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.

2 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. 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).

3 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)

4 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: 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.

5 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.

6 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 .

7 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. 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 .

8 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.

9 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.

10 12/12/06 9:40 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)