circuit note cn - 0300 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/cn0300 . devices connected /referenced aducm360 cortex - m3 based microcontroller with dual 24 - bit - adcs adp1720 - 3.3 low dropout linear regulator complete closed - loop precision analog microcontroller thermocouple measurement system with 4 ma to 20 ma output rev. a circuits from the lab? circuits from ana log dev ices have been designed and built by analog dev ices engineers. standard engineering pra ctices have been employed in the design and construction of each circuit, and their function and performance have been teste d and verified in a lab e nvironment a t room tempe rature. howeve r, you a re sole l y resp ons ib le fo r test in g the c irc u it and determ in in g i ts suitability and a pplicability for your use and a pplication. accordingly, in no event sha ll ana log dev ices be liable for direct, indirect, special, incide nta l, conse quential or punitive damages due t o 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 ? 2012 C 2013 analog devices, inc. all rights reserved. e valuation and design support circuit evaluation board cn - 0 300 evaluation board (eval - cn0300 - eb1z ) i ncludes analog devices j - link ob emulator (usb - swd/uart - emuz) design and integration files schematics, layout files, bill of materials , source code for aducm360 circuit function and benefits this circuit uses the aducm 360 precision analog microcontroller in an accurate thermocouple temperature monitoring application and controls the 4 m a to 20 ma output current accordingly . the aducm360 inte grate s dual 24 - bit sigma - delta ( - ) analog - to - digital converters ( adcs ) , dual programmable current sources, a 1 2 - bit digital - to - analog converter ( d ac ) , 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 the circuit, the aducm360 is connected to a type t thermocouple and a 100 ? platinum resistance temperature detector ( rtd ) . the rtd is used for cold junction compensation . the low power cortex - m3 core converts t he adc readings to a real temperature value. the type t temperature range supported is ? 200c to +350c , and this temperature range is converted to an output current range of 4 ma to 20 ma. the circuit provides a complete solution for thermocouple measurements with a minimum requirement for external components , and it is loop powered for loop voltages up to 28 v . figure 1 . aducm360 a s a temperature monitor controller with a thermocouple interface (simplified schematic, all connections not shown) aducm360 iexc iovdd adc0 ain9 ain8 vref+ r ref vref? adc1 5.6k? 0.1% 100? ptrtd ain2 thermocouple junction ain3 ain7/vbias agnd p2.2/bm dvdd_reg dac 10955-001 ferrite bead 600? at 100mhz murata blm31aj601sn1l 3.3v 10? 10? 1.6? 100k? 100k? rloop 47? npn bc548 vloop+ vloop? 0.01f 0.01f 0.1f 10f 10f 0.1f 10nf 10nf 0.47f 10f in vloop interface board connector reset gnd swdio out gnd adp1720-3.3 sout swclk sin nc iovdd p0.2/sout p0.1/sin current meter avdd 10k? 10k? reset swdio swclk iovdd reset sd reset
cn- 0300 circuit note rev. a | page 2 of 7 circuit d escription the following features of the adu cm360 are used in this application: ? the 12 - b it dac output with its flexible on - chip output buffer is us ed to control a n e xterna l npn transistor, bc548. by controlling the v be voltage of this transistor, the current passing through a 47 ? load resistor can be set to the desired value. when npn mode is selected, the buffered on chip 1.2 v reference voltage is present on ain8. ? the dac is 12 - bit monoto nic ; however, the accuracy of the dac output is typically around 3 lsbs. in addition , the bi - polar transistor introduce s linearity errors. to improve the accuracy of the dac output and to eliminate offset and gain end - point errors, adc0 measures , on ain9, a feedback voltage reflecting the voltage across the load resistor ( r load ) . based on this adc0 reading, the dac output is corrected by the source code . this provides 0.5 c accuracy on the 4 ma to 20 ma output. ? the 24 - bit - adc with a pga set for a gain of 32 in the sof tware for the thermocouple and the r t d. adc1 switches continuously between sampling the thermocouple and the rtd voltages. ? p rogrammable excitation current sources forc e a co nt r olled current through the rtd . the dual curr ent sources are configurable in steps from 0 a t o 2 ma. for this example, a 200 a setting is used to minimize the error introd uced by the rtd self - heating. ? an i nternal 1.2 v reference is provide d for the adc in the aducm360 . when measuring the thermocouple voltage , the internal voltage reference is used due to its precision. ? an e xternal voltage reference for the adc in the adu cm360 . when measuring the rtd resistance , a ratiometric setup was used where an external reference resistor (r ref ) wa s connected across the extern al v ref+ and v ref ? pins . the on - chip reference input buffer is enabled because the reference source in this circuit is high impedance. the on - chip reference buffer means no external buffer is required to minimize input leakage effects. ? a b ias voltage gener ato r (vb ias ) . the vb ias function is used to set the thermocouple common - mode voltage to a v dd _reg /2 (900 mv) . again, this removes the need for external r esistors to set the thermocouple common - mode voltage. ? 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 configures and controls the adc s and co nve rts the adc conversions from the thermocouple and rtd inputs to a final temperature value. it also controls th e dac output and continuously monitors this dac output using the closed - loop feedback from the voltage level on ain9. for extra debug purposes, i t also controls the communications over the uart/usb interface. ? the uart is used as the communication interface to the host pc. this is used to program the on - chip flash. it is also used as a debug port and for calibrating the dac and adc . ? two external switches are used to force the part into its flash boot mode . by h old ing s d lo w and toggling the r eset but ton , the aducm360 enter s boot mode instead of normal user m o de . in b o ot m ode, the internal flash can be re programmed th rough the uart interface. ? the j1 connector, an 8 - pin dual - in - line connector , connects to the analog devices j - link ob emulator that is provided with the cn0300 support hardware . t his allows programming and debugging of this application board. see figure 3 . both t he thermocouple and the rtd generate very small signals ; therefore , a programmable gain amplifier ( 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 ? 200 c 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 used 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 tempera ture variation of 0.385 ? / c. note that the reference resistor, r ref , must be a precision 5.6 k ? ( 0.1%). 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 tuto ria l m t - 031, grounding data converters and solving the mystery of "agnd" and "dgnd," tuto ria l m t - 101 , decoupling techniques , and the aducm360 tcz evaluation board layout). t he pcb used for evaluating this circuit is shown in figure 2 . figure 2 . eval - cn0300 - eb1z board used for this circuit 10955-002
circuit note cn- 0300 rev. a | page 3 of 7 figure 3 . eval - cn0300 - eb1z board c onnected to t h e analog devices j - link ob e mulator the analog devices j - link ob emulator (usb - swd/uart - emuz) supports the following: ? when plugged into a pc usb port, it can also be used to connect to a com port (virtual serial port) on the pc. this is required for running the calibration routines. ? provides sw (serial wire) debugging and programming for the aducm360 . ? this usb port can be used to program the part using the uart - based downloader. code description ? figure 4 shows a top view of the emulator board. j2 connector plugs into the e va l - cn0300 - eb1z board . the j2 connector pinout is shown in figure 5 figure 4 . analog devices j - link ob e mulator top view figure 5 . j2 connector for downloading and debugging, lk1, lk2, lk4, and lk6 must be inserted. lk3 and lk5 are required to c ommunicate via uart. required software for the j - link ob is included in the software installation. note that the j - link ob emulator replaces the j - link lite and related interface boards previously shipped with the aducm360 development system. for more details, see ug - 457 , aducm360 develo pment systems getting started tutorial . the source code used to test the circuit can be downloaded as a zip file from the aducm360 product page. the source code uses the function libraries provided with the example code. figure 6 shows the list of source files used in the project when viewed with the keil vis ion4 tools. figure 6 . source files viewed in vision4 calibration s ection of c ode the compiler #define values, calibrateadc1 and calibratedac , can be adjusted to enable or disable calibration routines for the adc and the dac. to calibrate either the adc or the dac, the analog devices j - l ink ob emulator (usb - s wd/ua rt - emuz ) must be connected to j1 and to the usb port on a pc. a com port viewer program , s u ch as hyperterminal , can be used to view the calibration menus and step thro ugh the calibration routines. when calibrating the adc, the source code prompts t he user to connect zero - scale and full - scale voltages to ain2 and ain3 . n ote that ain2 is the positive input. on completion of the ca libr ation routine, the new calibration va lues for the adc1intgn and adc1of registers are stored to the internal flash. 10955-003 10955-103 10955-104 10955-004
circuit note cn- 0300 rev. a | page 4 of 7 when calibrating the dac , connect the vloop + output through an accurate current meter . the first part of the dac calibration routine calibrates the dac to set a 4 ma output , and t he second part of the dac calibration routine calibrates the dac to set a 20 ma output. the dac code used to set a 4 ma and 20 ma output is stored to flash. the voltage measured at ain9 for the final 4 ma and 20 ma settings is a lso recorded and saved to flash. because the voltage at ain9 is linearly related to the current flowing across rloop, these values are used to calculate the adjustment factor for the dac . this closed - loop scheme means any linearity errors on the dac and tr ansistor based circuit are fine - tuned out using the on - chip 24 - bit - adc. the uart is configured for a baud rate of 9600, 8 data bits, no parity, and no flow control. if the circuit is connected directly to a pc, a communication port viewing application , such as hyperterminal , can be used to view the results sent by the program to the uart , as shown in figure 7 . to enter the characters required by the calibration routines, type the required character in the viewing terminal and this character will be received by the aducm360 uart po rt. figure 7 . output of hyperterminal w hen calibrating the dac temperature measurement section of c ode 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 - u p t ab le (see the ise, inc., its - 90 table for type t thermocouple) . these two vo lt ages 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 , converted 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 measurement. for the thermocouple , temperatures for a fixed number of voltage s are store d in an array . temperature values in between are calculated using a linea r interpolation between the adjacent points. figure 8 shows the error obtained when using adc1 on the aducm360 to measure 52 thermocouple voltages over the full thermocouple operating range . the overall worst - case error is less than 1c. figure 8 . error w hen using piecew ise linear approximation using 52 calibration points m easured by aducm360 / aducm361 the rtd temperature is calculated using lookup tables and is implemented 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 an d maximizing the performance of the rtd, refer to application note an - 0970 , rt d in t er f ac i ng and linearization using an aduc706x microcontroller . temperature - to - current output section of c ode onc e the final temperature has been measured, set the dac output voltage to the appropriate value that gives the required current across r loop . the input temperature range is expected to be ? 200 c to + 350 c . the code set s the output current to 4 ma for ? 200 c and 20 m a for + 350 c . the code implements a closed - loop scheme , as shown in figure 9 , where the feedback voltage on ain9 is measured by adc0 , and this value is used t o compensate the d ac o utput se tting. the finetunedac(void) function performs this correction. for best results, calibrate the dac before beginning performance testing of this circuit. 10955-005 ?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) temper a ture (c) 10955-006
circuit note cn- 0300 rev. a | page 5 of 7 figure 9 . closed - l oop co ntrol 4 ma to 20 ma dac output for debug purposes, the following strings are sent to the uart during normal operation ( s ee figure 10 ) . figure 10 . uart strings used for d ebug common variations 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 e rror, a thermistor c an be placed in contact with the actual 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 se nsor c an be used . f or example, the adt7410 c an connect to the aducm360 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 connecto r and this circuit is required , the adum3160 / adum4160 isolation devices must be added. circuit evaluation a nd test current output m easurements the dac and external - voltage - to - curre nt - convertor circuit performance tests were all completed together. a current meter was placed in series with the vloop+ connection , as shown in figure 1 . the meter used was a hp 34401a. the circuit performance when the initial calibration is perf ormed and when using the closed - loop control of the vdac output results in temperature values of 0.5c being reported by the d ac output circuit. non linearity errors from the dac and the e xternal transistor circuit are adjusted out thanks to the 24 - bit adc. because temperature is a slow changing input parameter, this closed scheme is ideal for this application . figure 11 shows the ide al d ac output in blue and the real dac output with no closed - l oop control (adc0 is not used to compensate the dac output). the error can be >10 c without closed - loop control. figure 11 . temperature in c vs. current out in ma ( bl ue = ideal value, open loop operation : dac output uncompensated) figure 12 shows the same information when the closed - loop control is used as is recommended. the error is tiny , less than 0.5 c from the ideal value . figure 12 . temperature in c vs. current out in ma ( blue = ideal v alue , closed - loop operation : dac output compensated by adc0 m easurement thermocouple measurement test the basic test setup is shown in figure 13 . t he thermocouple is connected to j 2 . 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 use d to measure the temperature of an ice bucket . then, it was used to measure the temperature of bo iling water . 100k? 100k? rloop 47? vloop+ vloop? ain9 ain8 (buffered v ref ) dac adc0 10955-007 npn bc548 10955-008 0 ?200 ?150 ?100 ?50 0 temperature (c) 50 100 150 200 250 350 5 10 15 current output (ma) 20 25 10955-009 ideal current actual current 0 5 10 15 20 25 ?200 ?100 0 100 200 300 400 actual current ideal current temperature (c) current output (ma) 10955-010
circuit note cn- 0300 rev. a | page 6 of 7 a wavetek 4808 multifunction calibrator was used t o fully evaluate the error , as shown in figure 13 . in this mode, the thermocouple was replaced with the calibrator as the voltage source . to evaluate the entire ra nge of a type t thermocouple, t he calibrator was used to set the equivalent th ermocouple voltage at 52 points between ? 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 ). figure 8 shows the test results. when doing performance checks and using the cn0300 circuit for normal operation, please ensure the j - l ink ob emulator is unplugged from the e va l - cn0300 - eb1z board only use the j - l ink ob when programming, calibrating and debugging the e va l - cn0300 - eb1z board. figure 13 . test setup used to calibrate and test the circuit o ver full thermocouple output voltage range r td measurement test to evaluate the rtd circuit and linearization source code, the rtd on the board was replace d 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 te st setup circuit for measuring the rtd is shown in figure 14 , and the error results for the rtd tests are shown in figure 1 5 . figure 14 . test setup for measuring rtd error figure 15 . error in c of rtd measurement using piecewise linearization code and adc0 measurements current measurement test s when operating normally, the entire circuit consumes 2.25 ma typically. when held in a reset state, the entire circuit consumes less than 600 a. when doing performance checks and using the cn0300 circuit for normal operation, please ensure the j - l ink ob emulator is unplugged from the e va l - cn0300 - eb1z board only use the j - l ink ob when programming, calibrating and debugging the e va l - cn0300 - eb1z board. for more details on the curre nt consumption figures for the aducm3 60 , see application n o te an - 1111 . ev al-cn0300-eb1z wavetek 4808 multifunction calibrator pc j2 ain7/vbias thermocouple junction see text usb cable 10955-0 1 1 1433-z decade resis t or aducm360 0.1f 0.01f 0.01f r ref 5.6k? 0.1% 10? 10? a vdd vref+ vref? iovdd a vdd iovdd 0.1f ain5/iexc ain0 ain1 10955-012 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 10955-013
circuit note cn- 0300 rev. a | page 7 of 7 learn more cn0 300 design support package: http://www.analog.com/cn0300 - designsupport adisim power design tool . kester, walt. 1999. sensor signal conditioning. analog devices. chapter 7, "temperature sensors." kester , walt . 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 . analog devices. mt - 023 tutorial, adc architectures iv: sigma - delta adc advanced concepts and applications. analog devices. mt - 031 tutorial, grounding da ta converters and solving 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 adp 1 720 data sheet revision history 5 /13 r e v. 0 to r e v. a changed usb - swd/uart and segger j - link lite b oard to j - l ink ob e mulator ......................................................... universal changes to circuit description section ........................................2 changes to figure 3 and calibration section of code section; added figure 4 a nd figure 5 , renumbered sequentially ............3 changes to figure 9 ........................................................................4 changes to thermocouple measurement test section and current measurement tests section .............................................6 change to data sheets and evaluation boards section ...............7 10 /1 2 revision 0: initial ve rsi o n (continue d from 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 des ign 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 an d reliable. however, circuits from the lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warrant y 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 right s of third parties that may result from their use. analog devices reserves the right to change any circuits from the lab circuits a t any time without notice but is under no obligation to do so. ? 2012 C 2013 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn10955 - 0 - 5/13(a)
|
