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a 12-bit, 65 msps if sampling a/d converter features 65 msps minimum sample rate 80 db spurious-free dynamic range if sampling to 70 mhz 710 mw power dissipation single 5 v supply on-chip t/h and reference twos complement output format 3.3 v or 5 v cmos compatible output levels applications cellular/pcs base stations multichannel, multimode receivers gps anti-jamming receivers communications receivers phased array receivers functional block diagram adc a th3 dac adc th2 buf th1 msb lsb d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0 digital error correction logic 7 gnd 6 ain ain v ref encode e ncode 2.4v reference internal timing av cc dv cc AD6640 general description the AD6640 is a high speed, high performance, low power, monolithic 12-bit analog-to-digital converter. all necessary functions, including track-and-hold (t/h) and reference, are included on-chip to provide a complete conversion solution. the AD6640 runs on a single 5 v supply and provides cmos compatible digital outputs at 65 msps. specifically designed to address the needs of multichannel, multimode receivers, the AD6640 maintains 80 db spurious- free dynamic range (sfdr) over a bandwidth of 25 mhz. noise performance is also exceptional: typical signal-to-noise ratio is 68 db. the AD6640 is built on analog devices?high speed complemen- tary bipolar process (xfcb) and uses an innovative multipass architecture. units are packaged in a 44-lead plastic quad flatpack (lqfp) specified from ?0 c to +85 c. product highlights 1. guaranteed sample rate is 65 msps. 2. fully differential analog input stage specified for frequencies up to 70 mhz; enables if sampling. 3. low power dissipation: 710 mw off a single 5 v supply. 4. digital outputs may be run on 3.3 v supply for easy interface to digital asics. 5. complete solution: reference and track-and-hold. 6. packaged in small, surface-mount 44-lead plastic lqfp. AD6640 rev. a 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.
dc specifications test AD6640ast parameter temp level min typ max unit resolution 12 bits accuracy no missing codes +25 ci guaranteed offset error full vi ?0 +3.5 +10 mv gain error full vi ?0 +4.0 +10 % fs differential nonlinearity (dnl) 1 +25 ci ?.0 0.5 +1.5 lsb integral nonlinearity (inl) 1 full v 1.25 lsb temperature drift offset error full v 50 ppm/ c gain error full v 100 ppm/ c power supply rejection ratio (psrr) full v 0.5 mv/v reference out (v ref ) 2 full v 2.4 v analog inputs (ain, ain ) 3 analog input common-mode range 4 full v v ref 0.05 v differential input voltage range full v 2.0 v p-p differential input resistance full iv 0.7 0.9 1.1 k ? differential input capacitance +25 cv 1.5 pf power supply supply voltage av cc full vi 4.75 5.0 5.25 v dv cc full vi 3.0 3.3 5.25 v supply current ia vcc (av cc = 5.0 v) full vi 135 160 ma id vcc (dv cc = 3.3 v) full vi 10 20 ma power consumption full vi 710 865 mw notes 1 encode = 20 msps 2 if v ref is used to provide a dc offset to other circuits, it should first be buffered. 3 the AD6640 is designed to be driven differentially. both ain and ain should be driven at levels v ref 0.5 v. the input signals should be 180 degrees out of phase to produce a 2 v p-p differential input signal. see driving the analog inputs section for more details. 4 analog input common-mode range specifies the offset range the analog inputs can tolerate in dc-coupled applications (see figure 17 for more detail). specifications subject to change without notice . digital specifications test AD6640ast parameter temp level min typ max unit logic inputs (encode, encode ) 1 encode input common-mode range 2 full iv 0.2 2.2 v differential input voltage full iv 0.4 v p-p single-ended encode 10 v p-p logic compatibility 3 ttl/cmos logic ??voltage full vi 2.0 5.0 v logic ??voltage full vi 0 0.8 v logic ??current (v inh = 5 v) full vi +500 +650 +800 a logic ??current (v inl = 0 v) full vi ?00 ?20 ?00 a input capacitance +25 cv 2.5 pf logic outputs ( d11 ?0) 4 logic compatibility cmos logic ??voltage (dv cc = 3.3 v) full vi 2.8 dv cc ?0.2 v logic ??voltage (dv cc = 3.3 v) full vi 0.2 0.5 v logic ??voltage (dv cc = 5.0 v) full iv 4.5 dv cc ?0.3 v logic ??voltage (dv cc = 5.0 v) full iv 0.35 0.5 v output coding twos complement notes 1 best dynamic performance is obtained by driving encode and encode differentially. see encoding the AD6640 section for more details. performance versus encode/ encode power is shown in tpc 12. 2 for dc-coupled applications, the encode input common-mode range specifies the common-mode range the encode inputs can tolerate when driven differentially by the minimum differential input voltage of 0.4 v p-p. for differential input voltage swings greater than 0.4 v p-p, the common-mode range will change. the minimum value ensures that the input voltage on either encode pin does not go below 0 v. the maximum value ensures that the input voltage on either en code pin does not go below 2.0 v or above av cc (e.g., for a differential input swing of 0.8 v, the min and max common-mode specs become 0.4 v and 2.4 v, respectively). 3 encode or encode may be driven alone if desired, but performance will likely be degraded. logic compatibility specifications are provided to sh ow that ttl or cmos clock sources will work. when driving only one encode input, bypass the complementary input to gnd with 0.01 f. 4 digital output load is one lcx gate. specifications subject to change without notice. rev. a (av cc = 5 v, dv cc = 3.3 v; t min = ?0 c, t max = +85 c, unless otherwise noted.) ? AD6640?pecifications (av cc = 5 v, dv cc = 3.3 v; t min = ?0 c, t max = +85 c, unless otherwise noted.)
test AD6640ast parameter (conditions) temp level min typ max unit maximum conversion rate full vi 65 msps minimum conversion rate 2 full iv 6.5 msps aperture delay (t a ) +25 cv 400 ps aperture uncertainty (jitter) +25 cv 0.3 ps rms encode pulsewidth high 3 +25 civ 6.5 ns encode pulsewidth low +25 civ 6.5 ns output delay (t od ) dv cc + 3.3 v/5.0 v 4 full iv 8.5 10.5 12.5 ns notes 1 all switching specifications tested by driving encode and encode differentially. 2 a plot of performance versus encode is shown in tpc 10. 3 a plot of performance versus duty cycle (encode = 65 msps) is shown in tpc 11. 4 outputs driving one lcx gate. delay is measured from differential crossing of encode and encode to the time when all output data bits are within valid logic levels. specifications subject to change without notice. test AD6640ast parameter (conditions) temp level min typ max unit snr analog input 2.2 mhz +25 cv 68 db @ ? dbfs 15.5 mhz +25 ci 6467 .7 db 31.0 mhz +25 cv 67 .5 db 69.0 mhz +25 cv 66 db sinad analog input 2.2 mhz +25 cv 68 db @ ? dbfs 15.5 mhz +25 ci 63.5 67.2 db 31.0 mhz +25 cv 67 .0 db 69.0 mhz +25 cv 65 .5 db worst harmonic 2 (2nd or 3rd) analog input 2.2 mhz +25 cv 80 dbc @ ? dbfs 15.5 mhz +25 ci 74 80 dbc 31.0 mhz +25 cv 79 .5 dbc 69.0 mhz +25 cv 78 .5 dbc worst harmonic 2 (4th or higher) analog input 2.2 mhz +25 cv 85 dbc @ ? dbfs 15.5 mhz +25 ci 74 85 dbc 31.0 mhz +25 cv 85 dbc 69.0 mhz +25 cv 84 dbc multitone sfdr (with dither) 3 eight tones @ ?0 dbfs full v 90 dbfs two-tone imd rejection 4 f1, f2 @ ? dbfs full v 80 dbc analog input bandwidth 5 +25 cv 300 mhz notes 1 all ac specifications tested by driving encode and encode differentially. 2 for a single test tone at ? dbfs, the worst-case spectral performance is typically limited by the direct or aliased second or third harmonic. if a system is designed such that the second and third harmonics fall out-of-band, overall performance in the band of interest is typically improved by 5 db. worst harmonic (fourth or higher) includes fourth and higher order harmonics and all other spurious components. reference tpc 6 for more detail. 3 see overcoming static nonlinearities with dither section for details on improving sfdr performance. to measure sfdr, eight tone s from 14 mhz to 18 mhz (0.5 mhz spacing) are swept from ?0 dbfs to ?0 dbfs. an open channel at 16 mhz is used to monitor sfdr. 4 f1 = 14.9 mhz, f2 = 16 mhz. 5 specification is small signal bandwidth. plots of performance versus analog input frequency are shown in tpcs 4, 5, and 6. samp ling wide bandwidths (5 mhz?5 mhz) should be limited to 70 mhz center frequency. specifications subject to change without notice. rev. a ? (av cc = 5 v, dv cc = 3.3 v; encode and encode = 65 msps; t min = ?0 c, t max = +85 c, unless otherwise noted.) (av cc = +5 v, dv cc = +3.3 v; encode and encode = 65 msps; t min = ?0 c, t max = +85 c, unless otherwise noted.) switching specifications 1 ac specifications 1 AD6640
AD6640 ? rev. a absolute maximum ratings 1 parameter min max unit electrical av cc voltage 0 7 v dv cc voltage 0 7 v analog input voltage 0 av cc v analog input current 25 ma digital input voltage (encode) 0 5 v digital output current ?0 +10 ma environmental 2 operating temperature range (ambient) ?0 +85 c maximum junction temperature 150 c lead temperature (soldering, 10 sec) 300 c storage temperature range (ambient) ?5 +150 c notes 1 absolute maximum ratings are limiting values to be applied individually and beyond which the serviceability of the circuit may be impaired. functional operability is not necessarily implied. exposure to absolute maximum rating conditions for an extended period of time may affect device reliability. 2 typical thermal impedances (44-lead lqfp); ja = 55 c/w. ordering guide model temperature range package description package option AD6640ast ?0 c to +85 c (ambient) 44-lead plastic quad flatpack (lqfp) st-44 AD6640st/pcb evaluation board with AD6640ast explanation of test levels test level i 100% production tested. ii 100% production tested at +25 c and sample tested at specified temperatures. ac testing done on sample basis. iii sample tested only. iv parameter is guaranteed by design and characterization testing. v parameter is a typical value only. vi all devices are 100% production tested at +25 c; sam ple tested at temperature extremes. caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the AD6640 features proprietary esd protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality.
AD6640 ? rev. a pin function descriptions pin no. name function 1, 2, 36, 37, 40, 41 dv cc 3.3 v/5 v power supply (digital). powers output stage only. 3 encode encode input. data conversion initiated on rising edge. 4 encode complement of encode. drive differentially with encode or bypass to ground for single-ended clock mode. see encoding the AD6640 section. 5, 6, 13, 14, 17, 18, 21, gnd ground 22, 24, 34, 35, 38, 39 7 ain analog input 8 ain complement of analog input 9v ref internal voltage reference. nominally 2.4 v. bypass to ground with 0.1 f + 0.01 f microwave chip capacitor. 10 c1 internal bias point. bypass to ground with 0.01 f capacitor. 11, 12, 15, 16, 19, 20 av cc 5 v power supply (analog) 23 nc no connect 25 d0 (lsb) digital output bit (least significant bit) 26?3 d1?8 digital output bits 42, 43 d9?10 digital output bits 44 d11 (msb) * digital output bit (most significant bit) * output coded as twos complement. pin configuration 3 4 5 6 7 1 2 10 11 8 9 40 39 38 41 42 43 44 36 35 34 37 29 30 31 32 33 27 28 25 26 23 24 12 13 14 15 16 17 18 19 20 21 22 pin 1 top view (not to scale) av cc av cc av cc av cc av cc d8 d7 d6 d5 d4 d3 d2 AD6640 dv cc dv cc encode encode gnd gnd ain nc = no connect v ref c1 av cc d1 d0 (lsb) gnd nc d11 (msb) gnd gnd gnd gnd d10 gnd gnd gnd gnd gnd d9 dv cc dv cc dv cc dv cc gnd ain
AD6640 ? rev. a definition of specifications analog bandwidth (small signal) the analog input frequency at which the spectral power of the fundamental frequency (as determined by the fft analysis) is reduced by 3 db. aperture delay the delay between a differential crossing of encode and encode and the instant at which the analog input is sampled. aperture uncertainty (jitter) the sample-to-sample variation in aperture delay. differential nonlinearity the deviation of any code from an ideal 1 lsb step. encode pulsewidth/duty cycle pulsewidth high is the minimum amount of time that the encode pulse should be left in logic ??state to achieve rated performance; pulsewidth low is the minimum time encode pulse should be left in low state. at a given clock rate, these specifications define an acceptable encode duty cycle. integral nonlinearity the deviation of the transfer function from a reference line mea- s ured in fractions of 1 lsb using a ?est straight line?determined by a least square curve fit. minimum conversion rate the encode rate at which the snr of the lowest analog signal frequency drops by no more than 3 db below the guaranteed limit. maximum conversion rate the encode rate at which parametric testing is performed. output propagation delay the delay between a differential crossing of encode and encode and the time when all output data bits are within valid logic levels. power supply rejection ratio the ratio of a change in input offset voltage to a change in power supply voltage. signal-to-noise-and-distortion (sinad) the ratio of the rms signal amplitude (set at 1 db below full scale) to the rms value of the sum of all other spectral compo- nents, including harmonics but excluding dc. signal-to-noise ratio (snr) the ratio of the rms signal amplitude (set at 1 db below full scale) to the rms value of the sum of all other spectral components, excluding the first five harmonics and dc. spurious-free dynamic range (sfdr) the ratio of the rms signal amplitude to the rms value of the peak spurious spectral component. the peak spurious component may or may not be a harmonic. may be reported in dbc (i.e., degrades as signal levels is lowered), or in dbfs (always related back to converter full scale). two-tone intermodulation distortion rejection the ratio of the rms value of either input tone to the rms value of the worst third order intermodulation product; reported in dbc. two-tone sfdr the ratio of the rms value of either input tone to the rms value of the peak spurious component. the peak spurious component may or may not be an imd product. may be reported in dbc (i.e., degrades as signal levels are lowered) or in dbfs (always related back to converter full scale). worst harmonic the ratio of the rms signal amplitude to the rms value of the worst harmonic component, reported in dbc.
? ? equivalent circuits?d6640 450 buf t/h buf 450 buf t/h ain ain v ref av cc v ch av cc v ch v cl v cl figure 2. analog input stage av cc av cc timing circuits r2 8k r2 8k r1 17k r1 17k encode encode av cc figure 3. encode inputs av cc av cc c1 current mirror v ref av cc figure 4. compensation pin, c1 v ref dv cc dv cc current mirror d0 d11 current mirror figure 5. digital output stage v ref av cc 0.5ma 2.4v av cc figure 6. 2.4 v reference n digital outputs ( d11 ?0) n ?1 n ?2 t a n + 1 t od analog inputs encode inputs (encode) n ain ain figure 1. timing diagram rev. a
AD6640 ? rev. a ?ypical performance characteristics frequency ?mhz 0 60 100 dc 32.5 power relative to adc full scale ?db 6.5 13.0 19.5 26.0 40 80 20 120 encode = 65msps ain = 2.2mhz 4 8 9 5 37 6 2 tpc 1. single tone at 2.2 mhz frequency ?mhz 0 60 100 dc 32.5 power relative to adc full scale ?db 6.5 13.0 19.5 26.0 40 80 20 120 encode = 65msps ain = 15.5mhz 48 95 37 62 tpc 2. single tone at 15.5 mhz frequency ?mhz 0 60 100 dc 32.5 power relative to adc full scale ?db 6.5 13.0 19.5 26.0 40 80 20 120 encode = 65msps ain = 31.0mhz 48 953 7 6 2 tpc 3. single tone at 31.0 mhz analog input frequency ?mhz 070 7 worst case harmonic ?dbc 14 21 28 35 42 49 56 63 81 79 78 77 80 t = +25 c t = ?0 c, +85 c encode = 65msps temp = ?0 c, +25 c, and +85 c tpc 4. harmonics vs. ain analog input frequency ?mhz 070 7 snr ?db 14 21 28 35 42 49 56 63 69 67 66 65 68 encode = 65msps temp = ?0 c, +25 c, and +85 c t = +25 c t = ?0 c t = +85 c tpc 5. noise vs. ain encode = 65msps analog input frequency ?mhz 90 80 30 1 100 10 snr, harmonics ?db, dbc 60 50 40 70 24 20 40 200 300 worst other spur harmonics (second, third) s nr tpc 6. harmonics, noise vs. ain
AD6640 ? rev. a frequency ?mhz 0 60 100 dc 32.5 power relative to adc full scale ?db 6.5 13.0 19.5 26.0 40 80 20 120 encode = 65msps ain = 15.0mhz, 16.0mhz no dither tpc 7. two tones at 15.0 mhz and 16.0 mhz analog input power level ?dbfs 100 0 ?0 0 ?0 worst case spurious ?dbc and dbfs ?0 ?0 ?0 ?0 ?0 ?0 90 60 40 20 10 80 70 50 30 encode = 65msps ain = 31.0mhz dbfs dbc sfdr = 80db reference line tpc 8. single tone sfdr input power level (f1 = f2) ?dbfs 100 0 ?0 0 ?0 worst case spurious ?dbc and dbfs ?0 ?0 ?0 ?0 ?0 ?0 90 60 40 20 10 80 70 50 30 encode = 65msps f1 = 15.0mhz f2 = 16.0mhz sfdr = 80db reference line dbfs dbc tpc 9. two tone sfdr sample rate ?msps dc 80 8 snr, worst case spurious ?db, dbc 16 24 32 40 48 56 64 72 80 70 65 60 75 ain = 19.5mhz snr worst spur 85 tpc 10. snr, worst spurious vs. encode encode duty cycle ?% 25 75 30 snr, worst full-scale spurious ?db, dbc 35 40 45 50 55 60 65 70 90 65 55 45 75 85 80 70 60 50 40 35 30 encode = 65msps ain = 2.2mhz worst spur snr tpc 11. snr, worst spurious vs. duty cycle encode power ?dbm ?5 15 ?2 snr, worst full-scale spurious ?db, dbc ? ? ? 0 3 6 9 12 90 65 55 45 75 85 80 70 60 50 40 35 30 encode = 65msps worst spur snr 2.2mhz 2.2mhz 69mhz 69mhz tpc 12. snr, worst spurious vs. encode power
AD6640 ?0 rev. a dc 0 ?0 ?20 ?0 ?00 ?0 ?0 power relative to adc full scale ?db encode = 65msps ain = 19.5mhz @ ?6dbfs no dither 6.5 13.0 19.5 26.0 32.5 frequency ?mhz tpc 13. 16k fft without dither analog input power level ?dbfs 100 0 ?0 0 ?0 worst case spurious ?dbc ?0 ?0 ?0 ?0 ?0 ?0 90 60 40 20 10 80 70 50 30 encode = 65msps ain = 19.5mhz no dither sfdr = 80db reference line tpc 14. sfdr without dither 0 ?0 ?20 ?0 ?00 ?0 ?0 power relative to adc full scale ?db encode = 50msps ain = 65.5mhz, 68.5mhz no dither 0 ?0 ?20 ?0 ?0 50 75 55 60 70 frequency ?mhz 65 aliased signals analog if filter mask tpc 15. if sampling at 70 mhz without dither 0 ?0 ?20 ?0 ?00 ?0 ?0 power relative to adc full scale ?db encode = 65msps ain = 19.5mhz @ ?6dbfs dither = ?2.5dbm dc 6.5 13.0 19.5 26.0 32.5 frequency ?mhz tpc 16. 16k fft with dither analog input power level ?dbfs 100 0 ?0 0 ?0 worst case spurious ?dbc ?0 ?0 ?0 ?0 ?0 ?0 90 60 40 20 10 80 70 50 30 encode = 65msps ain = 19.5mhz dither = ?2.5dbm sfdr = 80db reference line tpc 17. sfdr with dither 50 75 55 60 65 70 frequency ?mhz 0 ?0 ?20 ?0 ?00 ?0 ?0 power relative to adc full scale ?db encode = 50msps ain = 65.5mhz, 68.5mhz dither = ?2.5dbm ?0 ?0 ?20 ?0 0 analog if filter mask aliased signals tpc 18. if sampling at 70 mhz with dither
AD6640 ?1 rev. a theory of operation the AD6640 analog-to-digital converter (adc) employs a two- stage subrange architecture. this design approach ensures 12-bit accuracy, without the need for laser trim, at low power. as shown in the functional block diagram, the AD6640 has complementary analog input pins, ain and ain . each analog input is centered at 2.4 v and should swing 0.5 v around this reference (see figure 2). since ain and ain are 180 degrees out of phase, the differential analog input signal is 2 v p-p. both analog inputs are buffered prior to the first track-and-hold, th1. the high state of the encode pulse places th1 in hold mode. the held value of th1 is applied to the input of a 6-bit coarse adc. the digital output of the coarse adc drives a 6-bit dac; the dac is 12 bits accurate. the output of the 6-bit dac is subtracted from the delayed analog signal at the input of th3 to generate a residue signal. th2 is used as an analog pipeline to null out the digital delay of the coarse adc. the 6-bit coarse adc word and 7-bit residue word are added together and corrected in the digital error correction logic to generate the output word. the result is a 12-bit parallel digital cmos compatible word, coded as twos complement. applying the AD6640 encoding the AD6640 a valid encode clock must be present on the AD6640 before the application of av cc (5 v). best performance is obtained by driving the encode pins differentially. however, the AD6640 is also designed to interface with ttl and cmos logic families. the source used to drive the encode pin(s) must be clean and free from jitter. sources with excessive jitter will limit snr (see the first equation under the noise floor and snr section). 0.01 f ttl or cmos source encode encode AD6640 figure 7. single-ended ttl /cmos encode the AD6640 encode inputs are connected to a differential input stage (see figure 3). with no input signal connected to either encode pin, the voltage dividers bias the inputs to 1.6 v. for ttl or cmos usage, the encode source should be connected to encode, pin 3. encode should be decoupled using a low inductance or microwave chip capacitor to ground. if a logic threshold other than the nominal 1.6 v is required, the following equations show how to use an external resistor, rx, to raise or lower the trip point (see figure 3; r1 = 17 k ? and r2 = 8 k ? ). v l = 5 r 2 rx r 1 r 2 + r 1 rx + r 2 rx to lower logic threshold. 0.01 f encode source encode encode AD6640 rx v l 5v r1 r2 figure 8. lower logic threshold for encode v r r rrx rrx l = + + 52 2 1 1 to raise logic threshold. 0.01 f encode source encode encode AD6640 rx v l 5v r1 r2 av cc figure 9. raise logic threshold for encode while the single-ended encode will work well for many appli- cations, driving the encode differentially will provide increased performance. depending on circuit layout and system noise, a 1 db to 3 db improvement in snr can be realized. it is not recom- mended that differential ttl logic be used because most ttl families that support complementary outputs are not delay or slew rate matched. instead, it is recommended that the encode signal be ac-coupled into the encode and encode pins. the simplest option is shown below. the low jitter ttl signal is coupled with a limiting resistor, typically 100 ? , to the primary side of an rf transformer (these transformers are inexpensive and readily available; part number in figure 10 is from mini- circuits). the secondary side is connected to the encode and encode pins of the converter. since both encode inputs are self-biased, no additional components are required. ttl encode encode AD6640 100 t1?t 0.1 f figure 10. ttl source?ifferential encode a clean sine wave may be substituted for a ttl clock. in this case, the matc hing network is shown. select a transformer r atio to match source and load impedan ces. the input impedance of the AD6640 encode is approximately 11 k ? differentially. therefore the ?,?shown in the figure 11, may be any value that is convenient for available drive power.
AD6640 ?2 rev. a encode encode AD6640 r t1?t sine source figure 11. sine source?ifferential encode if a low jitter ecl clock is available, another option is to ac-couple a differential ecl signal to the encode input pins as shown in figure 12. the capacitors shown here should be chip capacitors but do not need to be of the low inductance variety. encode encode AD6640 ecl gate 0.1 f 0.1 f ? s 510 510 figure 12. differential ecl for encode as a final alternative, the ecl gate may be replaced by an ecl comparator. the input to the comparator could then be a logic signal or a sine signal. encode encode AD6640 0.1 f 0.1 f ? s 50 ad96687 (1/2) 510 510 figure 13. ecl comparator for encode driving the analog input because the AD6640 operates from a single 5 v supply, the analog input voltage range is offset from ground by 2.4 v. each analog input connects through a 450 ? resistor to the 2.4 v bias voltage and to the input of a differential buffer (figure 14). this resistor network on the input properly biases the followers for maximum linearity and range. therefore, the analog source driving the AD6640 should be ac-coupled to the input pins. since the differential input impedance of the AD6640 is 0.9 k ? , the analog input power requirement is only ? dbm, simplifying the drive amplifier in many cases. AD6640 450 2.4v reference ain 0.01 f 450 buf buf buf ain v ref 0.1 f figure 14. differential analog inputs to take full advantage of this high input impedance, a 20:1 trans- former w ould be required. this is a large ratio and could result in unsatisfactory performance. in this case, a lower step-up ratio could be used. for example, if r t were set to 260 ? , along with a 4:1 transformer, the input would match to a 50 ? source with a full-scale drive of 4 dbm (figure 15). note that the external load resistor, r t , is in parallel with the AD6640 analog input resistance of 900 ? . the external resistor value can be calculated from the following equation: r t = 1 1 z 1 900 where z is the desired impedance (200 ? for a 4:1 transformer with 50 ? input source). ain 0.01 f ain v ref 0.1 f r t 1:4 analog input signal AD6640 figure 15. transformer-coupled analog input signal if the lower drive power is attractive, a combination transformer match and lc match could be employed that would use a 4:1 transformer with an lc as shown in figure 16. this solution is useful when good performance in the third nyquist zone is required. such a requirement arises when digitizing high inter- mediate frequencies in communications receivers. ain 0.01 f ain v ref 0.1 f 1:4 AD6640 ?125 + j100 analog signal at ?dbm figure 16. low power drive circuit in applications where gain is needed but dc-coupling is not necessary, an extension of figure 16 is recommended. a 50 ? gain block may be placed in front of the lc matching network. such gain blocks are readily available for comm ercial applications. these low cost modules can have excellent nf and intermodulation performance. this circuit is especially good for the ?f?receiver application previously mentioned. in applications where dc-coupling is required, the circuit in figure 17 can be used. it should be noted that the addition of circuitry for dc-coupling may compromise performance in terms of noise, offset, and dynamic performance. this circuit requires an inverting and noninverting signal path. additionally, an offset must be generated so that the analog input to each pin is centered near 2.4 v. since the input is differential, small differences in the dc voltage at each input can translate into an offset for the circuit. the same holds true for gain mismatch. therefore, some means of adjusting the gain and offset between the sides should
AD6640 ?3 rev. a layout information the schematic of the evaluation board (figure 18) represents a typical implementation of the AD6640. the pinout of the AD6640 facilitates ease of use and the implementation of high frequency/high resolution design practices. all of the digital outputs are on one side while the other sides contain all of the inputs. it is highly recommended that high quality ceramic chip capacitors be used to decouple each supply pin to ground directly at the device. depending on the configuration used for the encode and analog inputs, one or more capacitors are required on those input pins. the capacitors used on the encode and v ref pins must be a low inductance chip capacitor as referenced previously in this data sheet. a multilayer board is recommended to achieve best results. care should be taken when placing the digital output runs. because the digital outputs have such a high slew rate, the capacitive loading on the digital outputs should be minimized. circuit traces for the digital outputs should be kept short and connect directly to the receiving gate (broken only by the insertion of the series resistor). digital data lines should be kept clear of analog and encode traces. evaluation boards the evaluation board for the AD6640 is very straightforward, consisting of power, signal inputs, and digital outputs. the evaluation board includes the option for an onboard clock oscil- lator for the encode. power to the analog supply pins is connected via banana jacks. the analog supply powers the crystal oscillator and the av cc pins of the AD6640. the dv cc power is supplied via j3, the digital interface. this digital supply connection also powers the digital gates on the pcb. by m ain ta ining separate analog and digital power supplies, degra- dation in snr and sfdr is kept to a minimum. total power requirement is approximately 200 ma. this configurat ion allows for easy evaluation of different logic families (i.e., connection to a 3.3 v logic board). the analog input is connected via j2 and is transformer-coupled to the AD6640 (see driving the analog input section). the onboard termination resistor is 270 ? . this resistor, in parallel with the AD6640? input resistance (900 ? ), provides a 50 ? load to the analog source driving the 1:4 transformer. if a different input impedance is required, replace r16 by using the equation r 16 = 1 1 z ? 1 900 where z is desired input impedance (200 ? for a 4:1 transformer with 50 ? source). the analog input range of the pcb is 0.5 v (i.e., signal ac-coupled to AD6640). the encode signal may be generated using an onboard crystal oscillator, u1. the oscillator is socketed and may be replaced by an external encode source via j1. if an external source is used, it should be a high quality ttl source. a transformer converts the single-ended ttl signal to a differential clock (see encoding the AD6640 section). since the encode is coupled with a transformer, a sine wave could have been used; note, however, that u5 requires ttl levels to function properly. be implemented. the addition of small value resistors between the ad9631 and the AD6640 will prevent oscillation due to the capacitive input of the adc. 62 signal source ad9631 15 467 0.1 f op279 (1/2) op279 (1/2) 750 1000 78 350 AD6640 ain v ref 425 467 0.1 f 0.01 f 127 350 ad9631 15 ain 350 figure 17. dc-coupled analog input circuit power supplies care should be taken when selecting a power source. linear supplies are strongly recommended as switching supplies tend to h ave radiated components that may be ?eceived?by the a d6640. each of the power supply pins should be decoupled as closely to the package as possible using 0.1 f chip capacitors. the AD6640 has separate digital and analog 5 v pins. the analog supplies are labeled av cc and the digital supply pins are labeled dv cc . although analog and digital supplies may be tied together, best performance is achieved when the supplies are separate. this is because the fast digital output swings can couple switching noise back into the analog supplies. note that av cc must be held within 5% of 5 v; however, the dv cc supply may be varied accord- ing to output digital logic family (i.e., dv cc should be connected to the same supply as the digital circuitry). the AD6640 is specified for dv cc = 3.3 v as this is a common supply for digital asics. output loading care must be taken when designing the data receivers for the AD6640. it is recommended that the digital outputs drive a series resistor (e.g., 348 ? ) followed by a gate like the 74 lcx574. to minimize capacitive loading, there should only be one gate on each output pin. an example of this is shown in the evaluation board schematic shown in figure 18. the digital outputs of the AD6640 have a constant rise time output stage. the output slew rate is about 1 v/ns w hen dv cc = 5 v. a typical cmos gate combined with pcb trace and through hole will have a load of approxi mately 10 pf. therefore, as each bit switches: 10 ma 10 pf 1 v 1 ns ? ? ? ? ? ? of dynamic current per bit will flow in or out of the device. a full-scale transition can cause up to 120 ma (12 bits 10 ma/bit) of current to flow through the digital output stages. the series resistor will minimize the output currents that can flow in the output stage. these switching currents are con- fined between ground and the dv cc pin. standard ttl gates should be avoided since they can appreciably add to the dynamic switching currents of the AD6640.
AD6640 ?4 rev. a AD6640 output data is latched using 74lcx574 (u3, u4) latches following 348 ? series resistors. the resistors limit the current that would otherwise flow due to the digital output slew rate. the resistor value was chosen to represent a time constant of ~25% of the data rate at 65 mhz. this reduces slew rate while not appreciably distorting the data waveform. data is latched in a pipeline configuration; a rising edge generates the new AD6640 data sample, latches the previous data at the con- verter output, and strobes the external data register over j3. note that power and ground must be applied to j3 to power the digital logic section of the evaluation board. table i. AD6640st/pcb bill of material item quantity reference description 12 +5 va, gnd banana jack 21 1c 7?9, c11?17, c19 ceramic chip capacitor 0805, 0.1 f 32 c4, c6 tantalum chip capacitor 10 f 41 j3 40-lead double row male header 53 j1, j2, j4 bnc coaxial pcb connector 61 r1 surface mount resistor 1206, 348 ? 72 5r 2?14, r20?25, r30?35 surface mount resistor 1206, 348 ? 81 r15 surface mount resistor 1206, 100 ? 91 r16 surface mount resistor 1206, 270 ? 10 2 t1, t2 surface mount transformer mini-circuits t4?t, 1:4 ratio 11 1 u1 clock oscillator (optional) 12 1 dut AD6640ast 12-bit?5 msps a/d converter 13 2 u3, u4 74lcx574 octal latch 14 1 u5 74lvq00 quad two input nand gate 15 1 c1, c18 ceramic chip capacitor 0508, 0.01 f low inductance 16 2 c2, c3 ceramic chip capacitor 0508, 0.1 f low inductance 17 2 cr1, cr2 1n2810 schottky diode digital wideband receivers introduction several key technologies are now being introduced that may forever alter the vision of radio. figure 25 shows the typical dual conversion superheterodyne receiver. the signal picked up by the antenna is mixed down to an intermediate frequency (if) using a mixer with a variable local oscillator (lo); the variable lo is used to ?une-in?the desired signal. this first if is mixed down to a second if using another mixer stage and a fixed lo. demodulation takes place at the second or third if using either analog or digital techniques.
AD6640 ?5 rev. a 29 30 31 32 33 27 28 25 26 24 23 3 4 5 6 7 2 10 11 8 9 1 40 39 38 41 42 43 44 36 35 34 37 12 13 14 15 16 17 18 19 20 21 22 2 9 8 7 6 3 4 5 12 13 14 15 16 17 18 19 74lcx574 (dv cc ) 8d 7d 5d 1d 4d 6d 3d 2d 5q 6q 7q 8q 2q 3q 4q 1q oe ck b06 b07 b08 b09 b10 b11 11 1 2 9 8 7 6 3 4 5 12 13 14 15 16 17 18 19 u4 74lcx574 (dv cc ) 8d 7d 5d 1d 4d 6d 3d 2d 5q 6q 7q 8q 2q 3q 4q 1q oe ck b00 b01 b02 b03 b04 b05 11 1 nc = no connect gnd gnd gnd dv cc dv cc 3 2 1 4 6 t4?t 100 1 2 3 u5 74lvq00 (+5va) 5 6 4 buflat analog input +5v analog supply +5va gnd common 0.1 f 1:4 0.01 f gnd gnd gnd gnd dv cc c6 10 f + c7 0.1 f c11 0.1 f c12 0.1 f c13 0.1 f c15 0.1 f c16 0.1 f +5va c4 10 f + c8 0.1 f c9 0.1 f c17 0.1 f av cc av cc av cc av cc av cc dut AD6640 dv cc encode encode ain v ref c1 av cc gnd gnd gnd gnd d10 d9 dv cc dv cc dv cc dv cc gnd 348 348 348 348 348 348 348 348 348 348 348 348 0.01 f d8 d7 d6 d5 d4 d3 d2 d1 gnd nc (lsb) d0 d11 dv cc +5va 3 2 1 4 6 t4?t 1:4 two complement buffered outputs e1 e2 270 0.1 f +5va 348 j1 j2 encode input 0.1 f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 31 30 29 28 27 26 25 24 23 22 21 32 33 34 35 36 37 38 39 40 gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd gnd dv cc (+3.3v or +5.0v) b11 b10 b09 b08 b07 b06 b05 b04 b03 b02 b01 b00 gnd gnd gnd gnd gnd 348 348 348 348 348 348 348 348 348 348 348 348 buflat 348 ain j4 j3 dv cc figure 18. AD6640st/pcb schematic
AD6640 ?6 rev. a figure 19. AD6640st/pcb top side silkscreen figure 20. AD6640st/pcb bottom side silkscreen figure 21. AD6640st/pcb top side copper figure 22. AD6640st/pcb bottom side copper (positive) note: evaluation boards are often updated; consult factory for latest version.
AD6640 ?7 rev. a figure 23. AD6640st/pcb ground layer (negative) figure 24. AD6640st/pcb ?plit?power layer (negative)
AD6640 ?8 rev. a adcs variable if 1 if 2 fixed narrowband filter narrowband filter i q lna rf e.g. 900mhz shared one receiver per channel figure 25. narrowband digital receiver architecture if demodulation takes place in the analog domain, then tradi- tional discriminators, envelop detectors, phase locked loops, or other synchronous detectors are generally employed to strip the modulation from the selected carrier. however, as general-purpose dsp chips such as the adsp-2181 become more popular, they will be used in many baseband- sampled applications like the one shown in figure 25. as shown in the figure, prior to adc conversion, the signal must be mixed down and filtered, and the i and q components separated. these functions are realizable through dsp techniques; however, several key technology breakthroughs are required: high dynamic range adcs such as the AD6640, new dsps (highly programmable with onboard memory, fast), digital tuners and filters such as the ad6620, wide band mixers, and amplifiers. wideband adc fixed wideband mixer wideband filter lna rf e.g. 900mhz shared "n" channels to dsp 12.5mhz (416 channels) channel selection digital tuner/filter dsp digital tuner/filter dsp figure 26. wideband digital receiver architecture figure 26 shows such a wideband system. this design shows that the front-end variable local oscillator has been replaced with a fixed oscillator and the back end has been replaced with a wide dynamic range adc, digital tuner, and dsp. this technique offers many benefits. first, many passive discrete components that formed the tuning and filtering functions have been elimi- nated. these passive components often require tweaking and special handling during assembly and final system alignment. digital components require no such adjustments; tuner and filter characteristics are always exactly the same. moreover, the tuning and filtering characteristics can be changed through software. since software is used for demodulation, different routines may be used to demodulate different standards such as am, fm, gmsk, or any other desired standard. in addition, as new standards arise or new software revisions are generated, they may be field installed with standard software update channels. a radio that performs demodulation in software as opposed to hardware is often referred to as a soft radio because it may be changed or modified simply through code revision. system description in the wideband digital radio (figure 26), the first down conver- sion functions in much the same way as a block converter does. an entire band is shifted in frequency to the desired intermediate frequency. in the case of cellular base station receivers, 5 mhz to 30 mhz of bandwidth are down-converted simultaneously to an if frequency suitable for digitizing with a wideband analog- to-digital converter. once digitized the broadband digital data stream contains all of the in-band signals. the remainder of the radio is constructed digitally using special purpose and general purpose programmable dsp to perform filtering, demodulation and signal conditioning not unlike the analog counter parts. in the narrowband receiver (figure 25), the signal to be received must be tuned. this is accomplished by using a variable local oscillator at the first mix down stage. the first if then uses a narrow-band filter to reject out-of-band signals and condition the selected carrier for signal demodulation. in the digital wideband receiver (figure 26), the variable local oscillator has been replaced with a fixed oscillator, so tuning must be accomplished in another manner. tuning is performed digitally using a digital-down conversion and filter chip fre- quently called a channelizer. the term channelizer is used because the purpose of these chips is to select one channel out of many within the broadband spectrum present in the digital data stream of the adc. decimation filter low-pass filter digital tuner cos sin decimation filter low-pass filter data i q figure 27. ad6620 digital channelizer figure 27 shows the block diagram of a typical channelizer, such as the ad6620. channelizers consist of a complex nco (numerically controlled oscillator), dual multiplier (mixer), and matched digital filters. these are the same functions that would be required in an analog receiver, but implemented in digital form. the digital output from the channelizer is the desired car- rier, frequently in i & q format; all other signals have been fil- tered and removed based on the filtering characteristics desired. since the channelizer output consists of one selected rf channel, one tuner chip is required for each frequency received, although only one wideband rf receiver is needed for the entire band. data from the channelizer may then be processed using a digital signal processor such as the adsp-2181 or the sharc proces- sor, the adsp-21062. this data may then be processed through software to demodulate the information from the carrier. system requirements figure 28 shows a typical wideband receiver subsystem based around the AD6640. this strip consists of a wideband if filter, amplifier, adc, latches, channelizer, and interface to a digital signal processor. this design shows a typical clocking scheme used in many receiver designs. all timing within the system is referenced back to a single clock. while this is not necessary, it does facilitate pll design, ease of manufacturing, system test, and calibration. keeping in mind that the overall performance goal is to maintain the best possible dynamic range, many con- siderations must be made. one of the biggest challenges is selecting the amplifier used to drive the AD6640. since this is a communications application, it is common to directly sample an intermediate frequency (if) signal. as such, if gain blocks can be implemented instead of baseband op amps. for these gain block amplifiers, the critical specifications are third order intercept point and noise figure. a
AD6640 ?9 rev. a b and-pass filter will remove harmonics generated within the amplifier, but intermods should be better than the performance of the a/d converter. in the case of the AD6640, amplifier intermods must be better than ?0 dbfs when driving full- scale power. as mentioned earlier, there are several amplifiers to choose from and the specifications depend on the end application. figure 29 shows a typical multitone test. frequency ?mhz 0 ?0 ?20 ?0 ?00 ?0 ?0 dc 32.5 6.5 power relative to adc full scale ?db 13.0 19.5 26.0 encode = 65msps figure 29. multitone performance two other key considerations for the digital wideband receiver are converter sample rate and if frequency range. since perfor- mance of the AD6640 converter is largely independent of both sample rate and analog input frequency (tpcs 4, 5, and 10), the designer has greater flexibility in the selection of these parameters. also, since the AD6640 is a bipolar device, power dissipation is not a function of sample rate. thus there is no penalty paid in power by operating at faster sample rates. all of this is good because, by carefully selecting the input frequency range and sample rate, some of the drive amplifier and adc harmonics can actually be placed out-of-band. for example, if the system has second and third harmonics that are unacceptably high, by carefully selecting the encode rate and signal bandwidth, these second and third harmonics can be placed out-of-band. for the case of an encode rate equal to 60 msps and a signal bandwidth of 7.5 mhz, placing the fun- damental at 7.5 mhz places the second and third harmonics out of band as shown in the table ii. table ii. encode rate 60 msps fundamental 7.5 mhz?5 mhz second harmonic 15 mhz?0 mhz third harmonic 22.5 mhz?0 mhz, 30 mhz?5 mhz another option can be found through band-pass sampling. if the analog input signal range is from dc to f s /2, then the amplifier and filter combination must perform to the specification required. however, if the signal is placed in the third nyquist zone (f s to 3f s /2), the amplifier is no longer required to meet the harmonic performance required by the system specifications since all harmonics would fall outside the pass-band filter. for example, the pass-band filter would range from f s to 3 f s /2. the second harmonic would span from 2 f s to 3 f s , well outside the pass- band filter? range. the burden then has been passed off to the filter design, provided that the adc meets the basic specifications at the frequency of interest. in many applications, this is a worth- while trade-off since many complex filters can easily be realized using saw and lcr techniques at these relatively high if fre- quencies. although harmonic performance of the drive amplifier is relaxed by this technique, intermodulation performance cannot be sacrificed since intermods must be assumed to fall in-band for both amplifiers and converters. noise floor and snr oversampling is sampling at a rate that is greater than twice the bandwidth of the signal desired. oversampling does not have anything to do with the actual frequency of the sampled signal; it is the bandwidth of the signal that is key. band-pass or if sampling refers to sampling a frequency that is higher than nyquist and often provides additional benefits such as down conversion using the adc and replacing a mixer with a track-and-hold. over- sampling leads to processing gains because the faster the signal is digitized, the wider the distribution of noise. since the integrated noise must remain constant, the actual noise floor is lowered by 3 db each time the sample rate is doubled. the effective noise density for an adc may be calculated by the equation v noise rms / hz = 10 ? snr /20 4 fs for a typical snr of 68 db and a sample rate of 65 msps, this is equivalent to 25 nv/ hz . this equation shows the relationship between the snr of the converter and the sample rate f s . this equation may be used for computational purposes to determine overall receiver noise. preselect filter lna 5mhz?5mhz pass band 348 cmos buffer d11 d0 +3.3v (d) +5v (a) AD6640 ain encode encode m/n pll synthesizer lo drive ref in 1900mhz reference clock 65mhz 12 ad6620 (ref. fig 27) i & q data clk adsp-2181 network controller interface ain figure 28. simplified wideband pcs receiver
AD6640 ?0 rev. a the signal-to-noise ratio (snr) for an adc can be predicted. when normalized to adc codes, the following equation accurately predicts the snr based on jitter, average dnl error, and thermal noise. each of these terms contributes to the noise within the converter. snr ft rms v analog noise rms = + + + () ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? log / 20 2 1 22 2 12 2 12 2 12 j where: f analog = analog input frequency t j rms =r ms jitter of the encode (rms sum of encode source and internal encode circuitry) = average dnl of the adc (typically 0.51 lsb) v noise rm s =v rms thermal noise referred to the analog input of the adc (typically 0.707 lsb) processing gain processing gain is the improvement in signal-to-noise ratio (snr) gained through oversampling and digital filtering. most of this processing gain is accomplished using the channelizer chips. these special purpose dsp chips not only provide channel selec- tion and filtering but also a data rate reduction. the required rate reduction is accomplished through a process called decimation. the term decimation rate is used to indicate the ratio of input data rate to output data rate. for example, if the input data rate is 65 msps and the output data rate is 1.25 msps, then the decimation rate is 52. large processing gains may be achieved in the decimation and filtering process. the purpose of the channelizer, beyond tuning, is to provide the narrow-band filtering and selectivity that tradi- tionally have been provided by the ceramic or crystal filters of a narrow-band receiver. this narrow-band filtering is the source of the processing gain associated with a wide band receiver and is simply the ratio of the pass-band to whole band expressed in db. for example, if a 30 khz amps signal is being digitized with an AD6640 sampling at 65 msps, the ratio would be 0.015 mhz /32.5 mhz . expressed in log form, the processing gain is ?0 log (0.015 mhz /32.5 mhz ) or 33.4 db. additional filtering and noise reduction techniques can be achieved through dsp techniques; many applications do use additional process gains through proprietary noise reduction algorithms. overcoming static nonlinearities with dither typically, high resolution data converters use multistage tech- niques to achieve high bit resolution without large comparator arrays that would be required if traditional ?lash?adc techniques were employed. the multistage converter typically provides better wafer yields meaning lower cost and much lower power. however, since the AD6640 is a multistage device, certain por- tions of the circuit are used repetitively as the analog input sweeps from one end of the converter range to the other. although the worst dnl error may be less than an lsb, the repetitive nature of the transfer function can play havoc with low level dynamic signals. spurious signals for a full-scale input may be ?0 dbc. at 36 db below full scale, however, these repetitive dnl errors may cause spurious-free dynamic range (sfdr) to fall below 80 dbfs as shown in tpc 14. a common technique for randomizing and reducing the effects of repetitive static linearity is through the use of dither. the purpose of dither is to force the repetitive nature of static linear- ity to appear as if it were random. then, the average linearity over the range of dither will dominate sfdr performance. in the AD6640, the repetitive cycle is every 15.625 mv p-p. to ensure adequate randomization, 5.3 mv rms is required; this equates to a total dither power of ?2.5 dbm. this will randomize the dnl errors over the complete range of the residue converter. although lower levels of dither such as that from previous analog stages will reduce some of the linearity errors, the full effect will only be gained with this larger dither. increasing dither even more may be used to reduce some of the global inl errors. however, signals much larger than the mvs proposed here begin to reduce the usable dynamic range of the converter. even with the 5.3 mv rms of noise suggested, snr would be limited to 36 db if injected as broadband noise. to avoid this problem, noise may be injected as an out-of-band signal. typically, this may be around dc but may just as well be at f s /2 or at some other frequency not used by the receiver. the bandwidth of the noise is several hundred kilohertz. by band-limiting and control- ling its location in frequency, large levels of dither may be intro- duced into the receiver without seriously disrupting receiver performance. the result can be a marked improvement in the sfdr of the data converter. tpc 17 shows the same converter shown earlier but with this injection of dither (see tpc 14). ad600 a a ref 2.2k 1 f 0.1 f 39 390 16k +15v nc202 noise diode (n oisecom) +5v ?v 1k 2k op27 optional high power drive circuit low control (0v?v) figure 30. noise source (dither generator) the simplest method for generating dither is through the use of a noise diode (figure 30). in this circuit, the noise diode nc202 generates the reference noise that is gained up and driven by the ad600 and op27 amplifier chain. the level of noise may be controlled by either presetting the control voltage when the system is set up, or by using a digital-to-analog converter (dac) to adjust the noise level based on input signal conditions. once generated, the signal must be introduced to the receiver strip. the easiest method is to inject the signal into the drive chain after the last down conversion as shown in figure 31.
AD6640 ?1 rev. a noise source (see figure 30) lpf ain 0.01 f ain v ref 0.1 f AD6640 combiner bpf from rf/if if amp figure 31. using the AD6640 with dither receiver example to determine how the adc performance relates to overall receiver sensitivity, the simple receiver in figure 32 will be examined. this example assumes that the overall down conversion process can be grouped into one set of specifications, instead of individu- all y examining all components within the system and summing them together. although a more detailed analysis should be employed in a real design, this model will provide a good approximation. in examining a wideband digital receiver, several considerations must be applied. although other specifications are important, receiver sensitivity determines the absolute limits of a radio excluding the effects of other outside influences. assuming that receiver sensitivity is limited by noise and not adjacent signal strength, several sources of noise can be identified and their overall contribution to receiver sensitivity calculated. rf/if AD6640 channelizer ref in dsp encode 61.44mhz gain = 30db nf = 10db bw =12.5mhz single channel bw = 30khz figure 32. receiver analysis the first noise calculation to make is based on the signal bandwidth at the antenna. in a typical broadband cellular receiver, the if bandwidth is 12.5 mhz. given that the power of noise in a given bandwidth is defined by p n = ktb , where b is bandwidth, k = 1.38 10 ?3 is boltzman? constant, and t = 300 k is absolute temperature, this gives an input noise power of 5.18 10 ?4 w or ?02.86 dbm. if our receiver front end has a gain of 30 db and a noise figure of 10 db, then the total noise presented to the adc input becomes ?2.86 dbm (?02.86 + 30 + 10) or 0.16 mv rms. comparing receiver noise to dither required for good sfdr, we see that in this example, our receiver supplies about 3% of the dither required for good sfdr. based on a typical adc snr specification of 68 db, the equiva- lent internal converter noise is 0.140 mv rms. therefore, total broadband noise is 0.21 mv rms. before processing gain, this is an equivalent snr (with respect to full scale) of 64.5 db. assuming a 30 khz amps signal and a sample rate of 61.44 msps, the snr through processing gain is increased by approximately 33 db to 97.5 db. however, if eight strong and equal signals are present in the adc bandwidth, then each must be placed 18 db below full scale to prevent adc overdrive. therefore 18 db of range is given away and the carrier-to-noise ratio (c/n) is reduced to 79.5 db. assuming that the c/n ratio must be 10 db or better for accurate demodulation, one of the eight signals may be reduced by 66.5 db before demodulation becomes unreliable. at this point, the input signal power would be ?0.5 dbm. referenced to the antenna, this is ?20.5 dbm. to improve sensitivity, several things can be done. first, the noise figure of the receiver can be reduced. since front end noise domi- nates the 0.16 mv rms, each db reduction in noise figure translates to an additional db of sensitivity. second, providing broadband agc can improve sensitivity by the range of the agc. however, the agc would only provide useful improvements if all in-band signals were kept to an absolute minimal power level so that agc could be kept near the maximum gain. this noise limited example does not adequately demonstrate the true limitations in a wideband receiver. other limitations such as sfdr are more restrictive than snr and noise. assume that the analog-to-digital converter has an sfdr specification of ?0 dbfs or ?6 dbm (full scale = +4 dbm). also assume that a tolerable carrier-to-interferer (c/i) (different from c/n) ratio is 18 db. this means that the minimum signal level is ?2 dbfs (?0 plus 18) or ?8 dbm. at the antenna, this is ?8 dbm. therefore, as can be seen, sfdr (single or multi-tone) would limit receiver performance in this example. however, as shown previously, sfdr can be greatly improved through the use of dither (tpcs 13 and 16). in many cases, the addition of the out-of-band dither can improve receiver sensitivity nearly to that limited by thermal noise. if sampling using the AD6640 as a mix-down stage since performance of the AD6640 extends beyond the baseband region into the third nyquist zone, the converter has many uses as a mix-down converter in both narrow-band and wideband applications. this application is called band-pass sampling. doing this has several positive implications in terms of the selection of the if drive amplifier. not only is filtering a bit easier, the selection of drive amplifiers is extended to classical if gain blocks. in the third nyquist zone and above, the second and third harmonics are easily filtered with a band-pass filter. now only in-band spurs that result from third order products are important.
AD6640 ?2 rev. a in narrow-band applications, harmonics of the adc can be placed out-of-band. one example is the digitization of a 201 mhz if signal using a 17.333 mhz clock. as shown in figure 33, the spurious performance has diminished due to internal slew rate limitations of the adc. however, the snr of the converter is still quite good. subsequent digital filtering with a channelizer chip such as the ad6620 will yield even better snr. for multicarrier applications, third order intercept of the drive amplifier is important. if the input network is matched to the internal 900 ? input impedance, the required full-scale drive level is ? dbm. if spurious products delivered to the adc are required to be below ?0 dbfs, the typical performance of the adc with dither applied, then the required third order intercept point for the drive amplifier can be calculated. for multicarrier applications, the AD6640 is useful up to about 80 mhz analog in. for single channel applications, the AD6640 is useful to 200 mhz as shown in the bandwidth charts. in either case, many common if frequencies exist in this range of frequencies. if the adc is used to sample these signals, they will be aliased down to baseband during the sampling process in much the same manner that a mixer will down-convert a signal. for signals in various nyquist zones, the following equations may be used to determine the final frequency after aliasing. f 1 nyquists = f sample ? f signal f 2 nyquists = abs ( f sample ? f signal ) f 3 nyquists = 2 f sample ? f signal f 4 nyquists = abs (2 f sample ? f signal ) using the converter to alias down these narrow-band or wide- band signals has many potential benefits. first and foremost is the elimination of a complete mixer stage along with amplifiers, f ilters, and o ther devices, reducing cost and power dissipation. in some cases, the elimination of two if stages is possible. tpcs 15 and 18 illustrate a multicarrier, if sampling system. by using dither, all spurious components are forced below 90 dbfs (tpc 18). the dashed line illustrates how a 5 mhz band-pass filter could be centered at 67.5 mhz. as discussed earlier, this approach greatly reduces the size and complexity of the receiver? rf/if section. frequency ?mhz 0 60 198 207 power relative to adc full scale ?db 199.8 201.6 203.4 205.2 40 80 20 100 aliased second harmonic aliased third harmonic analog if filter mask aliased signals figure 33. if sampling a 201 mhz input receive chain for a phased-array cellular base station the AD6640 is an excellent digitizer for beam forming in phased- array antenna systems. the price performance of the AD6640 and ad6620 channelizers allows for a very competitive solution. phased-array base stations allow better coverage by focusing the receivers?sensitivity in the direction needed. phased-array systems allow for the electronic beam to form on the receive antennas. a typical phased-array system may have eight antennas, as shown in figure 34. since a typical base station will handle 32 calls, each antenna would have to be connected to 32 receivers. if done with analog or traditional radios, the system grows quite rapidly. with a multicarrier receiver, however, the design is quite compact. each antenna would have a wideband down-converter with one AD6640 per receiver. the output of each AD6640 would drive 32 ad6620 channelizers, which are phase locked in groups of eight?ne per antenna. this allows each group of eight ad6620s to tune and lock onto a different user. when the incoming signal direction is determined, the relative phase of each ad6620 in the group can be adjusted so that the output signals sum together in a constructive manner, giving high gain and directivity in the direction of the caller. this application would not be possible with traditional receiver designs.
AD6640 ?3 rev. a AD6640 common lo antenna 1 eight wideband front ends ad6620 (1) sync 1 antenna 2 ad6620s (32 channels) AD6640 AD6640 antenna 3 antenna 4 ad6620s (32 channels) AD6640 AD6640 antenna 5 antenna 6 ad6620s (32 channels) AD6640 antenna 8 ad6620s (32 channels) AD6640 sum adsp-21xx (32) sum adsp-21xx (31) sum adsp-21xx (30) sum adsp-21xx (3) sum adsp-21xx (2) sum adsp-21xx (1) 32 channels out each channel is summation from eight antennas combine signals from eight antennas ad6620 (2) ad6620 (3) ad6620 (30) ad6620 (31) ad6620 (32) ad6620 (1) sync 1 ad6620 (2) ad6620 (3) ad6620 (30) ad6620 (31) ad6620 (32) ad6620 (1) sync 1 ad6620 (2) ad6620 (3) ad6620 (30) ad6620 (31) ad6620 (32) AD6640 antenna 7 ad6620 (1) sync 1 ad6620 (2) ad6620 (3) ad6620 (30) ad6620 (31) ad6620 (32) figure 34. receive chain for a phased-array cellular base station with eight antennas and 32 channels
AD6640 ?4 rev. a c00970??/03(a) printed in u.s.a. outline dimensions 44-lead plastic quad flatpack [lqfp] (st-44) dimensions shown in millimeters view a 7 3.5 0 0.20 0.09 1.45 1.40 1.35 0.15 0.05 0.10 max coplanarity view a rotated 90 ccw seating plane top view (pins down) 11 1 44 34 33 23 22 12 0.80 bsc 12.00 bsc 10.00 bsc 1.60 max seating plane 0.75 0.60 0.45 0.45 0.37 0.30 compliant to jedec standards ms-026bcb revision history location page 2/03?ata sheet changed from rev. 0 to rev. a. updated format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . universal changed to 44-lead plastic quad flatpack (lqfp) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . universal changes to absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 updated tpcs 13 and 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 text inserted in encoding the AD6640 section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 updated outline dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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