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Low-Voltage Current Loop Transmitter

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Low-Voltage Current Loop Transmitter ADC101S021,ADC121S021,LM94022,LMP8270, LMV951,LP2951 Low-Voltage Current Loop Transmitter Literature Number: SNOA865 8208Signal_Path_108 12/12/06 9:40 AM Page 1 SIGNAL PATH designer ® Tips, tricks, and techniques from the analog signal-path experts No. 108 Low-Voltage Current Loop Transmitter Feature Article....1-7 — By Walt Bacharowski, Applications Manager Pressure Force Load Testing............2 Current Loop Receiver Factory Current Loop + Loop Automation Transmitter Power VS Solutions..............4-5 W R1 - Supply I =4 to 20 mA Design Tools............8 Sensor VT L RR WR2 VOUT Figure 1. Current Loop Components and Connection he 4 to 20 mA current loop, which is used extensively in industrial and process control systems, creates challenges for maximizing the Toperating loop length. In some cases, a very long loop is required and the combination of limited loop-power supply voltage and excessive loop wire resistance prevents it use. This article discusses the use of low-voltage amplifiers 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 of the physical parameter being measured by the sensor. Figure 1 shows the basic components and connection of a current loop. The maximum distance between the transmitter and receiver is dependent on the power supply voltage (VS), and the sum of the loop drops, which are the minimum transmitter voltage (VT), the voltage drops across the wire resistance (WR1 and WR2), and receiver resistor (RR). In equation form: EQ1 VS = VWR1 + VT + VWR2 + VRR NEXT ISSUE: Generating Precision Clocks for >1 GSPS Interleaved ADCs 8208Signal_Path_108 12/12/06 9:40 AM Page 2 Solutions for Pressure Force Load Testing Bridge Sensor Application 3 +5V 1 6 SYNC DAC 5 SCLK 4 DIN 5V +V 2 3 5 180 + 1 A1 4 - 470 pF 2 0.2 pF +V -V 140K 2 1 8 7 SCLK 6D ADC OUT To µC A = 141 5 V 2K 3 470 pF 4 CS +V 0.2 pF ADC = ADC121S625 A1 = LMP2012 4 - 5 1 140K DAC = DAC081S101 A1 3 + 2 180 -V LMP2012 Precision Op Amp ADC121S625 12-Bit A/D Converter • Auto zero dual op amp • 12-bit analog-to-digital converter • Input offset voltage, VOS, (36 µV MAX) minimizes • True differential inputs signal amplification errors of original input • Guaranteed performance from 50 kSPS to 200 kSPS • TCVos of 15 nV/°C maintains a stable VOS over time • Reference voltage between 500 mV and 2.5V and temperature • Binary 2’s compliant • CMRR and PSRR greater than 120 dB ensures • SPI™/QSPI™/MICROWIRE™/DSP compatible accuracy over various common mode voltages and across its entire operating voltage range DAC081S101 8-Bit D/A Converter • Gain bandwidth product and slew rate are best in class at 3 MHz and 4 V/µs • Low power, 8-bit digital-to-analog converter • Also available: • ±0.75 LSB INL • LMP2011 (single) in SOIC-8 and SOT23-5 packaging • 3 µsec settling time • LMP2014 (quad) in TSSOP-14 packaging • Rail-to-rail voltage output • SPI™/QSPI™/MICROWIRE™/DSP compatible For FREE samples, datasheets, and more information, visit www.national.com/adc amplifiers.national.com 2 8208Signal_Path_108 12/12/06 9:40 AM Page 3 SIGNAL PATH designer Low-Voltage Current Loop Transmitter Substituting the loop current and loop resistances In this example, a temperature sensor, such as the into EQ1: LM94022, provides a signal for the transmitter. EQ2 VS = ILWR1 + VT + ILWR2 + ILRR The components A1, Q1, and R1 through R5 form a voltage-to-current converter. The nonin- Given the wire’s resistance in X Ohms per foot, the verting input of A1, pin 3, is the summing node for maximum loop current of 20 mA, the value of R R three signals, the loop current, offset current, and equal to 10Ω, and the equal lengths of wire, EQ2 sensor signal voltage. The resistor R2 is the current can be rearranged to calculate the maximum loop shunt that measures the current flowing in the loop distance in terms the loop parameters: and is fed back through R3. The total loop current is the sum of the currents flowing in resistor R2 VS − VT − 0.2 EQ3 ft = and R3, IL=IR2+IR3. The amplifier, A1, forces the 0.04 (X Ω/ft) voltages at its inputs, pins 3 and 4, to be equal by forcing more or less current through R2. The result EQ3 illustrates three ways to increase the is that R2 and R3 have the same voltage across maximum loop length: (1) increase the loop power them. The ratio of the currents in R2 and R3 is the supply voltage, (2) increase the wire gage, which inverse of the resistor ratio: will reduce the wire’s ohms per foot, or (3) reduce I R the minimum voltage required for the current loop EQ4 ( R 2 = 3 ) transmitter operation, which is the focus of the IR3 R 2 following section. The use of low voltage amplifiers, such as the This highlights that the current in R3 is also part of LMV951, and low drop out voltage regulators, the voltage-to-current conversion and is not an such as the LP2951, can reduce the minimum volt- error current. An error source that will affect the age required for the current loop transmitter. loop current is the offset voltage of amplifier A1, Figure 2 shows the schematic of a loop-powered which will add an error current to the loop current. 4 to 20 mA transmitter, which will function with a At the minimum loop current of 4 mA, the voltage minimum of 1.9V, and a 4 to 20 mA receiver. V2 is very close to 0.040V. 1 V 8 OUT V C3 IN +5 R5 V V3 10K 10nF REG C4 3 + Loop 4 4.7 µF ADJ SD C2 7 6 R6 1 µF VREF VS Power 3 20K GND – 4 Supply 1,2 7,8 100nF R7 4 R4 25.5K VIN 402K VDD W R1 +5 4.096V 3 3 VOUT 6 1 GS0 + 1 4 T 1 R 1 6 SCLK S R5 A1 Q1 R 5 4 10 + 180 GS1 – 5 R1 C1 5 3 6 CS To 100K 2N3904 A2 GND 2 4.7K ADC 1 µF 8 5 SDATA µP 2 W R2 – 3 4 V2 2 2 470pF I I R2 R3 R3 R2 100nF T = LM94022 10 S 10K I L V1 A1 = LMV951 ADC = ADC081S021 V = LP2951 4 to 20 mA 4 to 20 mA REG A2 = LMP8270 Current Loop Current Loop ADC101S021 Transmitter Receiver VREF = LM4140ACM-4.1 ADC121S021 Figure 2. Loop-Powered Transmitter Schematic signalpath.national.com/designer 03 8208Signal_Path_108 12/12/06 9:40 AM Page 4 Solutions for Factory Automation Resistance Temperature Detector Application +V 43 3 6 5 + LM4140A-2.500 1 W1 A1 4 1,2 7,8 2 - +V R2 R1 W2 3 5 10K 10K -V + 1 A2 RTD 4 2 - R5 -V 10K W3 +V +V 1 W4 3 5 180Ω 3 4 SCLK R7 + 1 3.205K 6 /CS To μP R8 A4 ADC 2 5 SDATA 2.5K 4 - V R6 -V OUT 470pF 2 +V 4 10K - 5 1 A3 A1, A2, A3, and A4 = LMP7701 3 ADC = ADC121S021 + 2 R4 R3 or one LMP7704 (quad op amp) -V 10K 10K Thermocouple Temperature Detector Application 180Ω 3 4 SCLK +V ADC 6 /CS 5 SDATA Cold Junction Av = 200 4 1 2 Temperature Full Scale ~ 500°C 470 pF VDD 1M 1 3 VOUT +5 GS0 LM4140ACM-4.1 To 5 4 GS1 5K 4.096V µP +V 3 6 100nF TCJR 2 1,2 7,8 1 + 5 4 SCLK Copper 3 180Ω 3 Type K A1 1 ADC 6 /CS Thermocouple 5 SDATA - Amplified 2 Thermocouple 2 Copper 5K 4 470 pF Output Cold Junction 1M Reference TCJR= LM94022 ADC = ADC081S021 A1 = LMP7701 ADC101S021 ADC121S021 For FREE samples, datasheets, and more information, visit www.national.com/adc www.amplifiers.national.com 4 8208Signal_Path_108 12/12/06 9:40 AM Page 5 Precision Op Amps IBIAS Max VOS TCVOS Specified Supply PSRR CMRR Gain GBWP Voltage Noise Room Product ID Room Temp (µV) (µV/°C) Voltage Range (V) (dB) (dB) (dB) (MHz) (nV/ Hz) Temp (pA) LMP2011/12/14 25 0.015 2.7 to 5.25 120 130 130 3 35 -3 LMP7701/02/04 200 1 2.7 to 12 100 130 130 2.5 9 0.2 LMP7711/12 150 -1 1.8 to 5.5 100 100 110 17 5.8 0.1 LMP7715/16 150 -1 1.8 to 5.5 100 100 110 17 5.8 0.1 Precision Current Sense Amps Product ID Input Voltage Range TCVOS (µV/°C) Fixed Gain (V/V) Supply Voltage (V) CMRR (dB) Packaging LMP8275 -2 to 16 30 20 4.75 to 5.5 80 SOIC-8 LMP8276 -2 to 16 30 20 4.75 to 5.5 80 SOIC-8 LMP8277 -2 to 16 30 14 4.75 to 5.5 80 SOIC-8 Low-Voltage Op Amps Typ Is/ Total Specified Max VOS Max IBIAS Over Typ GBW Product ID Channel (µA) Supply Range (V) (mV) Temperature CMVR (V) (MHz) Packaging LMV651 110 2.7 to 5.5 1 80 nA (typ) 0 to 4.0 12 SC70-5, TSSOP-14 LMV791 1150/0.14 1.8 to 5.5 1.35 100 pA -0.3 to 4.0 17 TSOT23-6, MSOP-10 LMV796 1150 1.8 to 5.5 1.35 100 pA -0.3 to 4.0 17 SOT23-5, MSOP-8 LMV716 1600 2.7 to 5.0 5 130 pA -0.3 to 2.2 5 MSOP-8 LPV531 425 2.7 to 5.5 4.5 10 pA -0.3 to 3.8 4.6 TSOT23-6 ADCs for Single-Channel Applications # of Pin/Function Throughput Max Power Max INL Min SINAD Product ID Res Inputs Compatible Rate (kSPS) Input Type 5V/3V (mW) Supply (V) (LBS) (dB) Packaging ADC121S101 12 1 500 to 1000 Single ended 16/4.5 2.7 to 5.25 ±1.1 70 SOT23-6, LLP-6 ADC121S051 12 1 200 to 500 Single ended 15.8/4.7 2.7 to 5.25 ±1.0 70.3 SOT23-6, LLP-6 ADC121S021 12 1 50 to 200 Single ended 14.7/4.3 2.7 to 5.25 ±1.0 70 SOT23-6, LLP-6 ADC101S101 10 1 500 to 1000 Single ended 16/4.5 2.7 to 5.25 ±0.7 61 SOT23-6 ADC101S051 10 1 200 to 500 Single ended 13.7/4.3 2.7 to 5.25 ±0.7 60.8 SOT23-6 ADC101S021 10 1 50 to 200 Single ended 12.6/4 2.7 to 5.25 ±0.6 60.7 SOT23-6 ADC081S101 8 1 500 to 1000 Single ended 16/4.5 2.7 to 5.25 ±0.3 49 SOT23-6 ADC081S051 8 1 200 to 500 Single ended 12.6/3.6 2.7 to 5.25 ±0.3 49 SOT23-6 ADC081S021 8 1 50 to 200 Single ended 11.6/3.24 2.7 to 5.25 ±0.3 49 SOT23-6 ADC121S625* 12 1 50 to 200 Differential 2.8 4.5 to 5.5 ±1.0 68.5 MSOP-8 ADC121S705* 12 1 500 to 1000 Differential 16.5 4.5 to 5.5 ±.95 69.5 MSOP-8 *Differential input, 200 to 500 kSPS thruput rate forthcoming 05 8208Signal_Path_108 12/12/06 9:40 AM Page 6 SIGNAL PATH designer Low-Voltage Current Loop Transmitter An offset voltage of 1 mV in A1 will cause an error The loop current generates a voltage drop across of about 2.5% in IR3: resistor R2 such that V1 is negative with respect to V2 and then fed back through R3.
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