Low-Voltage Current Loop Transmitter - TI.com

1 ADC101S021, adc121s021 ,LM94022,LMP8270,LM V951,LP2951 Low-Voltage Current Loop TransmitterLiterature Number: SNOA865 Feature Pressure Force Load Factory Automation Design PATH designer Tips, tricks, and techniques from the analog signal-path expertsThe 4 to 20 mA Current loop, which is used extensively in industrial and process control systems, creates challenges for maximizing theoperating loop length. In some cases, a very long loop is required andthe combination of limited loop-power supply voltage and excessive loopwire resistance prevents it use. This article discusses the use of low-voltageamplifiers to minimize the Transmitter s operating voltage requirements,which will maximize the operating loop length. Typically, the Current loop is powered from the receiver side while the Transmitter controls the Current flowing in the loop to indicate the value ofthe physical parameter being measured by the sensor.

2 Figure 1shows thebasic components and connection of a Current loop. The maximum distance between the Transmitter and receiver is dependenton the power supply voltage (VS), and the sum of the loop drops, which arethe minimum Transmitter voltage (VT), the voltage drops across the wireresistance (WR1and WR2), and receiver resistor (RR). In equation form:VS=VWR1+VT+VWR2+VRRLow- voltage Current Loop Transmitter By Walt Bacharowski, Applications ManagerNo. 108 NEXT ISSUE: Generating Precision Clocks for >1 GSPS Interleaved ADCsLoopPowerSupply+- Current LoopTransmitterSensorCurrent LoopReceiverVOUTRRIL=4 to 20 mAVSWR1WR2 VTFigure 1. Current Loop Components and ConnectionEQ18208 Signal_Path_108 12/12/06 9:40 AM Page 1 Solutions for Pressure Force Load Testing25 VAV = 141A1 = LMP20123452+V-V140K140K4352+ pF470 pF470 pF18031 ADC = ADC121S625+V84765 SCLKDOUTCSTo C1802KA1+-A1-+ADCDAC = DAC081S1012654 SYNCSCLKDINDAC+5V231 Bridge Sensor ApplicationLMP2012 Precision Op Amp Auto zero dual op amp Input offset voltage , VOS, (36 V MAX) minimizessignal amplification errors of original input TCVos of 15 nV/ C maintains a stable VOSover timeand temperature CMRR and PSRR greater than 120 dB ensuresaccuracy over various common mode voltages andacross its entire operating voltage range Gain bandwidth product and slew rate are best inclass at 3 MHz and 4 V/ s Also available.

3 LMP2011 (single) in SOIC-8 and SOT23-5 packaging LMP2014 (quad) in TSSOP-14 packagingADC121S625 12-Bit A/D Converter 12-bit analog-to-digital converter True differential inputs Guaranteed performance from 50 kSPS to 200 kSPS Reference voltage between 500 mV and Binary 2 s compliant SPI /QSPI /MICROWIRE /DSP compatibleDAC081S101 8-Bit D/A Converter Low power, 8-bit digital-to-analog converter LSB INL 3 sec settling time Rail-to-rail voltage output SPI /QSPI /MICROWIRE /DSP compatible For FREE samples, datasheets, and more information, 12/12/06 9:40 AM Page 2 Low-Voltage Current Loop TransmitterSIGNAL PATH designer03 Substituting the loop Current and loop resistancesinto EQ1:VS=ILWR1+VT+ILWR2+ILRRG iven the wire s resistance in X Ohms per foot, themaximum loop Current of 20 mA, the value of RRequal to 10 , and the equal lengths of wire, EQ2can be rearranged to calculate the maximum loopdistance in terms the loop parameters:EQ3 illustrates three ways to increase the maximum loop length: (1) increase the loop powersupply voltage , (2) increase the wire gage, whichwill reduce the wire s ohms per foot, or (3) reducethe minimum voltage required for the Current looptransmitter operation, which is the focus of the following section.

4 The use of low voltage amplifiers, such as theLMV951, and low drop out voltage regulators,such as the LP2951, can reduce the minimum volt-age required for the Current loop 2shows the schematic of a loop-powered 4 to 20 mA Transmitter , which will function with aminimum of , and a 4 to 20 mA receiver. In this example, a temperature sensor, such as theLM94022, provides a signal for the Transmitter . The components A1, Q1, and R1 through R5form a voltage -to- Current converter. The nonin-verting input of A1, pin 3, is the summing node forthree signals, the loop Current , offset Current , andsensor signal voltage . The resistor R2 is the currentshunt that measures the Current flowing in the loopand is fed back through R3. The total loop currentis the sum of the currents flowing in resistor R2and R3, IL=IR2+IR3. The amplifier, A1, forces thevoltages at its inputs, pins 3 and 4, to be equal byforcing more or less Current through R2.

5 The resultis that R2 and R3 have the same voltage acrossthem. The ratio of the currents in R2 and R3 is theinverse of the resistor ratio:This highlights that the Current in R3 is also part ofthe voltage -to- Current conversion and is not anerror Current . An error source that will affect theloop Current is the offset voltage of amplifier A1,which will add an error Current to the loop the minimum loop Current of 4 mA, the voltageV2 is very close to LoopTransmitterCurrent LoopReceiver4 to 20 mAWR1WR2A1162435+ 10 10K RRLoopPowerSupply+ VSC1C2C3C41 F1 F4 to 20 mATS = LM94022A1 = LMV951 VREG = LP29511,2 SCLKCSSDATATo P100nF1437,8 ADC = ADC081S021 ADC101S021 ADC121S021324656+ +5180 100nFADCA2 = LMP82701865432Q1V1V2V3IR3IR2 ILVINA2 VREF = + = (X /ft)2332 RRIIRR=( )Figure 2. Loop-Powered Transmitter SchematicEQ2EQ3EQ48208 Signal_Path_108 12/12/06 9:40 AM Page 34 Solutions for Factory Automation180 A2+-A3-+A4+-A1+ , A2, A3, and A4 = LMP7701 or one LMP7704 (quad op amp)34512347,81, +V34+ +V52-V1+V52-V1 VOUT312470pF465 SCLK/CSSDATATo P34+VADC = adc121s021 -V+5180 1 SCLK4/CS6 SDATA54470 pF311M341 Cold JunctionTemperatureTCJR6To PTCJR= LM94022 A1 = LMP7701 AmplifiedThermocoupleOutputADC = ADC081S021 ADC101S021 adc121s021 Type KThermocoupleCold JunctionReference2470 pF1+V+V25 CopperCopperVOUT1 MAv = 200 Full Scale ~ 500 C43180 3+-52A1GS0GS13 VDD1,27, Temperature Detector ApplicationThermocouple Temperature Detector ApplicationFor FREE samples, datasheets, and more information, 12/12/06 9.

6 40 AM Page 450 Product IDMax VOS Room Temp ( V)TCVOS( V/ C)Specified Supply voltage Range (V)PSRR (dB) CMRR(dB) Gain (dB) GBWP (MHz) voltage Noise(nV/ Hz)IBIASRoom Temp (pA) LMP2011/12 to LMP7701/02 to to to Op AmpsProduct IDInput voltage RangeTCVOS( V/ C)Fixed Gain (V/V)Supply voltage (V)CMRR (dB) PackagingLMP8275-2 to to to to to to Current Sense AmpsProduct IDTyp Is/Channel ( A) Total Specified Supply Range (V)Max VOS(mV) Max IBIASOverTemperatureTyp CMVR (V)GBW (MHz) Packaging to nA (typ)0 to , TSSOP-14 LMV7911150 to to , to to , to pA to to to Op AmpsADCs for Single-Channel ApplicationsProduct IDRes# ofInputsPin/FunctionCompatibleThroughput Rate (kSPS)Input TypeMax Power 5V/3V (mW)Supply (V)Max INL(LBS)Min SINAD(dB)PackagingADC121S101121500 to 1000 Single ended16 to , LLP-6 ADC121S051121200 to 500 Single to , LLP-6 ADC121S02112150 to 200 Single to , LLP-6 ADC101S101101500 to 1000 Single ended16 to to 500 Single to to 200 Single to to 1000 Single ended16 to to 500 Single to to 200 Single to *12150 to to *121500 to to.

7 *Differential input, 200 to 500 kSPS thruput rate forthcoming8208 Signal_Path_108 12/12/06 9:40 AM Page 56An offset voltage of 1 mV in A1 will cause an errorof about in IR3:Because the ratio of IR2to IR3is 1000 to 1, an errorof in IR3results in a error in theloop Current . The voltage supply requirements for the compo-nents in Transmitter must be evaluated in order todetermine the minimum operating voltagerequired by the Transmitter . For this example, afull-scale sensor input signal of is used andresults in a 10 mA per volt scale factor:The minimum voltage required for the transmit-ter (V3 V1) is the highest voltage requirementof the two paths from V3 to V1. Path one is fromV3 to Q1 and R2 to V1. At the maximum loopcurrent of 20 mA, the voltage drop across R2 (V2) and a collector emitter voltage of to stay out of saturation is a total of second path is V2 plus the output voltage ofthe voltage regulator and its dropout voltage .

8 Thefull-scale sensor input signal of requiresabout a output from the regulator and thedropout voltage of the voltage regulator is lessthen 50 mV. The path has a minimum voltagerequirement of ( + + ). Notethat the minimum operating voltage of theLMV951 is so the minimum transmittervoltage could be reduced to about byincreasing the scale factor to 18 mA per volt. Thisis supported by the voltage regulator, VR, whichcan be adjusted down to , and with a dropout voltage of 50 mV, the loop Transmitter canwork down to The Current loop transmitterfunctions by summing three signals: the loop Current (R3), the offset Current (R4), and the sensor (R5). The loop Current generates a voltage drop acrossresistor R2 such that V1 is negative with respect toV2 and then fed back through R3. V1=V2-R2(IL)The 4 to 20 mA Current loop uses the offset currentlevel of 4 mA to represent zero signal input.

9 This isused as an open loop fault condition since zero Current is a broken wire, Transmitter failure, oranother fault. The resistor R4 is connected to theoutput of the adjustable low drop-out voltage regulator to create the 4 mA offset Current . ResistorR4, at 402 k , sets approximately a 4 mA offsetcurrent when the output of the voltage regulator The variable resistor R6 is used to set theloop Current to 4 mA when the input signal is atzero volts. This adjustment compensates for error inthe voltage regulator s output and resistor tolerancein R4, R5, and R7. The offset can be calibrated to4 mA by measuring the voltage across RRandadjusting R6 until the voltage across RRis equal The value of resistor R4 can be calculatedfor other supply voltages by equating the voltagesat the amplifier s input pins and rearranging tosolve for R4:The resistor R5 is used to scale the signal inputvoltage to the 16 mA span of the loop Current , andin this example, it is assumed the input signal spanis The equation for calculating R5 can bedeveloped by equating the voltages at the amplifi-er s input pins and rearranging to solve for R5.

10 VINis the maximum signal input, for thisexample, and IR2is the change in output Current ,16mA:This equation also indicates that changing thevalue of R5 can change the full-scale input low resistance variable resistor could be used inLow- voltage Current Loop TransmitterSIGNAL = 3RR3 x VOUTR4 =R2 x IR2R3 x VINR5=R2 x IR2% VVEQ5EQ6EQ7EQ8EQ98208 Signal_Path_108 12/12/06 9:40 AM Page 607series with R5 to add a full-scale calibration asshown in the following schematic (Figure 3). In this example, a silicon temperature sensor isused as a signal source. The LM94022 is a low voltage , programmable gain temperature sensorthat can be used to measure temperature from 50 C to 150 C. The schematic in Figure 2showsthe LM94022 s gain select pins connected toground, or the lowest gain. With this gain, the sen-sor s output ranges from for a temperatureof 50 C to for a temperature of 150 C.