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 R_SENSE, 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.