0 n e 1 i) 0 ( 0 n e 1 i i2en1 i2en0 function 00 iexc2 current source off 01 iexc2 current source routed to the iout1 pin 10 iexc2 current source routed to the iout2 pin 11 reserved i3en1 i3en0 function 00 iexc3 current source off 01 iexc3 current source routed to the iout1 pin 10 iexc3 current source routed to the iout2 pin 11 reserved 5 1 g i f n o c4 1 g i f n o c3 1 g i f n o c2 1 g i f n o c1 1 g i f n o c0 1 g i f n o c9 g i f n o c8 g i f n o c ) 0 ( g i d 4 p) 0 ( g i d 3 p) 0 ( n e 2 p) 0 ( n e 1 p) 0 ( t a d 4 p) 0 ( t a d 3 p) 0 ( t a d 2 p) 0 ( t a d 1 p
rev. a ad7709 e17e bit bit location name description config16 i1en0 iexc1 current source enable bit config15 p4dig digital input enable. set by user to enable pin ain4/p4 as a digital input. a weak pull-up resistor is activated in this state. cleared by user to configure pin ain4/p4 as an analog input. config14 p3dig digital input enable. set by user to enable pin ain3/p3 as a digital input. a weak pull-up resistor is activated in this state. cleared by user to configure pin ain3/p3 as an analog input. config13 p2en sw2/p2 digital output enable bit. set by user to enable p2 as a regular digital output pin. cleared by user to three-state the p2 output. psw2 takes precedence over p2en. config12 p1en sw1/p1 digital output enable bit. set by user to enable p1 as a regular digital output pin. cleared by user to three-state the p1 output. psw1 takes precedence over p1en. config11 p4dat digital input port data bit. p4dat is read only and will return a zero if p4dig equals zero. if p4 is enabled as a digital input, the readback value indicates the status of pin p4. config10 p3dat digital input port data bit. p3dat is read only and will return a zero if p3dig equals zero. if p3 is enabled as a digital input, the readback value indicates the status of pin p3. config9 p2dat digital output port data bit. p2 is a digital output only. when the port is active as an output (p2en = 1), the value written to this data bit appears at the output port. reading p2dat will return the last value written to the p2dat bit. config8 p1dat digital output port data bit. p1 is a digital output only. when the port is active as an output (p1en = 1), the value written to this data bit appears at the output port. reading p1dat will return the last value written to the p1dat bit. config7 refsel adc reference input select. cleared by the user to select refin1(+) and refin1(e) as the adc reference. set by the user to select refin2(+) and refin2(e) as the adc reference. config6 ch2 adc input channel selection bit. it is used in conjunction with ch1 and ch0 as shown below. config5 ch1 adc input channel selection bit. it is used in conjunction with ch2 and ch0 as shown below. config4 ch0 adc input channel selection bit. it is used in conjunction with ch2 and ch1 as shown below. the buffer column indicates if the analog inputs are buffered or unbuffered. this determines the common-mode input range on each input. if the input is unbuffered (aincom), the common-mode input includes ground. table vii. configuration register bit designations (continued) ch2 ch1 ch0 positive input negative input buffer 0 00 ain1 aincom positive analog input 0 01 ain2 aincom positive analog input 0 10 ain3 aincom positive analog input 0 11 ain4 aincom positive analog input 1 00 ain1 ain2 positive and negative analog inputs 1 01 ain3 ain4 positive and negative analog inputs 1 10 aincom aincom none 1 11 ain2 ain2 positive and negative analog inputs i1en1 i1en0 function 00 iexc1 current source off 01 iexc1 current source routed to the iout1 pin 10 iexc1 current source routed to the iout2 pin 11 reserved
rev. a ad7709 e18e table viii. filter register bit designations table ix. update rate vs. sf word sf (dec) sf (hex) f adc (hz) t adc (ms) 13 0d 105.3 9.52 69 45 19.79 50.34 255 ff 5.35 186.77 table vii. configuration register bit designations (continued) bit bit location name description config3 uni unipolar/bipolar operation selection bit. set by the user to enable unipolar operation. in this mode, the device uses straight binary output coding i.e., 0 differential input will generate a result of 0000h and a full-scale differential input will generate a code of ffffh. cleared by the user to enable pseudo-bipolar operation. the device uses offset binary coding, i.e., a nega- tive full-scale differential input will result in a code of 0000h, a 0 differential input will generate a code of 8000h, while a positive full-scale differential input will result in a code of ffffh. config2 rn2 this bit is used in conjunction with rn1 and rn0 to select the analog input range as shown below. config1 rn1 this bit is used in conjunction with rn2 and rn0 to select the analog input range as shown below. config0 rn0 this bit is used in conjunction with rn2 and rn1 to select the analog input range as shown below. filter register (a1, a0 = 1, 0; power-on-reset = 45h) the filter register is an 8-bit register from which data can be read or to which data can be written. this register determines the amount of averaging performed by the sinc filter. table viii outlines the bit designations for the filter register. fr7 through fr0 indicate the bit location, fr denoting the bits are in the filter register. fr7 denotes the first bit of the data stream. the number in brackets indicates the power-on/reset default status of that bit. the number in this register is used to set the decima- tion factor and thus the output update rate for the adc. the filter register cannot be written to by the user while the adc is active. the update rate is calculated as follows: f sf f adc mod = 1 3 1 8 where: f adc is the adc output update rate. f mod is the modulator clock frequency = 32.768 khz. sf is the decimal value written to the sf register. the allowable range for sf is 13dec to 255dec. examples of sf values and corresponding conversion rate (f adc ) and time (t adc ) are shown in table ix. it should also be noted that the adc input channel is chopped to minimize offset errors. this means that the time for a single conversion or the time to the first con- version result is 2 t adc . adc data result register (a1, a0 = 1, 1; power-on-reset = 0000h) the conversion result is stored in the adc data register (data). this register is 16-bits wide. this is a read-only register. on completion of a read from this register, the rdy bit in the s tatus register is cleared. 7 r f6 r f5 r f4 r f3 r f2 r f1 r f0 r f ) 0 ( 7 f s) 1 ( 6 f s) 0 ( 5 f s) 0 ( 4 f s) 0 ( 3 f s) 1 ( 2 f s) 0 ( 1 f s) 1 ( 0 f s rn2 rn1 rn0 selected adc input range (v ref = 2.5 v) 00 0 20 mv 00 1 40 mv 01 0 80 mv 01 1 160 mv 10 0 320 mv 10 1 640 mv 11 0 1.28 v 11 1 2.56 v
rev. a ad7709 e19e configuring the ad7709 the four user-accessible registers on the ad7709 are accessed via th e serial interface. communication with any of these regis ters is initiated by first writing to the communications register. the ad7709 begins converting on power-up without the need to write to the registers. the default conditions are used, i.e., the ad7709 operates at a 19.79 hz update rate that offers 50 hz and 60 hz rejection. figure 10 outlines a flow diagram of the sequence used to con figure all registers after a power-up or reset on the ad7709. the flowchart shows two methods of determining when it is valid to read the data register. the first method is hardware polling of the rdy rdyt adc adc t c ad r tad r cr t c r dw adc cr w t rdy rdystats rdy n ardware n start stware n wernresetrad cnreandntae c serart wrtetcncatnsrestersettn neteratnteawrtette terresterwrtetrester wrtetterrestercnrn terereddaterate wrtetcncatnsrestersettn neteratnteawrtette cnratnrester wrtetrester readdatarttrester wrtetcncatnsrestersettn neteratnteareadrte statsresterwrtetrester wrtetcncatnsrestersettn neteratnteareadrte dataresterwrtetrester wrtetcncatnsrestersettn neteratnteareadrte dataresterwrtetrester readtdatarest readtdatarest readstatsrester anter read rdy w anter read canne cane rdy end end stware n ardware n n yes yes yes n yes n yes n yes n wrtetcnratnrestertseect tentcannentraneand reerencecrrentsrcesandrt canasecnred canne cane rdad
rev. a ad7709 e20e digital interface as previously outlined, ad7709 programmable functions are controlled using a set of on-chip registers. data is written to these registers via the part?s serial interface and read access to the on-chip registers is also provided by this interface. all com- munications to the part must start with a write operation to the c ommunications register. after power-on or reset , the device expects a write to its communications register. the data writ- ten to this register determines whether the next operation to the part is a read or a write operation and also determines to which register this read or write operation occurs. therefore, write access to any of the other registers on the part starts with a write operation to the communications register followed by a write to the selected register. a read operation from any other register on the part (including the output data register) starts with a write operation to the communications register followed by a read operation from the selected register. the ad7709 serial interface consists of five signals: cs sc dndtrdytdn dt sc dn d tsct rdy adrdy not to read from the device to ensure that a data read is not attempted while the register is being updated. cs ad ad cs ad rdyc t a d cs scdn dtad rdy s rt cs sc t a d cs tds s cs cs sc dstsc t reset dnaddn t t adc rt s addtdn a w pull-up resistor should be used on this single data line. in this case, if the interface gets lost, because the read and write operations share the same line, the procedure to reset it back to a known state is somewhat different than previously described. it requires a read operation of 24 serial clocks followed by a write operation where a logic 1 is written for at least 32 serial clock cycles to ensure that the serial interface is back into a known state. microcomputer/microprocessor interfacing the ad7709 flexible serial interface allows for easy interface to most microcomputers and microprocessors. the flowchart of figure 10 outlines the sequence that should be followed when interfacing a microcontroller or microprocessor to the ad7709. figures 11, 12, and 13 show some typical interface circuits. the serial interface on the ad7709 is capable of operating from just three wires and is compatible with spi interface protocols. the 3-wire operation makes the part ideal for isolated systems where minimizing the number of interface lines minimizes the number of opto-isolators required in the system. the serial clock input is a schmitt-triggered input to accommodate slow edges from opto-couplers. the rise and fall tim es of other digital inputs to the ad7709 should be no longer than 1 m s. some of the registers on the ad7709 are 8-bit registers, which facilitates easy interfacing to the 8-bit serial ports of microcon- trollers. the data register on the ad7709 is 16 bits and the configuration register is 24 bits, but data transfers to these registers can consist of multiple 8-bit transfers to the serial port of the microcontroller. dsp processors and microprocessors generally transfer 16 bits of data in a serial data operation. some of these processors, such as the adsp-2105, have the facility to program the amount of cycles in a serial transfer. this allows the user to tailor the number of bits in any transfer to match the register length of the required register in the ad7709. even though some of the registers on the ad7709 are only 8 bits in length, communicating with two of these registers in succes sive write operations can be handled as a single 16-bit data transfer if required. for example, if the filter register is to be updated, the processor must first write to the communications register (say- ing that the next operation is a write to the filter register), and then write 8 bits to the filter register. if required, this can all be done in a single 16-bit transfer because once the eight serial clocks of the write operation to the communications register have been completed, the part immediately sets itself up for a write operation to the filter register.
rev. a ad7709 e21e ad7709-to-68hc11 interface figure 11 shows an interface between the ad7709 and the 68hc11 microcontroller. the diagram shows the minimum (3-wire) interface with cs ad rdysr dra rdy adt rdy rdy cc t rdy t rdy rc cs ad cc cs tcc caw csc tad ad dtsc sad dn sc c dd ad ss sc s s reset dt dn cs sc dd adc ad aadc t cs ad c rdy sr drt rdy adt rdy rdy c t rdy c dd ad reset dn sc cs dt dd adc t rdy nt c cs adc cs tc a dtdn ad w pull-up resistor. the serial clock on the 8xc51 idles high between data transfers. the 8xc51 outputs the lsb first in a write operation, while the ad7709 expects the msb first so the data to be transmitted has to be rearranged before being written to the output serial register. similarly, the ad7709 outputs the msb first during a read operation while the 8xc51 expects the lsb first. therefore, the data read into the serial buffer needs to be rearranged before the correct data word from the ad7709 is available in the accumulator. adsp-2103/ adsp-2105 v dd ad7709 rfs sclk reset dout din sclk cs tfs dr dt figure 13. ad7709-to-adsp-2103/adsp-2105 interface ad7709-to-adsp-2103/adsp-2105 interface f igure 13 shows an interface between the ad7709 and the adsp-2103/adsp-2105 dsp processor. in the interface shown, t he rdy bit of the status register is again monitored to determine when the data register is updated. the alternative scheme is to use an interrupt-driven system, in which case the
rev. a ad7709 e22e rdy rads adst adsads trsts adsads adsadss c t cs ad rstsads adstads ads ad crctdescrtn tad ad t as a adc t 20 mv to 2.56 v. the input channels can be configured for either fully differential inputs or pseudo-differential input channels via the ch2, ch1, and ch0 bits in the configuration register. buffer ing the input channel allows the part to handle significant source impedances on the analog input, allowing r/c filtering (for noise rejection or rfi reduction) to be placed on the analog inputs if required. these input channels are intended for converting signals directly from sensors without the need for external signal conditioning. other functions contained on-chip that augment the operation of the adc include software configurable current sources, switchable reference inputs, and low-side power sw itches. the basic connection diagram for the ad7709 is shown in figure 14. an ad780/ref195, precision 2.5 v reference, provides the reference source for the part. a quartz crystal or ceramic resonator provides the 32.768 khz master clock source for the part. in some cases, it will be necessary to connect capacitors on the crystal or resonator to ensure that it does not oscillate at over- tones of its fundamental operating frequency. the values of capacitors will vary depending on manufacturer specifications. analog input channels the main adc has five associated analog input pins (labeled ain1 to ain4 and aincom) that can be configured as two fully differential input channels (ain1eain2 and ain3eain4) or four pseudo-differential input channels (ain1eaincom, ain2eaincom, ain3eaincom, and ain4eaincom). channel selection bits ch2, chi, and ch0 in the c onfigura tion register detail the different configurations. when the analog input channel is switched, the settli ng time of th e part must elapse before a new valid word is available from the adc. iout1 ain3/p3 aincom ain1 iout2 ain4/p4 refin1(e) ain2 refin1(+) refin2(e) refin2(+) reset cs dout din sclk p2/sw2 p1/sw1 xtal1 xtal2 5v chip select receive (read) serial clock 32.768khz crystal pwrgnd gnd ad780/ ref195 v in v out gnd v dd analog 5v supply 0.1 f 10 f 0.1 f 10 f analog 5v supply ad7709 serial data (write) figure 14. basic connection diagram the output of the adc multiplexer feeds into a high impedance input stage of the buffer amplifier. as a result, the adc inputs can handle significant source impedances and are tailored for direct connection to external resistive-type sensors like strain gauges or resistance temperature detectors (rtds). the absolute input voltage range on the adc inputs when buff- ered (ain1 to ain4) is restricted to a range between gnd + 100 mv and v dd e 100 mv. care must be taken in setting up the common-mode voltage and input voltage range so that these limits are not exceeded; otherwise, there will be a degradation in linearity and noise performance. the absolute input voltage range on the adc inputs when unbuffered (aincom) includes the range between gnd e 30 mv to v dd + 30 mv as a result of being unbuffered. the negative abso- lute input voltage limit does allow the possibility of monitoring small true bipolar signals with respect to gnd.
rev. a ad7709 ?3 programmable gain amplifier the output from the buffer on the adc is applied to the input of the on-chip programmable gain amplifier (pga). the pga can be programmed through eight different unipolar and bipolar ranges. the pga gain range is programmed via the range bits in the configuration register. with an external 2.5 v reference applied, the unipolar ranges are 0 mv to 20 mv, 0 mv to 40 mv, 0 mv to 80 mv, 0 mv to 160 mv, 0 mv to 320 mv , 0 mv to 640 mv, 0 v to 1.28 v, and 0 to 2.56 v, while bipolar ranges are 20 mv, 40 mv, 80 mv, 160 mv, 320 mv, 640 mv, 1.28 v, and 2.56 v. these are the ranges that should appear at the input to the on-chip pga. typical matching across ranges is shown in figure 15. here, the adc is configured in fully differential, bipolar mode with an external 2.5 v reference, while an analog input voltage of just greater than 19 mv is forced on its analog inputs. the adc continuously converts the dc voltage at an update rate of 5.35 hz, i.e., sf = ffh. a total of 800 conversion results are gath ered. the first 100 results gathered with the adc operating in the 20 mv. the adc range is then switched to 40 mv and 100 more results are gathered, and so on, until the last 100 samples are gathered with the adc configured in the 2.5 v range. from figure 15, the variation in the sample mean through each range, i.e., the range matching, is seen to be on the order of 2 v. 0 100 200 300 400 500 600 700 800 sample count adc input voltage ?mv 19.372 19.371 19.370 19.369 19.368 19.367 19.366 19.365 19.364 adc range 20mv 40mv 80mv 160mv 320mv 640mv 1.28v 2.56v figure 15. adc range matching bipolar/unipolar configuration the analog inputs on the ad7709 can accept either unipolar or bipolar input voltage ranges. bipolar input ranges do not imply that the part can handle negative voltages with respect to system gnd. unipolar and bipolar signals on the ain(+) input on the adc are referenced to the voltage on the respective ain(? input. ain(+) and ain(? refer to the signals seen by the modulator that come from the output of the multiplexer, as shown in figures 16 and 17. fully differential fully differential ain(+) ain(? ain1 ain2 ain3 ain4 ain1 ain2 ain3 ain4 mux adc channel figure 16. fully differential mode of operation ain3 ain3 ain4 ain4 ain1/aincom pseudo-differential input ain(? ain(+) ain1 ain1 ain2 ain2 aincom aincom adc channel mux ain2/aincom ain3/aincom ain4/aincom pseudo-differential input figure 17. pseudo-differential mode of operation for example, if ain(? is 2.5 v and the adc is configured for an analog input range of 0 mv to 20 mv, the input voltage range on the ain(+) input is 2.5 v to 2.52 v. if ain(? is 2.5 v and the ad7709 is configured for an analog input range of 1.28 v, the analog input range on the ain(+) input is 1.22 v to 3.78 v (i.e., 2.5 v 1.28 v). bipolar or unipolar options are chosen by programming the uni bit in the configuration register. this programs the adc for either unipolar or bipolar operation. programming for either unipolar or bipolar operation does not change any of the input signal conditioning; it simply changes the data output coding. data output coding when the adc is configured for unipolar operation, the output coding is natural (straight) binary with a zero differential input voltage resulting in a code of 000 . . . 000, a midscale voltage resulting in a code of 100 . . . 000, and a full-scale input voltage re sulting in a code of 111 . . . 111. the output code for any analog input voltage on the adc can be represented as follows: code ain gain v n ref = () () 2 1 024 . where: ain is the analog input voltage. gain is the pga gain, i.e., 1 on the 2.56 v range and 128 on the 20 mv range. n = 16.
rev. a ad7709 e24e when the adc is configured for bipolar operation, the coding is offset binary with a negative full-scale voltage resulting in a code of 000 . . . 000, a zero differential voltage resulting in a code of 100 . . . 000, and a positive full-scale voltage resulting in a code of 111 . . . 111. the output code from the adc for any analog input voltage can be represented as follows: code ain gain v n ref = () () + [] - 21 024 1 1 /. where: ain is the analog input voltage. gain in the pga gain, i.e., 1 on the 2.56 v range and 128 on the 20 mv range. n = 16. excitation currents the ad7709 also contains three software configurable constant current sources. iexc1 and iexc2 provide 200 m a of current while iexc3 provides 25 m a of current. all source current from v dd is directed to either the iout1 or iout2 pins of the device. these current sources are controlled via bits in the configuration register. the configuration bits enable the cur rent sources, and they can be configured to source current individu ally to both pins or a combination of currents, i.e., 400 m a, 225 m a, or 425 m a to either of the selected output pins. these current sources can be used to excite external resistive bridge or rtd sensors. crystal oscillator the ad7709 is intended for use with a 32.768 khz watch crys- tal. a pll internally locks onto a multiple of this frequency to provide a stable 4.194304 mhz clock for the adc. the modu- lator sample rate is the same as the crystal oscillator frequency. the start-up time associated with 32.768 khz crystals is typi cally 300 ms. the oscpd bit in the communications register can be used to prevent the oscillator from powering down when the ad7709 is placed in power-down mode. this avoids having to wait 300 ms after exiting power-down to start a conversion at the expense of raising the power-down current. reference input the ad7709 has a fully differential reference input capability for the channel. on the channel, the reference inputs can be refin1(+) and refin1(e) or refin2(+) and refin2(e). they provide a differential reference input capability. the common-mode range for these differential inputs is from gnd to v dd . the reference input is unbuffered and therefore ex cessive r-c source impedances will introduce gain errors. the nominal reference voltage, v ref , ((refin1(+) e refin1(e) or (refin2(+) e refin2(e)), for specified operation is 2.5 v, but the ad7709 is functional with refer ence voltages from 1 v to v dd . in applications where th e excita tion (voltage or current) for the trans ducer on the an alog i nput also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source will be removed because the application is ratiometric. if the ad7709 is used in a nonratiometric application, a low noise reference should be used. recommended reference voltage sources for the ad7709 include the ad780, ref43, and ref192. it should also be noted that the reference inputs provide a high impedance, dynamic load. because the input impedance of each reference input is dynamic, resistor/capacitor combinations on these inputs can cause dc gain errors, depending on the output impedance of the source that is driving the reference inputs. reference voltage sources like those recommended above (e.g., ad780) will typically have low output impedances and are therefore tolerant to having decoupling ca paci- tors on the refin(+) without introducing gain errors in the system. deriving the reference input voltage across an external resistor, as shown in figure 18, will mean that the reference input sees a significant external source impedance. external decoupling on the refin pins would not be recommended in this type of circuit configuration. reset input the reset ad rdy ad reset w reset ad reset d styadccr adtad t rdy ad rdy m a typical when the part is operated at 5 v with the oscillator run ning during power-down mode. with the oscillator shut down, the total i dd is 1.5 m a typical at 3 v and 6.5 m a typical at 5 v. grounding and layout since the analog inputs and reference inputs on the adc are differential, most of the voltages in the analog modulator are common-mode voltages. the excellent common-mode rejection of the part will remove common-mode noise on these inputs. t he digital filter will provide rejection of broadband noise on the power supply, except at integer multiples of the modulator sampling frequency. the digital filter also removes noise from the analog and reference inputs, provided these noise sources do not saturate the analog modulator. as a result, the ad7709 is m ore immune to noise interference than a conventional high resolution converter. however, because the resolution of the ad7709 is so high, and the noise levels from the ad7709 so low, care must be taken with regard to grounding and layout.
rev. a ad7709 e25e the printed circuit board that houses the ad7709 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. a minimum etch technique is generally best for ground planes as it gives the best shielding. it is recommended that the ad7709 gnd pin be tied to the agnd plane of the system. in any layout, it is important that the user keep in mind the flow of currents in the system ensuring that the return paths for all currents are as close as possible to the paths the currents took to reach their destinations. avoid forcing digital currents to flow through the agnd sections of the layout. the pwrgnd pin is tied internally to gnd on the ad7709. the pwrgnd pad internally has a resistance of less than 50 m w to the pwrgnd pin, while the resistance back to the gnd pad is less than 3 w . this means that 19.5 ma of the maximum speci- fied current (20 ma) will flow to pwrgnd with the remaining 0.5 ma flowing to gnd. pwrgnd and gnd should be tied together at the ad7709, and it is import ant to minimize the resistance on the ground return lines. avoid running digital lines under the device since these will couple noise onto the die. the analog ground plane should be allowed to run under the ad7709 to prevent noise coupling. t he pow er s upply lines to the ad7709 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. avoid crossover of digital and analog s ignals. traces on opposite sides of the board should run at right angles to each other, which will reduce the effects of feedthrough through the board. a microstrip technique is by far the best, but is not always possible with a double-sided board. in this technique, the component side of the board is dedicated to ground planes w hile signals are placed on the solder side. g ood decoupling is important when using high resolution adcs. the supply should be decoupled with 10 m f tantalum in p arallel with 0.1 m f capacitors to gnd. to achieve the best from these decoupling components, they have to be placed as close as possible; chips should be decoupled with 0.1 m f ceramic capacitors to dgnd. applications the ad7709 provides a low cost, high resolution, analog-to- digital function. because the analog-to-digital function is provided by a - architecture, it makes the part more immune to noisy environments, making it ideal for use in sensor measurement and industrial and process control applications. given the architec ture used in the ad7709, where the signal chain is chopped and the device is factory-calibrated at final test, field calibration is not needed due to the extremely low offset and gain drifts exhibited by this converter. it also provides a programmable gain amplifier and a digital filter. thus, it provides far more system-level func- tionality than off-the-shelf integrating adcs without the disadvantage of having to supply a high quality integrating ca pacitor. in addition, using the ad7709 in a system allows the system designer to achieve a much higher level of reso lution be cause noise perfo rmance of the ad7709 is significantly be tte r tha n that of integrating adcs. the on-chip pga allows the ad7709 to handle an analog input voltage range as low as 10 mv full scale with v ref = 1.25 v. the differential inputs of the part allow this analog in put range to have an absolute value anywhere between gnd + 100 mv and v dd e 100 mv. it allows the user to connect the transducer directly to the input of the ad7709. the programmable gain front end on the ad7709 allows the part to handle unipolar analog input ranges from 0 mv to 20 mv and 0 v to 2.5 v an d bipolar inputs of 20 mv to 2.5 v. because the part oper- ates from a single supply, these bipolar ranges are with respect to a biased-up differential input. pressure measurement one typical application of the ad7709 is pressure measurement. figure 18 shows the ad7709 used with a pressure transducer, the bp01 from sensym. the pressure transducer is arranged in a bridge network and gives a differential output voltage between its out(+) and out(e) terminals. with rated full-scale pres- sure (in this case 300 mmhg) on the transducer, the differential output voltage is 3 mv/v of the input voltage (i.e., the voltage between its in(+) and in(e) terminals). assuming a 5 v excitation voltage, the full-scale output range from the transducer is 15 mv. the excitation voltage for the bridge can be used to directly provide the reference for the adc as the reference input range includes the supply. alternatively, a suitable resistor divider can be implemented that allows the full dynamic range of the input to be utilized in this application. this implementation is fully ratiometric, so variations in the excitation voltage do not introduce errors in the system. choosing resistor values of 10 k w and 6 k w as per figure 18 gives a 1.875 v reference voltage for the ad7709 when the excitation voltage is 5 v. ad7709 in+ out+ oute ine 10k excitation voltage = 5v v dd ain1 ain2 refin1(+) p1 pwrgnd gnd refin2(e) 6k figure 18. pressure measurement using the ad7709 using the part with a programmed gain of 128 results in the full-scale input span of the ad7709 being 15 mv, which corre- sponds with the output span from the transducer.
rev. a ad7709 e26e a second key advantage to using the ad7709 in transducer -based applications is that the on-chip low-side power switch can be fully utilized in low power applications. the low-side power switch is connected in series with the cold side of the bridge. in normal operation, the switch is closed and measurements can be taken from the bridge. in applications where power is a concern, the ad7709 can be put into low power mode, substantially reducing the power burned in the application. in addition to this, the power switch can be opened while in low power mode, thus avoiding the unnecessary burning of power in the front end transducer. when taken back out of power-down, and the power switch is closed, the user should ensure that the front end circuitry is fully settled before attempting a read from the ad7709. the circuit in figure 19 shows a method that utilizes three pseudo-differential input channels on the ad7709 to temperature- compensate a pressure transducer. 5v out(+) out(e) in(e) in(+) i1 i2 pressure bridge xtal1 xtal2 iout1 6.25k v dd refin(+) refin(e) ain2 ain1 ain3 aincom gnd ad7709 250 figure 19. temperature-compensating a pressure transducer in this application, pseudo-differential input channel ain1/ aincom is used to measure the bridge output while pseuo- differential channels ain2/aincom and ain3/aincom measure the voltage across the bridge. the voltage measured across the bridge will vary proportionally with temperature, and the delta in this voltage can be used to temperature- compensate the output of the pressure bridge. temperature measurement th e ad7709 is also useful in temperature m easurement appli- cations. figure 20 shows an rtd temperature measurement application. refin(e) iout1 5v 6.25k ain2 ain1 ad7709 refin(+) iout2 v dd gnd pwrgnd drdy sclk din dout cs xtal1 xtal2 r ref rl1 rl2 rl3 rl4 r cm rtd 200 a controller figure 20. 4-wire rtd temperature measurement using the ad7709 in this application, the transducer is an rtd (resistive tem- perature device), a pt100. the arrangement is a 4-lead rtd configuration. there are voltage drops across the lead resistances rl1 and rl4, but these simply shift the common-mode voltage. there is no voltage drop across lead resistances rl2 and rl3 since the input current to the ad7709 is very low, looking into a h igh input impedance buffer. r cm is included to shift the analog input voltage to ensure that it lies within the common- mode range (gnd + 100 mv to v dd e 100 mv) of the adc. in the application shown, the on-chip 200 m a current source provides the excitation current for the pt100 and also generates the reference voltage for the ad7709 via the 6.25 k w resistor. variations in the excitation current do not affe ct the circuit since bot h the input voltage and the reference voltage vary ratiometrically with the excitation current. however, the 6.25 k w resistor must have a low temperature coefficient to avoid errors in the reference voltage over temperature.
rev. a ad7709 e27e figure 21 shows a further enhancement to the circuit shown in figure 20. generally, dc excitation has been accepted as the normal method of exciting resistive based sensors like rtds in temperature measurement applications. iout1 iout2 v dd ain2 ain1 ain3 ain4 ad7709 refin(+) mux1 r ref a a buf and pga 200 a i1 emf1 resistive t ransducer emf2 p1 p2 refin(e) figure 21. low resistance measurement with dc excitation, the excitation current through the sensor must be large enough so that the smallest temperature/resis- tance change to be measured results in a voltage change that is larger t han the system noise, offset, and drift of the system. the purpose of sw itching the excitation source is to eliminate dc-induced errors. dc errors (emf1 and emf2) due to para- sitic thermocouples produced by differential metal connections (solder and copper track) within the circuit are also eliminated when using this switching arrangement. this excitation is a form of synchronous detection where the sensor is excited with an alternating excitation source and the adc measures infor- mation only in the same phase as the excitation source. the switched polarity current source is developed using the on- chip current sources and external phase control switches (a and a add ad d t a t a adca t rtd adcd adc rtd adc t rr re adc wrtdc trtd tad m a current sources, is ideally suited to these applications. one possible 3-wire configuration using the ad7709 is shown in figure 22. refin(e) iout1 gnd 5v 6.25k ain2 ain1 ad7709 rl3 r cm refin(+) iout2 v dd drdy sclk din dout cs xtal1 xtal2 rl2 rtd 200 a 200 a rl1 controller figure 22. 3-wire rtd configuration using the ad7709 in this 3-wire configuration, the lead resistances will result in errors if only one current source is used since the 200 m a will flow through rl1, developing a voltage error between ain1 and ain2. in the scheme outlined below, the second rtd current source is used to compensate for the error introduced by the 200 m a flowing through rl1. the second rtd current flows through rl2. assuming that rl1 and rl2 are equal (the leads would normally be of the same material and of equal length) and that iout1 and iout2 match, the error voltage across rl2 equals the error voltage across rl1 and no error voltage is developed between ain1 and ain2. twice the voltage is developed across rl3 but, since this is a common-mode voltage, it will not introduce errors. r cm is included so the current flowing through the combination of rl3 and r cm develops enough voltage that the analog input voltage seen by the ad7709 is within the common - mode range of the adc. the reference voltage for the ad7709 is also generated using one of these matched current sources. this reference voltage is developed across the 6.25 k w resistor as shown, and applied to the differential reference inputs of the ad7709. this scheme ensures that the analog input voltage span remains ratiometric to the reference voltage. any errors in the analog input voltage due to the temperature drift of the rtd current source is compensated for by the variation in the reference voltage. the typical drift matching between the two rtd current sources is less than 20 ppm/ m a current source that can be used along with the two 200 m a current sources for v be measurement where a 17:1 ratio is required from the current sources.
rev. a ad7709 ?8 microcontroller v cc gnd ref out2 ref in 10 f 0.1 f v dd ain1 ain2 cs dout sclk din gnd refin(+) refin(? 0.1 f ref out1 clock latch data 4.7 f com c1 c2 c3 ad7709 ad421 boost v cc lv 0.01 f 1k 1000pf loop power 10 f 3.3v 1.25v dn25d 0.01 f loop rtn comp drive v ariables ain3 ain4 figure 23. smart transmitter employing the ad7709 smart transmitters smart transmitters are another key design-in area for the ad 7709. the - converter, single-supply operation, 3-wire interface capabilities, and small package size are all of benefit in smart transmitters. here, the entire smart transmitter must operate from the 4?0 ma loop. tolerances in the loop mean that the amount of current available to power the transmitter is as low as 3.5 ma. figure 23 shows a bl ock diagram of a smart transmitter that includes the ad7709. not shown in figure 23 is the isolated power source required to power the front end.
rev. a ad7709 e29e 24-lead thin shrink small outline package [tssop] (ru-24) dimensions shown in millimeters 24 13 12 1 6.40 bsc 4.50 4.40 4.30 pin 1 7.90 7.80 7.70 0.15 0.05 0.30 0.19 0.65 bsc 1.20 max 0.20 0.09 0.75 0.60 0.45 8 0 seating plane compliant to jedec standards mo-153ad 0.10 coplanarity outline dimensions
rev. a ad7709 e30e revision history location page 3/03?data sheet changed from rev. 0 to rev. a. changes to specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 change to communications register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 changes to table viii. filter register bit designations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 updated outline dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
e31e
e32e c02700e0e3/03(a) printed in u.s.a.

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