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Capacitive Wheatstone Bridge
Last modified by Microchip on 2023/11/09 09:03
A Capacitive Wheatstone bridge can be used as a sensor signal conditioning circuit that converts its impedance, at a specific frequency, to a voltage. The circuit below produces a change in differential voltage as a function of the capacitance change. An Alternate Current (AC) voltage source must drive the bridge; its frequency needs to be stable and accurate. R1 can be a digital potentiometer that is controlled to zero out the differential voltage, or it can be a regular resistor. R3 provides a means to bias the instrumentation amplifier correctly, and to keep the node between the capacitors from drifting over time. It needs to be much larger than C2's impedance (1/j𝜔C2).
Advantages:
Excellent common mode noise
Ratiometric output (with Analog-to-Digital Converter (ADC) using VDD as its reference voltage)
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