circuit note cn - 0357 circuits from the lab? reference designs are engineered and teste d 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/cn0357 . devices connected /ref erenced ada4528 -2 5.0 v , u ltralow n oise, z ero d rift, rrio, d ual o p a mp ad5270 -20 1024 -p osition, 1% r esistor tolerance e rror, 50- t p m emory d igital r heostat adr3412 micropower, 0.1% a ccurate , 1.2 v oltage r eference AD8500 micropower , rrio, o p a mp ad7790 low power, 16 -b it s igma -d elta, adc low noise, single - supply , toxic gas detector , using an electrochemical sensor with programmable gain tia for rapid prototyping rev. 0 circuits from the lab? reference designs from analog devices have been designed and built by analog devices engineers. standard engineering practices have been employed in the design and construction of each circ uit, 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. acco rdingly, 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 techno logy way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2014 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation boards cn - 0357 circuit evaluation board (eval - cn 0 3 57 - pmdz) sdp to pmod i nterposer board (pmd - sdp - ib1z) system demonstration platform (eval - sdp - cb1z) design and integration files schemati cs, layout files, and bill of materials circuit function and benefits the circuit shown in figure 1 is a single - supply, low noise , portable gas detector , using an electrochemical sensor . the alphasense co - ax c arbon m onoxide senso r is used in th is example . electrochemical sensors offer several advantages for instruments that detect or measure the concentration of many toxic gases. most sensors are gas specific and have usable resolutions under one part per million (ppm) of gas conc entration. the circuit shown in figure 1 uses the ada4528 - 2 , dual auto zero amplifier , which has a maximum offset voltage of 2 .5 v at room temperature and an industry leading 5.6 v/hz of voltage noise density. in addition, the ad5270 - 20 programmable rheostat is used rather than a fixed transimpedance resistor , allowing for rapid prototyping of different gas sensor sys tems , without changing the bill of materials. t he adr3412 precision, low noise, micropower reference e stablishes the 1.2 v common - mode , pseudo ground reference voltage with 0.1% accuracy and 8 ppm /c drift. for applications where measuring fractions of ppm gas concentration is important , using the ada4528 - 2 and the adr3412 makes the circuit performance suitable for interfacing with a 16 - bit adc , such as the ad7790 . 3.3v ain1(+) ain1(?) v in v out ref(+) v dd ref(?) gnd dout/rdy din sclk cs ad 77 90 to processor 1.2v c9 10f r8 100k? r10 3.3k? r9 3.3k? u3-b ad5270-20 r2 33? 3.3v r1 1m? r6 12.4k? r5 12.4k? r3 12.4k? r4 12.4k? g d q1 mmbfj270 s c2 0.02f w a c4 0.02f c3 0.02f c8 0.1f c10 0.1f 1.2v gnd vref ce re we u1 u8 co-ax agnd u2-b ada4528-2 u5 AD8500 u2-a ada4528-2 u4 adr3412 avcc c14 5.6nf r12 150? agnd avcc 12332-001 figure 1 . low noise gas detector circuit (simplified schematic: a ll connections and decoupling n ot shown)
cn- 0357 circuit note rev. 0 | page 2 of 5 circuit description figure 2 shows a simplified schematic of an electrochemical sensor measurement circuit. electrochemical sensors work by allowing gas to diffuse into the sensor through a membrane and by interacting with the working electrode (we). the sensor reference electrode (re) provides feedback to amplifier u2 - a, which maintain s a constant potential with the we terminal by varying the voltage at the counter electrode (ce). the direction of the c urrent at the we terminal depends on whether the reaction occurring within the sensor is oxidation or reduction. in the case of a carbon monoxide sensor , oxidation takes place ; therefore, the current flow s into the working electrode , which requires the cou nter electrode to be at a negative voltage (typically 300 mv to 400 mv) with respect to the working electrode. the op amp driving the ce terminal should have an output voltage range of approximately 1 v with respect to v ref to provide sufficient headroom for operation w ith different types of sensors (alphasense application note aan - 105- 03, designing a potentiostatic circuit , alphasense , ltd .). ce ? + sensor v ref v out v ref i we i we i we re we rf 12332-002 figure 2. simplified electrochemical sensor circuit the current into the we terminal i s less than 100 na per ppm of gas concentration ; therefore, converting this current into an output voltage requires a transimpedance amplifier with a very low input bias current. the ada4528 - 2 o p amp has cmos inputs with a maximum input bias current of 2 20 pa at room temperature, making it a very good fit for this application . the adr3412 establishes the pseudo ground reference for the c ircuit , which allows for single - supply operation while consuming very little quiescent current (100 a maximum) . amplifier u2 - a sink s enough current from the ce terminal to maintain a 0 v potential between the w e terminal and the r e terminal on the sensor . the re terminal is connected to the inverting input of a mplifier u2 - a ; therefore, no current flow s in or out of it . this means that the current come s from the w e terminal and it change s linearly with gas concentration. transimpedance a mplifier u2 - b conv erts the sensor current into a voltage proportional to the gas concentration . the sensor selected for this circuit is an alphase nse co - ax c arbon m onoxide sensor. table 1 shows the typical specifications associated with carbon monoxide sensors of this general type. warning: c arbon m onoxide is a toxic gas, and concentrations higher than 2 50 ppm can be dangerous ; therefore, take extreme care when testing this circuit . table 1 . typical carbon mo noxide sensor specifications parameter value sensitivity 55 na/ppm to 100 na/ppm (65 na/ppm typ ical ) response t ime ( t 90 from 0 ppm to 400 ppm co) < 30 s ec onds range (ppm ) co, guaranteed performance ) 0 ppm to 2,000 ppm over range limit ( specifications n ot guaranteed) 4,000 ppm the output voltage of the transimpedance amplifier is v o = 1.2 v + i we r f (1) w here i we is the current into the we terminal, and r f is the transimpedance feedback resistor ( s hown as the ad5270 - 20 u3 - b rheostat in figure 1 ). the maximum response of the co - ax sensor is 100 na/ppm, and its maximum input range is 2000 ppm of carbon monoxide . these values result in a maximum output cur rent of 200 a and a maximum out p ut voltage determined by the transimpedance resistor , as shown in equation 2 . f o r v + = ppm na 100 ppm 2000 v 2 . 1 v o = 1.2 v + 200 a r f (2) applying 1.2 v to v ref of the ad7790 allows a usable range of 1.2 v at the output of the transimpedance a mplifier , u2 - b. selecting a 6.0 k? resistor for the transimpedance feedback resistor gives a maximum output voltage of 2.4 v . equation 3 shows the circuit output voltage as a function of ppm of c arbon monoxid e , using the typical response of the sensor of 65 na/ppm. ppm v 390 v 2 . 1 + = o v (3) the ad5270 - 20 has a nominal resistance value of 20 k?. there are 1024 resistance positions, result ing in resistance s tep sizes of 19.5 ?. the 5 ppm/c resistance temperature coefficient of the ad5270 - 20 is better than that of most discrete resistors, and its 1 a of supply current is a very sma ll contributor to the overall power consumption of the system . resistor r4 keeps the noise gain at a reasonable level. selecting the value of this resistor is a compromise between the magnitude of the noise gain and the sensor settling time errors , when ex posed to high concentrations of gas. for the example shown in equation 4 , r4 = 33 ?, which results in a noise gain of 183. 183 33 k 6.0 1 = + = ng (4)
circuit note cn- 0357 rev. 0 | page 3 of 5 the input noise of the transimpedance amplifier appear s at the output , amplified by the noise gain. for this circuit, only low frequency noise is of interest because the frequency of oper ation of the sensor is very low. the ada4528 - 2 has a 0.1 hz to 10 hz input voltage noise of 97 n v p - p; therefore, the noise at the output is 18 v p - p , as shown in equation 5. v outputnoise = 97 n v ng = 18 v p - p (5) b ecause this is a very low frequency 1/f noise, the noise is very hard to filter. however, because the sensor response is also very slow , it is possible to use a very low frequency , low - pass filter (r5 and c6) with a cutoff frequency of 0.16 hz. even with such a low frequency filter, its effect on the sensor response time is negligible , when compared to the 30 second response time of the sensor . the noise free code resolution of the system is determined from the peak - to - peak output no ise. the maximum output of the ada4528 - 2 is 2.4 v , so the noise free number of counts is 64,865 p p v 18 v 2.4 = ? = counts free noise total (6) the noise free code resolution becomes noise free code resolution = log 2 (64,8 65) = 15.9 bits (7) t o take advantage of the entire adc range available (1.2 v) , the AD8500 micropower, rail - to - rail input/output amplifier is chosen to drive the input of the ad7790 . if the entire range is not necessary , the AD8500 can be removed and the internal buffer of the ad7790 can be used in its place. one important characteristic of electrochemical sensors is their very long time constant. when first powered up, it can take several minutes for the output to settle to its fin al value. when exposed to a mid scale step i n concentration of the target gas, the time required for the sensor output to reach 90% of its final value can be in the order of 25 seconds to 40 seconds. i f the voltage between the re terminal and the w e terminal has a sudden change in magnitude, it ca n take several minutes for the output current of the sensor to settle. t his long time constant also applies when cycling power to the sensor . to avoid very long start - up times , p - channel jfet q 1 shorts the r e terminal to the w e terminal when the supply volt age drops below the g ate - to - source threshold voltage (~2. 0 v) of the jfet . common variations electrochemical sensors operate with very small amounts of current, making them well suited for portable, battery - powered instruments. if lower power is required, t he ada4505 - 2 amplifier has a maximum input bias current of 2 pa and consumes only 10 a per amplifier. howeve r, the noise of the ada 4505- 2 is greater than that of the ada4528 - 2 . t he adr291 precision voltage reference consumes only 12 a and can be substituted for th e adr3412 , if lower power is required. more information can be found in circuit note cn - 0234. circuit e valuation a nd test the circuit s hown in figure 1 uses the e va l - cn0357 - pmdz circuit evaluation board, the pmd - sdp - ib1z interposer board, and the e va l - sdp - cb1z system demonstration platform (sdp - b ) controller board. in addition, the e va l - cn0357 - pmdz c omes in a pmod form factor, making it possible to connect the board to any pmod controller board , allowing for rapid prototyping. the cn - 0357 e valuation s oftware communicates with the sdp board to capture data from the e va l - cn0357 - pmdz circuit evaluation board. equipment n eeded the following equipment is needed for evaluating the cn - 0357 circuit: ? pc with usb port and windows ? xp , or wind ows vista (32 - bit), or windows 7 (32 - bit) ? e va l - cn0357 - pmdz e valuation board ? pmd - sdp - ib1z i nterposer board ? e va l - sdp - cb1z c ontroller board ? cn - 0357 e valuation s oftware ? calibration g as ( less than 250 ppm) ? e va l - cftl - 6v - pwrz or equivalent 6 vdc power supply getting started load the evaluation software by placing the cn - 0 357 evaluation software cd in to the cd drive of the pc. using my computer , locate the drive that contains the evaluation software cd and run the setup.exe . follow the on screen prompts for installing and using the evaluation software. functional block diagram a functional block diagram of the test setup is shown in figure 3 . there is a complete schematic of the evaluation board contained in the cn - 0357 design support package , including printouts of the gerber files , as well as a bill of materials. eval-sdp-cb1z con a or con b usb pc usb eval-cn0357-pmdz 120-pin sdp j1 +6v power pmd-sdp-ib1z 12-pin 120-pin sdp sensor co-ax 12332-003 figure 3. test setup functional block diagram
cn- 0357 circuit note rev. 0 | page 4 of 5 setup connect the electrochemical sensor to the socket on the e va l - cn0357 - pmdz circuit evalu ation board. connect the e va l - cn0357 - pmdz to the interposer board and ensure that the jumper on the interposer board selects +3.3 v as the configuration. next, connect the sdp - b board to the interposer board and apply power to the interposer board via the dc barrel jack. connect the usb cable supplied with the sdp - b board to the usb port on the pc and to the sdp - b board. the software can communicate with the sdp board if the analog device s system development platform driver appears in the device manager . when usb communications are established, the sdp board can now send, receive, and capture serial data from the e va l - cn03 57- pmdz circuit evaluation board. test navigate to the installation location of the cn - 0357 eval u a tion software and open the cn0357.exe file. ( the file should be located in a folder name d analog devices in the start menu of the operating system.) when the a pplication opens, the software automatically connect s to the sdp - b board. if multiple sdp - b controller boards are connected, the selected board will have a light emitting diode ( led ) bl inking. the cn - 0357 evaluation software user guide contains i nformation and details regarding how to use the evaluation software for data capture. the input signal for this board is gas concentration ; therefore, a calibration gas source is required. when testing with carbon monoxid e , consider that 250 ppm is the ma ximum short term exposure limit. the software is designed to be used with any electrochemical sensor, so it is important to in put the correct specifications for the chosen sensor . the maximum sensor sensitivity and sensor range are needed to calculate and set the resistance of the ad5270 - 20 digital rheostat. this value is a signed value. positive values are used to describe sensors than sink current, where as negative values are used to describe sensors that source current. the typical sensor sensitivity is used to calculate the system conversion coefficient of ppm/mv . pres sing the run button begin s acquisition of concentration data at 1 second intervals. figure 4 shows the circuit response after rapidly removing the sensor from the 1 75 ppm co atmosphere and is a better representation of circuit pe rformance. a photo of the e va l - cn0357 - pmdz board is shown in figure 5 . 12332-004 figure 4. response to a 175 ppm to 0 ppm s tep of carbon monoxi de 12332-005 figure 5 . eval - cn0357 - pmdz evaluation board
circuit note cn- 0357 rev. 0 | page 5 of 5 learn more cn - 0357 design support package: http://w ww.analog.com/cn0357 - designsupport an - 1114, lowest noise zero - drift amp lifier has 5.6 nv/hz voltage noise density, analog devices. ms - 2066 article, low noise signal conditioning for sensor - based circuits, analog devices. mt - 035 tutorial, op amp inputs, outputs, single - supply, and rail - to - rail issues , analog devices. cn - 0234 circuit note, single supply, micropower toxic gas detector using an electrochemi cal sensor, analog devices. alphasense application note aan - 105- 03, designing a potentiostatic circuit , alphasense limited. data sheets and evaluation boards cn - 0357 circuit evaluation bo a r d ( e va l - cn0357 - pmdz) system demonstration platform (eval - sdp - cb1z) ada4528 - 2 data sheet ad5270 - 20 data sheet adr3412 data sheet AD8500 data sheet a d7790 data sheet revision history 7 / 14 revision 0: initial version (continued from first page) circuits from the lab reference designs are intended only for use with analog devices products and are the intellectual property of analog devices or its l icensors. while you may use the circuits from the lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectua l property by application or use of the circuits from the l ab reference designs . information furnished by analog devices is believed to be accurate and reliable. however, circuits from the lab reference designs are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but n ot limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by analog devices for their use, nor for any infringements of patents or other rights of third parties that may res ult from their use. analog devices reserves the right to change any circuits from the lab reference designs at any time without notice but is under no obligation to do so. ? 2014 analog devices, inc. all righ ts reserved. trademarks and registered trademarks are the property of their respective owners. cn12332 - 0 - 7/14(0)
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