circuit note cn - 0267 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/cn0267 . devices connected /referenced aducm360 low power , precision analog microcontroller ad5421 16- bit , loop powered , 4 ma to 20 ma dac AD5700 low power hart modem complete 4 ma to 20 ma loop p owered field instrument with hart interface rev. a 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 tested and verified in a lab environment at room temperature. however, you are solely responsible for testing the circuit and determining its s uitability 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 ? 2012 C 2013 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation board cn0267 circuit evaluation board ( demo - AD5700d2z ) de sign and integration files schematics, layout files, bill of materials, code example circuit function and benefits the circuit shown in figure 1 is a com plete smart industrial , loop powered field instrument with 4 ma to 20 ma analog output and a highway addressable remote transducer (hart?) interface. hart is a digital 2 - way communication in which a 1 ma peak - to - peak frequency - shift - keyed (fsk) signal is m odulated on top of the standard 4 ma to 20 ma analog current signal. this allows features such as remote calibration, fault interrogation, and transmission of process variables, which are necessary in applications such as temperature and pressure control. this circuit has been compliance tested, verified , and registered by the hart communication foundation (hcf) . this successful registration provides circuit designers with a high level of confidence using one or all of the components in the circuit. the ci rcuit uses the aducm360 , an ultralow power, precision analog microcontroller, the ad5421 , a 16 - bit, 4 ma to 20 ma , loop powered digita l- to - analog converter ( dac ) , and the AD5700 , the industrys lowest power and smallest footprint hart - compliant ic modem . figure 1. 4 ma to 20 ma , loop powered field instrum ent with hart i nterface ( simplified schematic: all connections and decoupling not shown ) 50? dac loop? c in hart_out adc_ip dvdd com ad 542 1 aducm 36 0 reg in primary sensor secondary sensor ad 570 0 adc v loop temperature sensor ref iovdd xtal1 cortex m3 + sram flash + dma uart spi i 2 c clock reset watchdog i exc watchdog timer reg out 20m? 1m? 4700pf 1.6? 10f 10f 470? 1f l* l* l* tvs 40v low leakage 4.7v low leakage 10? 10f 10f 1.2m? 1.2m? 300pf 150pf 150k? 0.068f 0.22f 1f sclk sdo sdin sync ain4 ain0 ain3 ain2 0.1f 0.01f 0.01f 0.1f ain7 1k? 1k? 1k? 1k? 1k? 0.1f 1k? 1k? 0.1f 0.01f 0.01f ain1 gnd_sw avdd avdd adc1 rxd txd cd rts sclk0 miso0 mosi0 cs0 sin p0.5 sout ldac xtal2 v cc 3.8664mhz 1f reg_cap agnd dgnd exposed pad agnd dgnd agnd 0.47f dvdd_ reg avdd_ reg vref? vref+ pt100 vref 0.47f 0.1f 10f 10f dv dd refout1 refout2 p0.4 adc0 rref 5.62k? 10ppm spi interface uart interface loop+ loop? notes 1. l* = ferrite bead, 0.3? @ dc, 1k? @ 100mhz. 2. the aducm360 exposed pad is connected to dgnd. vref dvdd voltage regulator 10551-001
cn-0267 circuit note rev. a | page 2 of 8 circuit description analog front - end interface the aducm360 analog front - end incorporates dual, high p erformance 24 - bit sigma - delta ( - ) analog - to - digital converters (adcs) . it also integrates programmable gain instrumentation amplifiers , a precision band - gap reference, programmable current sources, a flexible multiplexer, and many other features. it allo ws a direct interface to multiple analog sensors, such as pressure sensor bridges, resistive temperature sensors, thermocouples, and many other types of sensors used in the industry. the circuit in figure 1 shows a n example connection for a primary b ridge type sensor and a secondary resistive temperature sensor ; however the aducm360 flexible front - end allows many other configurations to accommodate any t ype of precision analog sensor application. primary sensor input the aducm360 on - chip adc0 measures th e field instrument primary sensor, shown as a bridge transducer in figure 1 . the sensor connects to the analog input pins , ain0 and ain1 , via an rc filter network for improved system electromagnetic immunity . the common - mode filter bandwidth is approximately 16 khz, and the differential - mode bandwidth i s 800 hz. t he aducm360 v ref+ and v ref ? voltage reference inputs sens e the bridge excitation voltage and enabl e the circuit to work in a ratiometric mode, making the measurement independent of the exact value of the sensor power supply voltage. the on - chip ground switch can dynamically disconnect the bridge excitation and save power when required by the application. secondary sensor input the circuit uses a platinum (pt) 100 resistive tempera ture device (rtd) as a secondary sensor. the rtd can sense the temperature of the primary sensor and thus allow for temperature compensation of the primary sensor if required. the aducm360 pro grammable current source supplies the rtd via the ain4 pin . the adc1 on the aducm360 measures the voltage across the rtd using the ain3 and ain2 pins configured as a differential input. the exa ct value of the current flowing through the rtd is sensed by a precision resistor ( rref ) and is measured by the adc1 using the ain7 pin . the adc1 uses the on - chip , band - gap voltage reference. digital data processing, algorithm , and communications all the field instrument digital functions are provided by the aducm360 32- bit arm cortex? m3 risc processor, with integrated 128 k bytes of nonvolatile flash/ee memory, 8 k bytes of sram , and a n 11- channel direct memory access (dma ) controller that support s wired ( 2 spi, uart, i2c) communication peripherals. the demo n stration s oftware performs the initialization and configuration, process es data from the analog inputs, controls the analog outpu t, and perform s the hart communication. analog output the ad5421 integrates a low power precision 16 - bit dac with a 4 ma to 20 ma , loop powered output driver and provides all functions required for the field in strument analog output. the ad5421 interface s with the aducm360 controller via the spi interface. the ad5421 also includes a range of diagnostic functions related to the 4 ma to 20 ma loop . the auxiliary adc can measure the voltage across the instruments loop terminals via the 20 m /1 m resistive divider connected to the v loop pin. the adc can also measure the chip temperature via the integrated sensor . the aducm360 controller can configure and read all the diagnostics of the ad5421 , but the ad5421 can also operate autonomously. as an example, if the communication between the controller and the ad5421 fails, the ad5421 automatically set s its analog output to a 3. 2 ma alarm current after a defined period . this alarm current indicate s to the host that the field instrument failed to operate. t he software controls any change of the output current from one value to another to prevent disturbance of the hart communication. ( see the analog rate of change section ). hart communication the AD5700 integrates a complete hart fsk modem. the modem is connected to the aducm360 controller via a standard uart interface, complemented by request to send (rts ) and carrier detect (cd ) signals. the hart output is scaled to the required amplitude by the 0.068 f/ 0.22 f capacitive divider and coupled to the ad5421 c in pin, where it is combined with the dac output to drive and modulate th e output current. the hart input is coupled from loop+ via a simple passive rc filter to the AD5700 adc_ip pin. the rc filter works as the first stage , band - pass filter for the hart demodulator and also improve s the system electromagnetic immunity, which is important for robust applications working in harsh industrial environment s. the AD5700 low power oscillator generates the clock for the hart modem with a 3.8664 m hz external crystal connected directly to the xtal1 and xtal2 pins.
circuit note cn-0267 rev. a | page 3 of 8 output protection a transient voltage suppressor (tvs) protects the 4 ma to 20 ma hart interface from overvoltage . its voltage rating should prevent exceeding the ad5421 absolute maximum voltage of 60 v on the reg in pin. note that the tvs leakage current can affect the current output accuracy ; therefore , pay attention to the leakage current at a given loop voltage and temperature range when sel ecting this component. an external depletion - mode fet can be used with the ad5421 to increase the loop voltage maximum the circuit is protected against reversed polarity by a pair of diodes in series with loo p output. the ferrite beads in series with the loop together with the 4700 pf capacitor improve the system emc performance. do not use a higher capacitance across the loop terminals because of the hart network specifications . the 4.7 v, low leakage , zene r diode protects the ad5421 on - chip , 50 ? loop sense resistor in the event of an accidental external voltage between the ad5421 com pin and loop ? pin (for e xample , when programming the aducm360 or de bugging the circuit ). power supplies and power management the complete field instrument circuitry , including the sensor drive current, must operate on the limited amount of power available from the 4 ma to 20 ma loop . this is a common challenge in any loop powered field instrument design . the circuit in figure 1 provide s an example of delivering both a l ow power and high performance solution. a ll three integrated circuits used in the application are designed for low power, and t he circuit leverages their integrated features to deliver a flexible power management structure and an optimum loop - powered solution . the ad5421 is powered by the 4 ma to 20 ma loop voltage and pro vides a regulated low voltage for the rest of the circuit. the ad5421 reg out voltage is pin programmable from 1.8 v to 12 v depending on circuit requirements. the circuit in figure 1 uses the 3.3 v supply voltage option as an example for the input sensors used. however, the aducm360 and the AD5700 have a wider power supply voltage range ; theref ore , a different power supply voltage can be used to suit the application. the reg out rc filter (10 f/ 10 / 10 f) helps to prevent any interference coming from the loop affecting the sensor analog front - end. it also prevents any interference generated by the circuit, specifically by the controller and the digital circuitry, from coupling back to the loop, which is important for a reliable hart communication. the AD5700 hart modem is supplied through an additional rc filter (470 /1 f). this filter is very important in the loop powered application because it pr events current noise from the AD5700 from coupling to the 4 ma to 20 ma loop output, which would otherwise affect the hart communication . the 4 ma to 20 ma loop noise performance is specifically addressed by th e hart in - band , noise during silence test. the AD5700 mo dem uses the external crystal with 8.2 pf capacitors to ground on the xtal1 and xtal2 pins, which is the option using the least possible power. the aducm360 has very flexible internal power management, with many options for powering and clocking all the internal blocks and, when utilized by the software , allows an optimal balance between the req uired function, performance , and power for the specific instrument application. re fer to the aducm360 product page and the an - 1111 application note . the analog front - end avdd is supplied from another filter ( 10 f/ferrite bead/1.6 /10 f) to minimize power supply noise for better performance with respect to low voltage sensor signals . the gnd_sw ground switch pin of the aducm360 controls the excitation/power supply for the primary sensor. the switch is off as a default at the instrument power up. this default allows the system to be fully configured , including appropriate power modes, before turning on the sensor, a nd thus minimiz es any possible power - up spike s on the 4 ma to 20 ma loop output. similarly, the secondary sensor is supplied from the programmable current source of the aducm360 , and therefore , its power is f ully controlled by the software . aducm360 software a basic code example that demonstrates the functionality and performance of the circuit can be found in the cn- 0267 design support package . the code example includes a basic hart slave command response to demonstrate the hardware function and capability. however, the code example does not include the protocol layers of the hart communication. common variations t he aducm360 has a high performance and very flexible analog front - end, with 12 analog input pins and extra pins for voltage reference and ground switch. it allows direct interface to multiple analog sensors o f varying types, such as any resistive bridge sensors , resistive temperature sensors, or thermocouples. therefore, do not limit the field instrument solution to temperature - compensated pressure measurement only because it can be used for almost any sensor field instrument. the aducm361 can be used as an alternative to the aducm360 in applications that need only one - adc in the analog front - end. aside from the second adc, the aducm361 contains all the features of the aducm360 .
cn-0267 circuit note rev. a | page 4 of 8 the aducm361 on - chip dac with an external t ransistor can be used to control the 4 ma to 20 ma loop, refer to cn - 0300 for details. the ad5421 can be connected via the protection directly to the loop. alternat ively, a depletion mode n - channel mosfet can be connected between the ad5421 and the loop power supply, as shown in figure 2 . the use of the additional moseft in this configuration kee ps the voltage drop across the ad5421 at approximately 12 v, lowers the power dissipated in the ad5421 package , and therefore improves the 4 ma to 20 ma analog outp ut accuracy. it also increases the maximum voltage allowed in the loop to the level of the mosfet rating. the additional mosfet has no effect on the hart c ommunication. figure 2. moseft c onnected to the ad5421 loop power supply the AD5700 is used with a 3.8664 mhz crystal in this circuit , which is the configuration achieving the lowest power consumption. alternatively, the AD5700 -1 , with an integrated 0.5 % precision internal oscillator , can be used. the internal oscillator increase s the modem power supply current by 225 a maximum, compared to the crystal oscillator, but because no external cr ystal is needed , this option provides both cost savings and reduced board area requirements . for the applications that are not loop powered, the ad5410 , ad5420 , ad5422 , or ad5755 are good choices for the 4 ma to 20 ma dac. circuit evaluation a nd test circuit hardware the circuit shown in figure 1 is built on the demo - AD5700d2z printed circuit board shown in figure 3 . figure 3 . demo - AD5700d2z printed circuit board (pressure sensor not included) t he demo - AD5700d2z circuit board include s some additional feat ures for easy system evaluation . the 0.1 inch - pitch connector fo otprints allow optional primary and secondary sensor connection s . there are test points for hart rts and cd that may be needed for hart compliance tests . a connector on the edge of the demo - AD5700d2z m akes the aducm360 single wire and uart download/ debug signals accessible allowing easy software development, code download , and in - circuit debugging and emulation. the connector , with a small header extender included with the demo - AD5700d2z board , is compatible with all analog devices, inc., cortex - m3 based development tools, such as the e va l - a ducm360q spz e valuation kit (the evaluation kit is not included with the demo - AD5700d2z board). these features are not shown in the simplified diagram in figure 1 ; howe ver, they can be seen in the complete circuit schematic in the cn - 0267 design support package . the design support package also includes a full field instrument c - code example, which enables comple te verification and evaluation of all hardware blocks and features of the circuit, and a limited verification of the hart interface functionality. for detailed information about hart interface specifications and resources, contact the hart communication fo undation. hart compliance the demo - AD5700d2z has been verified to be compliant with hart fsk physical layer specification (hcf_spec - 054, revision 8.1), using methods and equipment specified in the hart physical layer test specification (hcf_test -2, revision 2.2). the board was submitted to the hart communication foundation and was successfully registered. the registered circuit can be found on the hart communication foundation (hfc) web site in the produ ct catalog as demo - AD5700d2z . th e results of two of the tests involved the output noise during silence and the analog rate of change . 200 k? dn254 0 b sp 12 9 loop? com ad 542 1 reg in drive 20m? 1m? 4700pf l l tvs 40v low leakage 4.7v low leakage to hart input filter loop+ loop? v loop 10551-002 10551-003
circuit note cn-0267 rev. a | page 5 of 8 output noise during silence test when a ha rt device is not transmit ting ( s ilence), do not couple noise onto the network. excessive noise may interfere with reception of hart signals by the device itself or other devices on the network. the voltage noise measured across a 500 load in the loop must contain no more than 2. 2 mv rms of combined broadband and correlated noise in the hart extended frequency band . in addition, t he noise should not exceed 138 mv rms outside the hart extended frequency band . this noise was measured by a true rms meter across the 500 load . this noise was measured directly for the out - of - band noise and measured through the hcf_tool - 31 filter for the in - band noise. an oscilloscope was also used to examine the noise waveform . the noise was measured at the worse condition, which was 4 ma output curre nt. the captured noise waveform is shown in figure 4 , and the results are summarized in table 1 . figure 4. output noise during silence waveform table 1 . output noise during silence output noise easured mv reuired mv outside extended frequency range 4.13 <138 inside extended frequency range 1.03 <2.2 analog rate of change test this specification ensures that when a device regulates the analog outpu t current, the maximum rate of change of analog current does not interfere with hart communications. step changes in current disrupt hart signaling. the worst - case change in the analog output current must not produce a disturbance higher than 15 mv peak , m easured across a 500 load in the hart extended frequency band. the ad5421 dac and output driver are relatively fast. therefore , to meet the required system specification, the output current change is control led by combining hardware slew- rate limit ing implemented at the ad5421 and a digital filter implemented in the aducm360 software. the hardware slew- rate limit is set by the capacitance connected to the ad5421 c in pin. when a large step change is required in the analog output current value, t he aducm360 software splits the output current change sent to the ad5421 dac in to a number of smaller subsequent steps. th is test was performed using an oscilloscope coupled to the 500 load through the hcf_tool - 31 filter. the result is sho wn in figure 5 . waveform ch1 shows the periodic steps between 4 ma and 20 ma, sensed directly across the 500 load. waveform ch2 is the signal captured on the hcf_tool - 31 filter output, amplified 10, within the 1 50 mv p eak limits. figure 5. analog r ate of change waveform circuit power consumption t wo methods were used to evaluate the circuit power consumption performance. in the first method, the current from the ad5421 integrated voltage regulator output was measured . considering t he minimum analog output current of 4 ma and hart output ac modulation of 0. 5 ma p eak , the maximum current consumed by the circuit in normal mode operation must be less than 3.5 ma. the ad5421 requires a 0.3 ma maximum for its own operation, which leaves approximately 3.2 ma maximum current for the ad5421 reg out output. for ease of in - circuit measurement, the demo - AD5700d2z has test points (t5, t6) on each side of the 10 resistor in the reg out output filter , as shown in figure 6 . this setup allows the voltage drop across the resistor to be measured , and the current to be calculated without interrupting the supply current or disturbing the circuit . figure 6. measuring the ad5421 reg out current using test points ch1 20.0mv b w m 100ms ch1 ?8.00mv <10hz 1 ch1 p-p 44.8mv ch1 cyc rms 4.64mv? ch1 none ch1 none ch2 off none measure 10551-004 ch1 5.00v ch2 50m v b w ch1 4ma to 20ma across 500? ch2 output of filter 10 limit = 150mv m 25.0ms ch1 6.80v ch1 p-p 8.60v ch1 freq 8.123hz? ch2 p-p 254mv ch2 max 134mv ch2 min ?120mv measure 10551-005 1 2 voltmeter + t6 t5 10? 10f 10f reg out rest of circuit power supply ad5421 voltage regulator 10551-006
cn-0267 circuit note rev. a | page 6 of 8 the results are shown in table 2 and were measured at the followi ng conditions: ? reg out = 3.3 v ? aducm360 m3 core clock = 2 mhz ? b oth adc s converting at 50 samples per second ? adc0 has both buffers on and g ain = 8 ? adc1 has both buffers on and g ain = 16 ? rtd excitation current = 200 a ? spi communicating to ad5421 with serial clock = 100 khz ? hart communicating the circuit with all relevant analog and digital blocks, including the input sensor, consumes power supply current within the budget allowed at the minimum 4 ma loop current. table 2 . power supply current from ad5421 , reg out = 3 .3v input sensor voltage t5 to t6 max imum (mv) current reg out max imum (ma) none 2 4.4 2.44 24pcdfa6d (5 k, 0.66 ma at 3.3 v) 31.0 3.10 in t he second method f or assessing the circuit power consumption, the circuit was verified to function as expected with the analog output current set to the minimum of 4 ma while performing hart comm unication. the result showed that the circuit delivered the 4 ma current and showed no distortion of the hart output signal. primary sensor input performance the aducm360 integrates most of the analog front -e nd on chip ; therefore , the performance of the analog input is primarily determined by the specifications of the aducm360 . the level of noise is the main factor that c an be influenced by the interaction of th e analog front - end with the rest of the circuitry on the board. thus, tests were carried out to focus on the noise and related resolution performance of the system. the demonstration was configured to transmit data from the primary analog input, expressed as pressure in kpa, over the hart communication. one hundred samples were captured, and a basic data analysis to quantify the performance was completed. two of the tests involved the following : ? the first test was performed with a standard pressure sensor (honeywell 24pcdfa6d) soldered directly on the board . ? a second test was performed with the primary input signal generated by a set of fixed and variable resistors , as shown in figure 7 . figure 7. primary input signal generated by a s et of resistors aducm 36 0 ain0 n ? n ? n ? n ? n ? 0.1f 0.01f 0.01f ain1 avdd vref? gnd_sw vref+ adc0 10551-007
circuit note cn-0267 rev. a | page 7 of 8 the performance summary can be seen in table 3 , and the signal plots are shown in figure 8 and figure 9 . tabl e 3 . primary sensor input noise and resolution parameter pressure sensor resistive network full s cale 2 07 kpa 246 kpa noise rms 1.3 pa 0.68 pa peak -to - peak noise 6.8 pa 3.6 pa resolution effective ( rms ) 17.2 bit 18.5 bit nois e- free resolution ( p-p) 14.9 bit 16.1 bit figure 8. pressure sensor input signal plot figure 9. resistive network as primary input signal plot secondary sensor input performance similar to th e primary sensor, the performance of the secondary sensor input is mainly determined by the analog front - end of the aducm360 with the exception of noise performance. t he analog input was configured to transmi t temperature in degrees celsius (c) to a master over the hart communication path . a nalysis was performed on two tests of 100 samples to quantify the performance. the first test was performed using the p latinum 100 sensor on the board , and t he second test was performed with the sensor replaced on the board by a standard (fixed) 100 1% resistor. the performance summary is shown in table 4 , and the signal plots are shown in figure 10 and figure 11. table 4 . secondary sensor input noise performance parameter pressure sensor resistive network noise rms 0.037c 0.033 c noise p eak to peak 0.19c 0.16c figure 10 . rtd (p latinum 100 ) sensor input signal plot figure 11 . fixed 100 1% resistor as the secondary input signal plot 20 25 30 35 40 0 10 20 30 40 50 60 70 80 90 100 pressure (pa) sample 10551-008 ?10 ?5 0 5 10 0 10 20 30 40 50 60 70 80 90 100 pressure (pa) sample 10551-009 23.0 23.5 24.0 24.5 25.0 0 10 20 30 40 50 60 70 80 90 100 tempera ture (c) sample 10551-010 ?1.0 ?0.5 0.0 0.5 1.0 0 10 20 30 40 50 60 70 80 90 100 tempera ture (c) sample 10551-0 11
cn-0267 circuit note rev. a | page 8 of 8 learn more cn - 0267 design support package: http://www.analog.com/cn0267-designsupport cn - 0270, complete 4 ma to 20 ma hart solution cn - 0278, complete 4 ma to 20 ma hart solution with additional voltage output capability cn - 0300 , complete closed- loop precision analog microcontroller thermocouple measurement system with 4 ma to 20 ma output an - 1111, options for minimizing power consumption when using the aducm360/aducm361 hart? communication foundation data sheets and evaluation boards aducm360 data sheet and evaluation board ad5421 data sheet and evaluation boards AD5700 data sheet and evaluation board revision history 2 /1 3r ev. 0 to rev. a changes to circuit hardware section and figure 3 capt ion ...... 4 12 /12 revision 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 an alog devices or its licensors. while you may use the circu its from the lab circuits in the design of your product, no other license is granted by implication or otherwise under any pa tents or other intellectual property by application or use of the circuits from the lab circuits. information furnished by analog d evices is believed to be accurate and 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 warranty of merchantability, noninfri ngement 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. ? 2012 C 2013 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn10551 -0- 2/13(a)
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