circuit note cn - 0217 circuits from the lab? reference circuits are engineered and tested for quick and easy system integration to help solve todays analog, mixed - signal, and rf design challenges. for more i nformation and/or support , visit www.analog.com/cn0217 . devices connected /referenced ad5933 1 msps, 12- bit impedance converter, network analyzer ad5934 250 ksps, 12 - bit impedance converter, network analyzer ad8606 precision, low noi se, dual cmos op amp high accuracy impedance measurements using 12 - bit impedance converters rev. a circuits from the lab? circuits 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 sol ely 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 circuits from the lab circuits. (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 ? 2011 C 2013 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation boards ad5933 evaluation board ( eval - ad5933ebz ) design and integration files schematic s, layout files, bill of materials ci rcuit function and b enefits the ad5933 and ad5934 are high precision impedance converter system solutions that combine an on - chip programmable frequency generator with a 12 - bit, 1 msps ( ad5933 ) or 250 ksps ( ad5934 ) analog - to - digital conv erter (adc). the tunable frequency generator allows an external complex impedance to be excited with a known frequency. the circuit shown in figure 1 yields accurate impedance measurements extending from the low ohm range to seve ral hundred k? , and it also optimizes the overall accuracy of the ad5933 / ad5934 . 50k? 50k? 50k? 50k? r fb 20k ? 20k ? 47nf z unknown v dd v dd v dd + + ? ? a1 a2 a1, a2 are ? ad8606 1.48v 1.98v p-p v dd /2 1.98v p-p vdd/2 dac scl sda dvdd a vdd mclk agnd dgnd r out vout ad5933/ad5934 rfb vin 1024-point dft i 2 c inter f ace imagina ry register rea l register oscill at or dds core (27 bits) tempera ture sensor transmit side output amplifier adc (12 bits) lpf gain v dd v dd 09915-001 i-v figure 1 . optimized signal chain for impedance measurement accuracy (simplified schematic, all connections and decoupling not shown)
cn- 0217 circuit note rev. a | page 2 of 6 circuit description the ad5933 and ad5934 have four programmable output voltage ranges; each range has an output impedance associated with it. for example, the output impedance for a 1.98 v p - p output voltage is typically 200 (see table 1 ). table 1 . output series resistance (r out ) vs. excitation range for v dd = 3.3 v supply voltage range output excitation amplitude (v p - p) output resistance ( r out ) range 1 1.98 200 typ ical range 2 0.97 2.4 k typ ical range 3 0.383 1.0 k typ ical range 4 0.198 600 typ ical th e output impedance affects the impedance measurement accuracy, particularly in the low k? range, and m ust be taken into account when calculating the gain f actor. r efer to the ad5933 or ad5934 data sheet s for more details on the gain factor calculation. a simple buffer in the signal chain prevent s the output impedance from affecting the unknown impedance measurement . select a low output impedance amplifier with sufficient band width to accommodate the ad5933 / ad5934 excitation frequency. an example of the low output impedance achievable is shown in figure 2 for the ad8605 / ad8606 / ad8608 family of cmos op amps. the output impedance for this amplifier for an a v of 1 is less than 1 ? up to 100 khz, which i s the maximum operating range of the ad5933 / ad5934 . frequenc y (hz) 100 90 0 1k 100m 10k 100k 1m 10m 80 70 20 60 50 30 10 40 a v = 100 a v = 10 a v = 1 v s = 2.7v 09915-002 output impedance (?) figure 2 . output impedance of ad8605 / ad8606 / ad8608 matching the dc bias of transmit stage to receive stage the four programmable o utput voltage ranges in the ad5933 / ad5934 have four associated bias voltages ( see table 2 ). for example , the 1. 98 v p - p excitation voltage has a bias of 1.48 v. however, the current - to - voltage (i - v) receive stage of the ad5933 / ad5934 is set to a fix ed bias of v dd /2 as shown in figure 1 . th e refore , for a 3.3 v supply, the transmit bias voltage is 1.48 v, a nd the receive bias voltage is 3.3 v/2 = 1.65 v. this potential difference polarizes the impedance under test and can cause i naccuracies in the impedance measurement. one solution is to add a simple high - pass filter with a corner frequency in the low hz range. removing the dc bias from the transmit stage and re biasing the ac signal to v dd /2 keeps the dc level constant throughou t the signal chain. table 2 . output levels and respective dc bias for v dd = 3.3 v supply voltage range output excitation amplitude (v p - p) output dc bias level (v) 1 1.98 1.48 2 0.97 0.76 3 0. 383 0.31 4 0. 198 0.173 selecting an optimized i - v buffer for the receive stage the i - v amplifier stage of the ad5933 / ad5934 can also add minor inaccuracies to the si gnal chain. the i - v conversion stage is sensitive to the amplifier's bias current, offset voltage, and common - mode rejection ratio ( cmrr ) . by selecting the proper external discrete amplifier to perform the i - v conversion , the user can choose an amplifier with lower bias current and off set voltage specifications along with excellent cmrr, making the i - v conversion more accurate. the internal amplifier can then be configured as a simple inverting gain stage. selection of the r fb resistor still depends on the gain through the system as descri bed in the ad5933 / ad5934 data sheet s. optimized signal chain for high accuracy impedance measurements figure 1 shows a proposed configurati on for measuring low impedance sensors. the ac signal is high - pass filtered and re biased b efore buffering with a very low output imped ance amplifier. the i - v conversion is completed externally before the signal returns to the ad5933 / ad5934 receive stage. key specifications that determine the required buffer are very low output impedance, the single - supply capability, low bias current, low offs et voltage, and excellent cmrr performance. some suggested parts are the ada4528 - 1 , ad8628 , ad8629 , ad8605 , and ad8606 . depending on board layout, use a single - channel or dual - channel amplifier. use precision 0.1% r esistors for both the biasing resistors (50 k?) and gain resistors ( 20 k? and r fb ) to reduce inaccuracies.
circuit note cn- 0217 rev. a | page 3 of 6 circuit evaluation a nd test the schematic in figure 1 was developed to improve impedance measurement accuracy , and some ex ample measurements were taken. the ad8606 dual - channel amplifier buffer s the signal on the transmit path and convert s the receive signal from current to v oltage. for the three examples shown, the g ain factor is calculated for each frequency increment to remove frequency dependent errors. a complete desig n package including schemat ics , bill of materials, layout, and gerber files is available for this solution at www.analog.com/cn0217 - designsupport . the software used is the same software that is available with evaluation boards and is accessible from the ad5933 and ad5934 product pages. example 1: low impedance range table 3 . low impedance range setup for v dd = 3.3 v supply voltage parameter value voltage peak - to - peak (v p - p) 1.98 v (range 1) number of settling time cycles 15 mclk 16 mhz r cal 20.1 r fb 20.0 excitation frequency range 30 khz to 30.2 khz unknown impedances r1 = 10.3 , r2 = 30.0 , c3 = 1 f (z c = 5.3 at 30 khz) the results of the low impedance measurements are shown in figure 3 , figure 4 , and figure 5 . figure 5 is for the 10.3 ? measurement and is shown on an expanded vertical scale. the accuracy achieved is very much dependent on how large the unknown impedance range is relative to the calibration resistor, r cal . therefore, in this example, the unknown impedance of 10.3 measured 10.13 , an approximate 2% error. choosing an r cal closer to the unknown impedance achieves a more accura te measurement; that is, the smaller the unknown impedance range is centered on r cal is the more accurate the measurement. consequently, for large unknown impedance ranges, it is possible to switch in various r cal resistors to break up the unknown imp edance range using external switches. the r on error of the switch is removed by calibration during the r cal gain factor calculation. using a switch to select various r fb values can optimize the dynamic range of the signal seen by the adc. in addition, note that to achieve a wider range of measurements a 200 mv p - p range was used. if the unknown z is a small range , a larger output voltage range can be used to optimiz e the adc dynamic range. 35 30 25 20 15 10 5 0 29.95 30.00 30.05 30.10 30.15 30.20 10.3? 30? 1f 30.25 frequency (khz) magnitude (?) 09915-003 figure 3 . measured low impedance magnitude resu lts 20 0 ?20 ?40 ?60 ?80 ?100 29.95 30.00 30.05 30.10 30.15 30.20 1f 30.25 frequency (khz) phase (degrees) 09915-004 10.3?, 30? figure 4 . measu r ed low impedance phase results 10.22 10.20 10.18 10.16 10.14 10.12 10.10 10.08 10.06 10.04 29.50 30.00 30.05 30.10 30.15 30.20 30.25 frequency (khz) magnitude (?) 09915-005 figure 5 . measured 10.3 ? magnitude results (expanded scale)
cn- 0217 circuit note rev. a | page 4 of 6 example 2: k impedance range using an r cal of 99.85 k, a wide range of unknown impedances were measured according to the setup conditions listed in table 4 . figure 6 to figure 10 document accuracy results . to improve the overall accuracy, select an r cal value closer to the unknown impedance. for example, in figure 9 , an r cal closer to the z c value of 217.5 k is required. if the unknown impedance range is large, use more than one r cal resistor. table 4 . k impedance range setup for v dd = 3.3 v supply voltage parameter value voltage peak - to - peak (v p - p) 0.198 v (range 4) numb er of settling time cycles 15 mclk 16 mhz r cal 99.85 k r fb 100 k excitation frequency range 30 khz to 50 khz unknown impedances r0 = 99.85 k , r1 = 29.88 k , r2 = 14.95 k , r3 = 8.21 k , r4 = 217.25 k , c5 = 150 pf , (z c = 26.5 k at 40 khz) , c6 = 47 pf (z c = 84.6 k at 40 khz) 60 70 80 90 100 1 10 120 30 35 40 frequenc y (khz) impedance magnitude (k?) 45 50 measured idea l 09915-006 figure 6. magnitude result f or z c = 47 pf, r cal = 99.85 k ? ?90.3 ?90.2 ?90.1 ?90.0 ?89.9 ?89.8 ?89.7 ?89.6 ?89.5 ?89.4 ?89.3 30 35 40 frequenc y (khz) phase (degrees) 45 50 09915-007 figure 7. phase result f or z c = 47 pf, r cal = 99.85 k ? 30 35 40 frequenc y (khz) 45 50 8160 8180 8200 8220 8240 8260 8280 impedance magnitude (?) r3 idea l 09915-008 figure 8. z c = 8.21 k? , r ca l = 99.85 k ? 30 35 40 frequenc y (khz) 45 50 impedance magnitude (k?) r4 09915-009 213.5 214.0 214.5 21.50 215.5 216.0 216.5 217.0 217.5 218.0 218.5 idea l figure 9. z c = 217. 2 5 k? , r cal = 99.85 k ?
circuit note cn- 0217 rev. a | page 5 of 6 0 20 40 60 80 100 120 30 32 34 36 38 40 42 44 46 48 50 impedance magnitude (k?) frequenc y (khz) r1 r3 c5 r0 c6 r2 09915-010 figure 10 . magnitude results for example 2 : r1, r2, r3, c5, c6 example 3 : parallel r - c ( r||c ) measurement an r||c type measurement was also made using the configuration , u sing an r cal of 1 k, an r of 10 k , and a c of 10 nf, measur ed across a frequency range of 4 khz to 100 khz. the magnitude a nd phase results v s . ideal are plotted in figure 11 and figure 12 . table 5 . r||c impedance range setup for v dd = 3.3 v supply voltage parameter value voltage peak - to - peak (v p - p) 0.383 v (range 3) number of settling time cycles 15 mclk 16 mhz r cal 1 k r fb 1 k excitation fre quency range 4 khz to 100 khz unknown impedance r||c r = 10 k , c = 10 n f 500 0 1000 1500 2000 2500 3000 3500 4000 4 24 44 64 84 104 impedance magnitude (?) frequenc y (khz) idea l measured 09915-0 1 1 figure 11 . magnitude results for z c = 10 k? ||10 nf, r cal = 1 k? 4 24 44 64 84 104 frequenc y (khz) idea l measured 09915?012 ?95 ?90 ?85 ?80 ?75 ?70 ?65 ?60 phase (degrees) figure 12 . phase results for z c = 10 k? ||10 nf, r cal = 1 k? s etup and test the e valuation board software is t he software used on the e va l - ad5933ebz . r efer to the technical note available on the cd provided with the evaluation board for details on the board setup . note that there are alterations to the schematic. link co nnections on the e va l - ad5933ebz board are listed in table 4 . i n addition, note that the location for r fb is located at r3 on the evaluation board , and the location for z unknown is c4. table 6 . link connecti ons for e val - ad5933ebz link number default position lk1 open lk2 open lk3 insert lk4 open lk5 insert lk6 a complete setup and operation for the hardware and software for the evaluation board can be found in user guide ug - 364. common va riations other op amp s can be used in the circuit, such as the ada4528 - 1 , ad8628 , ad8629 , ad8605 , and ad8608 . switching options for system applications for this particular circuit, the z unknown and r cal were interchanged m anually. however , in production, use a low on - resistance switch . the choice of the switch depends on how lar ge the unknown impedance range is and how accurate the measurement result needs to be. the examples in t his circuit note use just one calibration resistor, and so a low on - resistance switch , such as the adg849 , can be us ed as shown in figure 13 . multi channel switch solutions , such as the quad adg812 , can also be used. the errors caused by the switch resistance on the z unkn ow n are removed during cali bration, but by choosing a very low r on switch, the effects can be further minimized.
cn- 0217 circuit note rev. a | page 6 of 6 50k? z unknown r cal s1 d s2 r fb v dd in adg849 50k? a1 a2 09915?013 figure 13 . switching between r cal and u nknown z using the adg849 ultra l ow r on spdt switch (simplified schematic, all connections and decoupling not shown) learn more cn - 0217 design support package: http://www.analog.com/cn0217 - designsupport mt - 085 tutorial, "fundamentals of direct digital synthesis (dds)," analog devices. buchanan, david, "choosing dacs for direct digital synthesis," an - 237 application note, analog devices . riordan, liam, "ad5933 evaluation board example measurement," an - 1053 applic ation note, analog devices. ug - 364 user guide for ad5933 evaluation board adisimdds design and evaluation tool ad5933/ ad593 4 demonstration and design tool data sheets and evaluation boards ad593 3 data sheet ad5933 evaluation board ad593 4 data sheet ad5934 evaluation board ad860 6 data sheet adg84 9 data sheet adg81 2 data sheet revision history 3/13 rev. 0 to rev. a updated table numbers ; renumbered sequentially .................... 3 changes to evaluation and design support section .................... 1 changes to setup and test section and table 6 ............................ 5 changes to learn more section ...................................................... 6 6 /11 revision 0: initial version (continued from first page) circuits from the lab circuits are intended only for use with analo g devices products and are the intellectual property of analog devices or its licensors. while you may use the circuits from the lab circuits 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 circuits . information furnished by analog devices is believed to be accurate and reliable. however, circuits from the lab circuits are supplied "as is" and without warranties of any k ind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement o r fitness for a particular purpose and no responsibility is assumed by analog devices for their use, nor for any infringements of p atents or other rights of third parties that may result from their use. analog devices reserves the right to change any circuits from the lab circuits at any time without notice but is under no obligation to do so. ? 2011 C 2013 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn09915 - 0 - 3/13(a)
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