Current loop transmits ac measurement

Mark Fazio, David Scott, and Bob Clarke, Analog Devices, Wilmington, MA -January 28, 2013

Originally published in the Aug 6, 1992, issue of EDN

Process-control applications use current loops to send information as an analog signal over long distances with high noise immunity. Using the three-chip circuit in Figure 1, you can measure alternating current or voltage and transmit the results on a 4- to 20-mA current loop. The circuit accepts a 0- to 10-mV ac RMS input and provides a 4- to 20-mA output.

Figure 1 This circuit measures alternating current or voltage and transmits the results on a 4- to 20-mA current loop.

The input signal creates a floating voltage across sensing resistor RSENSE, whose size produces 0 to 10 mV RMS from the expected sensed current. This floating voltage is the input to a differential-input, single-ended AD22050 sensor interface, IC1.

IC1 operates at a gain of approximately 20 and drives the low-impedance (8-kΩ) input (pin 1) of the AD736 RMS-to-dc converter (IC2). This converter’s full-scale range is 200 mV RMS. IC2’s output drives IC3, an AD694 voltage to 4- to 20-mA current-loop interface.

Because of their low power consumption, both IC1 and IC2 can operate from the 10V supplied by IC3’s reference output at pin 7. IC3, and hence the entire circuit, operates from the standard 24V loop supply. Because this circuit operates from a single supply, you must bias IC2’s common input at one-half of IC3’s 10V output, or 5V. The voltage divider comprising R1 and R2 divides the 10V to 5V. R2 is in parallel with a 10-kΩ resistor inside IC3.

IC3’s internal buffer amplifies the difference between IC2’s output at pin 6 and the 5V rail. This difference ranges from 0 to 200 mV dc for a 0- to 10-mV RMS input and produces a 4- to 20-mA current output from IC3. R3 allows you to adjust the circuit’s gain. R4 and R5 set the gain of IC3’s internal amplifier to 10. R5 matches R4 to prevent offsets due to the internal amplifier’s input-bias current. This circuit’s accuracy is 1.2% of readings from 20 Hz to 40 Hz and 1% of readings from 40 Hz to 1 kHz. The −3-dB bandwidth is 33 kHz.

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