circuit note cn- 0188 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/cn0188 . devices connected /referenced ada4051 -2 microp ower , zero - drift , rail - to - rail input and output, dual op amp ad7171 low power, 16 - bit, sigma - delta adc adr 3 81 2.5 v, low noise, high accuracy, band g ap voltage reference ADUM5402 quad - channel isolator with integrated dc - to - dc converter low cost, level shifted low side current monitor for negative high voltage rails rev . b circuits from the lab? circuits 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 circuit, and their function and performance have been tes ted 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 f or 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 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation boards cn - 01 8 8 circuit evaluation board (eval - cn01 8 8 - sdpz) system demonstration platform (eval - sdp - cb1z) design and integration files schematics, layout files, bill of materials circuit function and bene fits the circuit shown in figure 1 monitors current in individual channels of ? 48 v to better than 1% accuracy . the load current passes through a shunt resistor , which is external to the circuit. the shunt resistor value is chosen so that the shunt voltage is approximately 50 mv at maximum load current. figure 1. low side current monitor for negative high voltage rails (all c onnections and decoupling not shown) s c l k p drs t d o u t v d d g n d r e f i n (+ ) a i n + a i n g n d i s o g n d i s o g n d i s o + 3 . 3 v i s o adr 381 vi n v o u t g n d l o a d so urce + 48v + ? v s hunt r s hunt i l o a d ad 7171 d 1 5 . 6 v r 2 1k 1 % r 4 1k 1 % 48. 7k , 1 % r 1 24. 9k u 1 a u 1 b u 1 : ada 4051- 2 q 1 + 2 . 5 v d 2 r 7 4 . 99k 1 % r 5 4 . 99k 1 % r 6 1k d 1 : mms z 5232b -7 -f d 2 : mbra120et 3 q 1 : z xmn 7 a 11g t a , 70v n -ch mo sf et 0 . 1 f r e f i n ( ? ) r 3 0 . 1 f 10 f 09570 - 001 v o a v i a v o b v i c v o c v i b v dd1 g n d 1 g n d i s o g n d 1 g n d i s o v i s o + 3 . 3 v ad u m 5402 n o t e: o pt ion a l adu m5402 pr o vi d es g a l v ani c i so l a t ion f o r d igi t a l signal s and i s o l a t ed po w er f o r c i rcui t
cn- 0188 circuit note rev. b | page 2 of 5 t he measurement result from the ad7171 is provided as a digital code utilizing a simple 2 - wire, spi - compatible serial interface . the entire circuit operates on a single +3.3 v supply. optional galvanic isolation is provided by the ADUM5402 quad channel isolator. in addition to isolating t he output data, the ADUM5402 digital isolator can also supply isolated + 3.3 v for the circuit. the ADUM5402 is not required for normal circuit operation unless galvanic isolation is needed. this combination of parts provides a accurate high voltage negative rail current sense solution with a small component count, low cost, and low power. the accuracy of the me asure - ment is primarily determined by resistor tolerances and the accuracy of the band gap reference , and is typically better than 1%. circuit description the circuit is designed for a full - scale shunt voltage of 50 mv at maximum load current i max . theref ore , the value of the shunt resistor is r shunt = ( 50 mv ) / ( i max ). the " ground " for the op amp stage is connected to the common C mode source voltage ( ? 48 v) . the voltage for the op amp stage is supplied by the "floating" 5.6 v z ener diode , which is biased at a current of approximately 2 ma. this eliminates the need for a separate power supply. the circuit will operate with a source voltage from ?60 v to ?10 v with no modifications. the shunt voltage is amplified by a factor of 49.7 using u 1a, where g = 1 + r3/r2. the zero - drift ada4051 - 2 has a low offset voltage (15 v maximum) and does not contribute significant error to the measurement. a full - scale shunt voltage of 50 mv produces a full - scale output voltage from u1a of 2.485 v (referenced to the common - mode source voltage) . an n - c hannel mosfet transistor with a large v ds breakdown (70 v) inside the feedback loop of u1b applies the output voltage of u1a across resistor r5, and the resulti ng current flows through r6 and r7. the full - scale voltage from u1a of 2.485 v produces a full - scale current of 0. 498 ma , which generates a full - scale voltage of 2.485 v across resistor r 7 . the voltage across r 7 is applied to ain? of the adc. resistor r 6 and the schottky diode d2 provide input protection for the ad7171 in the event the mosfet shorts out. notice that the power supply voltage for the adr381 , the ad7171 , and the floating zener diode is supplied by the isolated power output (+ 3.3 v iso ) of the ADUM5402 quad isolator. the reference voltage for the ad7171 is supplied by the adr381 precision band gap reference. the adr381 has an initial accuracy of 0. 24 % and a typical temperature coef ficient of 5 ppm/c. although it is possible to operate both the ad7171 vdd and refin(+) from the 3.3 v power supply , using a separate reference provides better accuracy. a 2.5 v reference is chosen to provide sufficient headroom. the input voltage to the ad7171 adc is converted into an offset binary code at the output of the adc. the ADUM5402 provides the is olation for the dout data output, the sclk input , and the pdrst input. the code is processed in the pc by using the sdp hardware board and labview software . the graph in f igure 2 shows how the circuit tested achieves an error of 0.3 % o ver the entire input voltage range (0 mv to 50 mv) . a comparison is made between the code seen at the output of the adc, recorded by l ab view , and an ideal code calculated based on a perfect system. figure 2. plot of output and error vs. shunt voltage in order to calculate this ideal code, there are several assumptions which must be made about the performance of the system . first, t he op amp gain stage must multiply the input signal by exactly 49.7 . depending on resistor tolerances (1%) , this value will vary by 2% worst case . secondly, the current sink resistor (r5) and the adc input resistor (r7) are assumed to be identical. in the circuit, t hese particular resistors have a tolerance of 1% . since they are the same value, the matching will probably b e better than 1% . resistors with tighter tolerances can be used , which will increase the accuracy and the cost of the circuit. several items have been implemented on the pcb, which are not crucial to the function or performance of t he circuit but are required to e nsure user and hardware safety. as an example, if q1 breaks down or shorts out, the adc, sdp board, user , and users pc are all at risk due to the large negative voltage potential. the safety items included are passive elements r6, d2 , which p rotect the ad7171 , and the ADUM5402 quad - channel digital isolator , which protects the ci r cuits on the sdp board , a s well as the user's pc. ?1.0 ?0.8 ?0.6 ?0.4 ?0.2 0 0.2 0.4 0.6 0.8 1.0 32768 37768 42768 47768 52768 57768 62768 65536 5 0 10 15 20 25 shunt vo lt age (mv) adc code error (%) 30 35 40 45 50 input output adc code error (%) 09570-002
circuit note cn- 0188 rev. b | page 3 of 5 pcb layout consideration s in any circuit where accuracy is crucial, it is important to co nsider the power supply and ground return layout on the board. the pcb should isolate the digital and analog sections as much as possible. this pcb was constructed in a four layer stack up wi th large area ground plane layers and power plane polygons. see the mt - 031 tutorial for more discussion on layout and grounding and the mt - 101 tutor ia l for information on decoupling techniques. the power supply to the ad7171 and ADUM5402 should be decoupled with 10 f and 0.1 f capacitors to properly suppress noi se and reduce ripple. the capacitors should be placed as close to the device as possible with the 0.1 f capacitor having a low esr value . c eramic capacitor s are advised for all high frequency decoupling . care should be taken in considering the isolation gap between the primary and secondary sides of the ADUM5402 . the e va l - cn0188 - sdpz board maximizes this distance by pulling back any polygons or components on the top layer and aligning them with the pins on t he ADUM5402 . power supply lines should have as large a trace width as possible to provide low impedance paths and reduce glitch effects on the supply line. clocks and other fast switching digital signals shou ld be shielded from other parts of the board by digital ground. a complete design support package for this circuit note , including board layouts , can be found at http://www.analog.com/cn0188 - designsupport . common variations there are a number of solutions available for high - side sensing of positive sources. ic solutions using current sense amplifiers, difference amplifiers, or a combination of these are available. high - side current sensing: difference amplifier vs, current - sense amplifier , analog dialogue , january 2008 , describes the use of current sense and difference amplifiers . the article is available at www.analog.com/highside_currentsensing . the following urls link to analog devices products which are useful in solving the current sense problem: current sense amplifiers: www.analog.com/currentsenseamps difference amplifiers: www.analog.com/differenceamps instrumentation amplifiers: www.analog.com/instrumentationamps figure 3. alternate galvanically isolated negative rail current sense circuit powered directly from ?48 v source and ADUM5402 isolator (all connections and decoupling not shown) sclk pdrst dout vdd gnd refin(+) ain+ ain? gnd iso +3.3v iso +3.3v iso gnd iso gnd iso adr381 vin vout gnd load source + ?48v +2.5v + ? v shunt r shunt i load ad7171 r2 1k? 1% r4 1k? 1% 48.7k?, 1% u1 u1: ada4051-1 refin(?) r3* ? gnd iso r1* 1.24m? 1% 09570-003 v oa v ia v ob v ic v oc v ib v dd1 gnd 1 gnd iso gnd 1 gnd iso v iso +3.3v ADUM5402 *r1 adds 100mv offset t o u1 output and allows measurement a t zero load curren t . if r1 is used, reduce r3 t o 46.4k? t o main t ain 2.5v full-scale
cn- 0188 circuit note rev. b | page 4 of 5 figure 3 shows an alternate circuit which can be used when galvanic isolation is required . the "ground" for the entire circuit is connected to the ?48 v source. the isolated +3.3 v from the ADUM5402 is used to power the circuit. note that this configuration does not require the op amp/mosfet level shifter (see figure 1) because the level shifting function is accomplished by the ADUM5402 isolator which allows a new ground reference (gnd1) to be established for the digital signals. a single zero - drift ada4051 - 1 provides a gain of 49.7 to the shunt voltage. resistor r1 provides a positive offset voltage of 100 mv at the op amp output that allows the circuit to operate down to zero load current. if this offset is added, then r3 should be reduced to 4 6 .4 k? to maintain a full - scale adc input voltage of 2.5 v for a 50 mv shunt voltage. without the offset, the ada4051 - 1 output will become nonlinear for output voltages less than about 40 mv. circuit evaluation a nd test this circuit uses the eval - cn0188 - sdpz circuit board and the eval - sdp - cb1z system demonstration platform (sdp) evaluation board. the two boards have 120 - pin mating connectors, allowing fo r the quick setup and evaluation of the circuits performance. the eval - cn0188 - sdpz board contains the circuit to be evaluated, as described in this note, and the sdp evaluation board is used with the cn0188 evaluation software to capture the data from the eval - cn0188 - sdpz circuit board. equipment needed ? pc with a usb p ort and windows? xp or windows vista? ( 32- bit), or windows? 7 (32 - bit) ? e va l - cn018 8 - sdpz c ircuit e valuation b oard ? e va l - sdp - cb1z sdp e valuation b oard ? cn018 8 e valuati on s oftware ? power supply: +6 v, or +6 v wall wart ? shu n t resistor with maximum voltage of 50 mv at the maximum load current . ? electro nic l oad getting started load the e valuation s oftware by placing the cn0 188 evaluation s oftware d isc in the cd drive of the pc. using "my c omputer, " locate the drive that co ntains the evaluation software disc and open the readme file. follow the instructions contained in the r eadme file for installing and using the evaluation software. functional block diagram see f igure 1 of this circu it note for the cir cuit block diagram and the e va l - cn 0188 - sdpz - sch pdf file for the circuit schematics. this file is contained in the cn01 8 8 design support p ackage . setup connect the 120 - pin connector on the eval - cn 0 1 8 8 - sdpz circuit board to the connector marked con a on the e va l - sdp - cb1z evaluation (sdp) board. nylon hardware should be used to firmly secure the two boards, using the holes provided at the ends of the 120 - pin connectors . co nnect a shunt resistor across the input terminals (r shunt ) with a load to ground as indicated in f igure 1. w ith power to the supply off, connect a + 6 v pow er supply to the pins marked + 6 v and gnd on the board. if available , a + 6 v " wall wart " can be connected to the barrel connector on the b oard and used in place of the + 6 v power supply. connect the usb cable supplied with the sdp board to the usb port on the pc. note : d o no t connect the usb cable to the m ini usb connector on the sdp board a t this time . it is important to connect the system ground and the pcb isolated ground to guarantee correct voltage levels and operation. test point 31 and test point 32 give access to the gnd_iso required to properly make this connection. test apply power to the + 6 v supply (or wall wart ) connected to the e va l - cn 0 188 - sdpz circuit board. launch the e valuation software and connect the usb cable from the pc to the usb mini - connector on the sdp board. once usb communications are est ablished, the sdp board c an be used to send, receive, and capture serial data from the e va l - cn 0 188 - sdpz board. data can be recorded for various values of load current as the electronic load is stepped. information and details regarding how to use the evaluation software for data capture can be found in the cn 0 188 e valuation s oftware read m e file. information regarding the sdp board can be found in the sdp user guide . learn more cn0 188 design support package: http://www.analog.com/cn0188 - designsupport sino, henri . high - side current sensing: difference amplifier vs.current - sense amplifier , analog dialogue 42 - 01, january (2008) . cantrell, mark. application note an - 0971, recommendations for control of radiated emissions with isopower devices . analog devices.
circuit note cn- 0188 rev. b | page 5 of 5 chen, baoxing , john wynne, and ronn kliger. high speed digital isolators using microscale on - chip transformers , analog devices, 2003. chen , baoxing. i coupler? products with iso power? technology: signal and power transfer across isolation barrier using microtransformers , analog devices, 2006 chen, baoxing. microtransformer isolation benefits digital control. power electronics technology . octobe r 2008. ghiorse, rich. application note an - 825, power supply considerations in icoupler? isolation products , analog devices. krakauer, david. digita l isolation offers compact, low - cost solutions to challenging design problems. analog dialogue. volume 40, december 2006. mt - 022 tutorial, adc architectures iii: sigma - delta adc basics , analog devices. mt - 023 tutorial, adc architectures iv: sigma - delta adc advanced concepts and applications , analog devices. mt - 031 tutorial, grounding data converters and solving the m ystery of "agnd" and "dgnd , " analog devices. mt - 101 tutorial, decoupling techniques , analog devices. wayne, scott. i coupler ? digital isolators protect rs - 232, rs - 485, and can buses in industrial, instrumenta tion, and computer applications. analog dialogue . volume 39, october 2005 . data sheets and evaluation boards cn0188 circuit evaluation board (eval - cn0188 - sdpz) system demonstration platform (eval - sdp - cb1z) ada4051 - 2 data sheet ada4051 - 2 evaluation board ada4051 - 1 data sheet ada4051 - 1 evaluation board ad7171 data sheet ad7171 evaluation board adr3 81 data sheet ad u m5402 data sheet ad u m5402 evaluation board revision histor 11/11 rev. a to rev. b change to figure 3 ............................................................................ 3 changes to circuit evaluation and test ......................................... 4 6/11 rev. 0 to rev. a changes to circuit note title .......................................................... 1 changes to circuit function and benefits ..................................... 1 changes to figure 1 .......................................................................... 1 changes to circuit description ....................................................... 2 changes to common variations ..................................................... 3 added figure 3 .................................................................................. 3 4 /1 1 rev ision 0: initial version (continued from first page) circuits from the lab circuits are intended only for use with analog 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" are suppl ied "as is" a nd without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merc hantability, 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 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 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn09570 - 0 - 11/11(b)
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