circuit note cn - 0221 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 , visi t www.analog.com/cn0221 . devices connected /referenced aduc m360 / aducm361 cortex -m3 based microcontroller with dual 24 - bit - adcs adp1720 - 3.3 low dropout linear regulator usb - based temperature monitor using the aducm360 precision analog microcontroller and an external thermocouple rev. 0 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 desi gn 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 fo r 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. (conti nued 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 ? 2012 analog devices, inc. all rights reserved. e valuation and design support circuit evaluation board cn - 0221 evaluation board (eval - aducm360tcz) design and integration files schematics, la yout files, bill of materials , source code for aducm360 circuit function and benefits this circuit uses the aducm360 / adu cm36 1 precision analog microcontroller in a n accurate thermocouple temperature monitoring application. the aduc m360 / aduc m3 61 integrate s dual 24 - bit sigma - delta ( - ) analog - to - digital converters ( adcs ), dual programmable current sources, a 1 2- bit digital - to - analog converter ( dac ) , and a 1.2 v internal reference , as well as an arm cortex - m3 core, 126 kb flash, 8 kb sram, and various digital peripherals such as uart, timers, spi s , and i 2 c interfaces. in th e circuit, the aducm360 / aducm361 is connected to a thermocouple and a 100 ? platinum resistance temperature detector ( rtd ) . the rtd is used for cold junction compensation . in the source code, an adc sampling rate of 4 hz is chosen. when the adc input programmable gain amplifier ( pga ) is configu red for a gain of 32, th e noise - free code resolution of the aduc m360 / aduc m36 1 is greater than 18 bits. aducm360 ain5/iexc avdd iovdd ain0 rxd rxd txd txd reset p2.2/bm reset sd v re f + r ref v re f ? ain1 5.6k ? 0.1% 100 ? ptrtd ain2 thermocouple junction ain3 j1 ain7/vbias agnd 09985-001 bead 3.3v bead usb header 10? 10? 10? 0.01f 0.01f 0.1f 4.7f 4.7f 0.1f 10f in 5v d? d+ gnd out gnd adp1720-3.3 FT232R bead ferrite beads: 1k? @ 100mhz taiyo yuden bk2125hs102-t 0.1f shield p0.2/so ut p0. 1/sin figure 1. aducm360 / aducm361 a s a temperature monitor controller with a thermocouple interface (simplified schematic, all connections not shown)
cn- 0221 circuit note rev. 0 | page 2 of 5 circuit d escription the following features of the aducm360 / aducm361 are used in this application: ? a 24- bit - adc with a pga set for a gain of 32 in the sof tware for the thermocouple and rtd. the adc 1 was switched continuously between sampling the thermocouple and the rtd voltages. ? p rogrammable excitation current sources for forcing a controlled current through the rtd . the dual curr ent sources are configurable in from 0 a to 2 ma. for this example, a 200 a setting was used t o minimize the error introd uced by the rtd self - heating. ? an i nternal 1.2 v reference for the adc in the aducm360 / aducm361 . it measur es the thermocouple voltage ; the internal voltage reference was used due to its precision. ? an e xternal voltage reference for the adc in the aducm360 / aducm361 . it measur es the rtd resistan ce ; a ratiometric setup was used where an external reference resistor (r ref ) w a s connected across the external v ref+ and v ref ? pins. ? a b ias voltage generator (vb ias ) . the vb ias function was used to set the thermocouple common - mode voltage to av dd /2 . ? the arm cortex - m3 core . the powerful 32 - bit arm core with integrated 126 kb flash and 8 kb sram memory runs the user code that con figures and controls the adc, processes the adc conversions from the rtd, and controls the communications over the uart/usb interface. ? the uart was used as the communication interface to the host pc. ? two external switches are used to force the part into i ts flash boot mode . by holding s d low and toggling the reset button , the aducm360 / aducm361 enter s boot mode instead of normal user mode. in boot mode, the inter nal flash can be reprogrammed through the uart interface. both the thermocouple and the rtd generate very small signals ; therefore , a pga is required to amplify those signals. the thermocouple used in this application is a t ype t (copper - constantan ) that has a temperature range of ? 200c to + 350c. its sensitivity is approximately 40 v/ c , which means that the adc in bipolar mode , with a pga gain of 32 , can cover the entire temperature range of the thermocouple. the rtd was used for cold junction compensation. the particular one us ed in this circuit was a platinum 100 ? rtd, enercorp pcs 1.1503.1. it is available in a 0805 , s urface - mount package. this rtd has a temperature variation of 0.385 ? / c. note that the reference resistor, r ref , should be a precision 5.6 k ? (0.1%). the usb interface to the aducm360 / aducm361 is implemented with an FT232R uart to usb transceiver, which converts usb signals directly to the uart. in addition to the d ecoupling shown figure 1 , the usb cable itself must have a ferrite bead for added emi/rfi protection. the ferrite beads used in the circuit were taiyo yuden, #bk2125hs102 - t, which have an impedance of 1000 ? at 100 mhz. construct t he circuit on a multilayer printed circuit board (pcb ) with a large area ground plane. use p roper layout, grounding, and decoupling techniques to achieve optimum performance (see tut orial mt - 031, grounding data converters and solving the mystery of "agnd" and "dgnd," tutor ia l mt - 101, decoupling te chni ques , and the aducm360tcz evaluation b oard layout). t he pcb used for evaluating this circuit is shown in figure 2 . 09985-002 figure 2. eval - aducm360tcz board used for this circuit
circuit note cn- 0221 rev. 0 | page 3 of 5 co de description the source code used to test the circuit can be downloaded as a zip file from the aducm360 product page. the uart is configured for a baud rate of 9600, 8 data bits, no parity, and no flow con trol. if the circuit is connected directly to a pc, a communication port viewing application , such as a hyperterminal , can be used to view the results sent by the program to the uart , as shown in figure 3 . 09985-003 figure 3 . output of hyperterminal communication port viewing application t o get a temperature reading , measure the temperature of the thermocouple and the rtd . the rtd temperature is converted to its equivalent thermocouple voltage via a look - up table (see the ise, inc., its - 90 table for type t thermocouple) . these two voltages are added together to give the absolute value at the thermocouple. first , the voltage measured between the two wires of the thermocouple (v1) . the rtd voltage is measured , conve rted to a temperature via a look - up table , and then, this temperature is converted to its equivalent thermocouple voltage (v2). v1 and v2 are then added to give the overall thermocouple voltage , and this is then converted to the final temperature measureme nt. 20 0 ?20 ?40 ?60 ?80 ?100 ?210 ?140 ?70 0 70 140 210 280 350 error (c) temperature (c) 09985-004 figure 4. error when using simple linear approximation initially, this was done using a simple linear assumption that the voltage on the thermocouple was 40 v/c. it can be seen from figure 4 that this gives an acceptable error only for a small range , around 0c. a better way of calculating the thermocouple temperatures is to use a six - order polynomial for the positive temperatures and a seventh - order polynomial for the negative tempe ratures . this requires mathematical operations that add to computational time and code size. a suitable compromise is to calculate the respective temperatures for a fixed number of voltages. these temperatures are stored in an array , and values in between are calculated using a linear interpolation between the adjacent points. it can be seen from figure 5 that the error is drastically reduced using this method. figure 5 gives the algorithm error using ideal thermocouple voltages. 0.30 0.25 0.20 0 0.05 0.10 0.15 ?0.05 ?210 ?140 ?70 0 70 140 210 280 350 error (c) temperature (c) 09985-005 figure 5. error when using piecewise linear approximation using 52 calibration points and ideal measurements
cn-0221 circuit note rev. 0 | page 4 of 5 figure 6 shows the error ob tained when using adc1 on the aducm360 to measure 52 thermocouple voltages over the full thermocouple operating range . the overall worst - case error is <1c. ?0.5 ?0.4 ?0.3 ?0.2 ?0.1 0 0.1 0.2 0.3 0.4 0.5 ?210 ?140 ?70 0 70 140 210 280 350 error (c) tempera ture (c) 09985-006 figure 6. error when usi ng piecewise linear approximation using 52 calibration points m easured by aducm360 / aducm361 the rtd temperature is calculated using lookup tables and is implem ented for the rtd the same way as for the thermocouple . note that the rtd has a different polynomial describing its temperatures as a function of resistance. for details on linearization and maximizing the performance of the rtd, refer to application note an - 0970 , rtd interfacing and linearization using an aduc706x microcontroller . common variations the adp1720 regulator can be replaced with the adp120 , which has the same operating temperature range (?40c to +125c) and consumes less power (typically 35 a vs. 70 a) but has a lower maximum input voltage . note that the aducm3 60 / aducm361 can be programmed or debugged via a standard s erial wire interface. for a standard uart to rs - 232 interface, the FT232R transceiver c an be replaced with a device such as the adm3202 , which requires a 3 v power supply. for a wider temperature range , a different thermocouple can be used , such as a t ype j. to minimize the cold junction compensation error, a thermistor c an be placed in contact with the act ual cold junction instead of on the pcb . instead of using the rtd and external reference resistor for measuring the cold junction temperature, a n external digital temperature sensor c an be used . f or example, the adt7410 c an connect to the aducm360 / aducm361 via the i 2 c interface. for more details on cold junction compensation, refer to sensor signal conditioning, analog devices, chapter 7, temperature sensors. if isolation between the usb connector and this circuit is required , the adum3160 / adum4160 isolation devices must be added. circuit evaluation a nd test to test and evaluate the circuit, the thermocouple measurements and the rtd measurements were evaluated separately. thermocouple measurement test the basic test setup is shown in figure 7 . t he thermocouple is connected to j5, and jumper j1 must be installed to allow the ain7/vbias pin to set the thermocouple common - mode voltage . the circuit board receives its power from the usb connecti on to the pc. two methods were used to evaluate the performance of the circuit. initially , the circuit was tested with the thermocouple at tached to the board and it was used to measure the temperature of an ice bucket . then, it was used to measure the tem perature of boiling water . a wavetek 4808 multi f unction calibrator was used t o fully evaluate the error , as shown in figure 4 and figure 6 . in this mode, the thermocouple w as replaced with the calibrator as the voltage source , as shown in figure 7 . to evaluate the entire range of a type t thermocouple, t he calibrator was used to set the equivalent thermocouple voltage at 52 points be tween ? 200 c to +350 c for the negative and positive range s of the t - type thermocouple ( see the ise, inc., its - 90 table for type t thermocouple ). to evaluate the accuracy of the lookup algorithm, 551 voltage r eadings, equivalent to temperatures in the rang e of ? 200c to +350c spaced at + 1c, were passed onto the temperature calculation functions. errors were calculated for the linear method and the piecewise linear approximation method as is shown in figure 4 and figure 5 . ev al-aducm360tcz wavetek 4808 multifunction calibrator pc j5 j1 ain7/vbias thermocouple junction see text usb cable 09985-007 figure 7. test setup used to calibrate and test the circuit o ver full thermocouple output voltage range
circuit note cn-0221 rev. 0 | page 5 of 5 rtd measurement test to evaluate the rtd circuit and linearization source code, the rtd on the board was replacement with an accurate, adjustable resistance source. the instrument used was the 1433 - z decade resistor. the rtd values are from 90 ? to 140 ?, which represents an rtd temperature range of ? 25 c to + 114 c. the test setup circuit is shown in figure 8 , and the error results for the rtd tests are shown in figure 9 . 1433-z decade resist or aducm360 0.1f 0.01f 0.01f r ref 5.6k? 0.1% 10? 10? a vdd v ref + v ref ? iovdd a vdd iovdd 0.1f 09985-008 ain5/iexc ain0 ain1 figure 8 . test setup for measuring rtd error 0 ?0.01 ?25 ?5 error (c) temperature (c) ?0.02 ?0.03 ?0.04 ?0.05 ?0.06 ?0.07 ?0.08 ?0.09 ?0.10 15 35 55 75 95 115 09985-009 figure 9 . error in c of rtd measurement using piecewise linearization code and adc0 measurements learn more cn0 2 21 design support package: http://www.analog.com/cn0221-designsupport adisim power design tool . kester, walt. 1999. sensor signal conditioning . analog devices. chapter 7, "temperature sensors." kester , wa lt . 1999. s ensor signal conditioning . analog devices. chapter 8, "adcs for signal conditioning." looney, mike. rtd interfacing and linearization using an aduc706x microcontroller. an- 0970 application note. analog devices. mt - 022 tutorial, adc architectures iii: sigma - delta adc basics . analo g devices. mt - 023 tutorial, adc architectures iv: sigma - delta adc advanced concepts and applications. analog devices. mt - 031 tutorial, grounding data converters and so lving the mystery of "agnd" and "dgnd. " analog devices. mt - 101 tutorial, decoupling techniques . analog devices. its - 90 table for t ype t t hermocouple . data sheets and evaluation boards aduc m360 / aducm 361 data sheet aducm360 / aducm361 evaluation kit adm3202 uart to rs232 transceiver data sheet adp120 data sheet adp 1720 data sheet revision history 5 /1 2 revision 0: initial version (continued fr om first page) circuits from the lab circuits are intended only for use with analog devices products and are the intellectual property of an alog devices or its licensors. while you may use the circuits from the lab circuits in the design of your product, n o other license is granted by implication or oth erwise 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 i s" 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 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 s o. ? 2012 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn09985 -0- 5/12(0)
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