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Signals from a pressure sensor or an ECG monitor belong to a class of analog signals that come in pairs. With this type of signal, the phase and frequency content between the two are equal, but the magnitude changes tend to be equal and opposite. For instance, the four-element piezoresistive pressure sensor is configured in a resistive bridge. Assuming that there is pressure applied to the sensor, the effects of the voltage (or current) excitation of the bridge are equally applied to each element. When a pressure excitation is applied to the bridge a small differential voltage (100's of mV) appears between the mid-points of the two resistive legs.

The signal conditioning challenge for this type of application is to subtract the two signals, apply a gain and produce a single-ended result that can easily be digitized in a single-supply environment. Implementation of the subtraction and gain function can be done in either the analog and digital domains, but the foundation for understanding the nuances of common-mode rejection, gain, input impedance matching, level shifting and dynamic range are easily demonstrated with three standard analog circuit examples. These analog circuits are the differential amplifier, the two op amp instrumentation amplifier and the three op amp instrumentation amplifier. Every one of these configurations take two opposing input signals, subtract them and also apply gain. The subtraction process inherently rejects common-mode voltages. Combined with these functions the signal is level-shifted, making it synergistic with the signal supply environment.

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