circuit note CN-0072 ci rcuit designs using analog devices products apply these product pairings quickly and with confidence. for more information and/or support call 1 -800- analogd (1 -800-262-5643) or visit www.analog.com/circuit . devic es connected /referenced adl5513 1 mhz to 4 ghz , 80 db logarithmic detector/controller ad8368 800 mhz linear - in - db variable gain amplifier (vga) exten ding the dynamic range of the adl5513 logarithmic detector using the ad8368 variable gain amplifier rev. 0 circuits from the lab from analog devices have been designed and buil t by analog devices engineers. standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. however, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. accordingly, in no event shall analog devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any circuit from the lab . (continued on last page) one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2009 analog devices, inc. all rights reserved. circuit function and benefits this circuit provides a solution for increasing the dynamic range of the adl5513 logarithmic detector. this is ac complished by adding an ad8368 v ariable gain a mplifier (vga) at the input of the adl5513. the v out signal from th e adl5513 is attenuated and fed back to the gain control input of the ad8368, providing precise control of the power at the input of the adl5513. using this approach , the dynamic range of the circuit is increased to 95 db with excellent temperature stabili ty (see figure 2 ). the output voltage is linear in db with respect to the input signal. 08175-001 inpt e nbl vps i ocom icom mode vps i vps i vps o vps o vps i outp gain deto hpfl deti ocom decl decl icom icom icom decl vps i ad 8368 vpos1 10k? 215? 0 ? 0 ? 10 nh 10 nf 10 nf 1nf 0.1f c10 1nf 5.6pf 1nf c12 1nf c15 0.1f input 10 nf vpos3 0 ? c12 1nf c15 0.1f vpos2 band -pass 120 mhz 52 .3 ? 1 vpos 2 i nh i 3 inlo 4 vpos 11vset 12 t ad j vout vout 10comm 9 5 6 7 8 nc nc nc nc nc nc clpf nc 15 16 14 13 ad l 551 3 z1 vpos gnd 1k? 47 nf 100 pf c7 1000 pf 0.1f 100 pf 0.1f vt ad j = 0. 89v vpos vpos 1k? 1k? vpos vpos1 vpos vpos2 vpos3 figure 1. adl5513 logari thmic detector and ad8368 vga providing 95 db dynamic r ange at 120 mhz (si mplified schematic ) note: vpos = +5 v
CN-0072 circuit note rev. 0 | page 2 of 3 circuit description the adl5513 logarithmic detector dynamic range can be ext ended by adding a separate vga whose gain control input is derived directly from v out . this extends the dynamic range by the gain cont rol ra nge of the vga. for the overall measurement to remain linear in db, the vga must provide a linear - in - db (exponential) gain control function. the vga gain must decrease with an increase in its gain bias in the same way as the adl5513. alternatively, an inve rting op amp with suitable level shifting can be used. it is convenient to select a vga that needs only a single 5.0 v supply and is capable of generating a single - ended output. all of these conditions are met by the ad8368. figure 1 shows the simplified schematic. using the inverse gain mode (mode pin low) of the ad8368, its gain decreases on a slope of 37.5 m v/db to a minimum value of ?12 db for a gain voltage (vgain) of 1.0 v. the voltage, vgain , required by the ad8368 is 50% of the output of the adl5513. to scale this voltage, it is necessary to insert a voltage divider at the output of the adl5513. over the 1.5 v range from the output of the adl5513, the gain of the ad8368 varies by (0.5 1.5 v) (37.5 mv/db), or 20 db. combined with the 75 db gain span (at 120 mhz) of the adl5513, this results in a 1.5 v change in v out for a 95 db change at the input to the ad8368. due to the amplification of out - of - band noise by the ad8368, a band - pass filter is used between th e ad8368 and adl5513 to increase the low - signal sensitivity. the vga amplifies low power signals and attenuates high power signals to fit them in the detectable range of the adl5513. if an amplifier with higher gain and lower noise figure is used, better t han 90 db sensitivity can be achieved for use in a receive -signal- strength - indicator (rssi) application. figure 2 shows the data results of the extended dynamic range at 120 mhz and the corresponding error in v out . due to the high frequencies and wide dynamic ranges involved, the circuit must be constructed on a multilayer pc board that has at least one layer for the ground plane. each ic power pin must be decoupled with a 0.1 f low inductance ceramic capacitor as shown in figure 1 . some power pins have an additional smaller value decoupling capacitor (low esl) for extremely high frequencies. the 0 ? resistors on the schematic represent optional small resistor values (less than 10 ? ) or ferrite beads that may be added for more decoupling. the data sheets for the adl5513 and the ad8368 show the optimal layouts of the respective evaluation boards. on the underside of both the adl5513 and the ad8368 chip scale package there is an exposed paddle. this paddle is internally connected to the ground of the chip. sol der the paddle to the low impedance ground plane on the pcb to ensure specified electrical performance and to provide thermal relief. it is also recommended that the ground planes on all layers under the paddle be stitched together with vias to reduce ther mal impedance. 1.750 1.625 1.500 1.375 1.250 1.125 1.000 0.875 0.750 0.625 0.500 0.375 0.250 3.0 2.0 1.0 0 ?1.0 ?2.0 ?3.0 2.5 1.5 0.5 ?0.5 ?1.5 ?2.5 ?90 ?70 ?50 ?30 ?10 0 20 ?80 ?60 ?40 ?20 10 p in (dbm) v out (v) error (db) 08175-002 v out +25c v out ?40c v out +85c error +25c error ?40c error +85c figure 2 . output and conformance for the ad8368/adl5513 extended dynamic range circuit at 120 mhz c ommon variations the application circuit described here can be modified for any if frequency within the operatin g range of the ad8368 and adl5513 by selecting the appropriate band - pass filter. for operation above 800 mhz, the adl5330 can be used in place of the ad8368. the adl5330 vga can be used at frequencies between 10 mhz and 3 ghz. using the adl5330 also requires an inverter between the adl5330 gain pin and the gain control voltage from the adl5513. a single - supply operational amplifier such as the ad8061 can be configur ed for use as an inverter in this application. learn more mt - 073 tutorial, high speed variable gain amplifiers (vgas) . analog devices. mt - 077 tutorial, log amp basics . analog devices. mt - 078 tutorial, high frequency log amps . analog devices. mt - 101 tutorial, decoupling techniques . analog devices. data sheets ad8061 data sheet. ad8368 data sheet. ad l5330 data sheet. adl5513 data sheet. revision history 7/09 revision 0: initial version
circuit note CN-0072 rev. 0 | page 3 of 3 (continued from first page) "circuits from the lab" are intended only for use with analog devices products and are the in tellectual property of analog devices or its licensors. while you may use the "circuits from the lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "circuits from the lab". information furnished by analog devices is believed to be accurate and reliable. however, "circu its from the lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibilit y is assumed by analog devices for their use, nor for any infringements of patents or other rights of third parties that may resul t from their use. analog devices reserves the right to change any "circuits from the lab" at any time without notice, but is under no obligation to do so. trademarks and registered trademarks are the p roperty of their respective owners. ? 2009 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn08175 -0- 7/09(0)
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