Charles Kitchin and Lew Counts
Analog Devices

The primary benefit of an in-amp circuit is it provides common-mode rejection. While the AD8221 and AD8225 both have an extended CMR frequency range, most in-amps fail to provide decent CMR at frequencies above the audio range.

A composite instrumentation amplifier features an extended frequency range over which the instrumentation amplifier has good common-mode rejection. The circuit consists of three instrumentation amplifiers. Two of these, U1 and U2, are correlated to one another and connected in antiphase. It is not necessary to match these devices because they are correlated by design. Their outputs, OUT1 and OUT2, drive a third instrumentation amplifier that rejects common-mode signals and amplifiers' differential signals. The overall gain of the system can be determined by adding external resistors. Without any external resistors, the system gain is 2.