Electronic Design

Single-Supply Circuit Handles Bipolar Sensor Signals

I recently had the task of designing an analog front-end circuit for a microcontroller application. A full-bridge sensor in a standard configuration produced the input signal. An instrumentation amplifier then amplified the low-level signal and fed it to an analog-to-digital converter (ADC). After that, a microcontroller read and processed the data.

Due to the nature of the sensor and the apparatus to which it was connected, the sensor's output voltage was bipolar. I had to amplify and digitize that signal, but I couldn't find a single-supply op amp able to handle a bipolar input. Even worse, none of the single-supply ADCs that I found could handle a bipolar input.

After struggling for two days, I found parts that would do the job or create a decent alternative solution. Talking to engineers from major IC manufacturers didn't help either.

Suddenly, the light bulb went on and revealed an interesting solution. I used two instrumentation amplifiers (IA1 and IA2) with reversed inputs connected to the bridge's resistive sensor (see the figure). The output of each IA drove one of the ADC channels. (You need at least two available channels for this trick.)

Software handled the rest of the task. By monitoring the voltages on the two channels, I was able to predict the direction and measure the magnitude of the signal produced by the sensor.

The original circuit used an INA122 instrumentation amplifier and an LTC1594 four-channel ADC. Adding an extra INA122 chip and some code modifications eliminated the need for a bipolar power supply, all new op amps, and a different ADC.

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