Instrumentation amplifier with digital gain programming and common-mode rejection trim

A design for an instrumentation amplifier with digitally programmable gain and common-mode rejection (CMR) trim is presented. A standard multiplying D/A converter (DAC) is used as the differential feedback control element, and the common-mode rejection is provided by four resistors in a configuration equivalent to a balanced bridge. Digital CMR adjustment is established after another bridge-balancing DAC has been added to the basic circuit. Detailed circuit analysis proves that excellent offset and drift cancellation follows from the proper pairing of four op amps used in the circuit. Feasibility of amplifier integration into a single chip is also discussed