 Instrumentation amplifier

An instrumentation amplifier amplifies a differential input while presenting high input impedance and rejecting a common component. A useful ideal relation isIts common-mode rejection ratio describes finite unwanted response to common-mode voltage. This is distinct from the amplifier input common-mode range over which the circuit can operate without internal saturation.^[ref]

For an original example, stipulate $v_{+}=2.501\mathrm{V}$, $v_{-}=2.499\mathrm{V}$, $G=100$ and $V_{\mathrm{ref}}=2.5\mathrm{V}$. The differential signal is $2\mathrm{mV}$ and the ideal output is $2.7\mathrm{V}$, assuming all input and output limits are satisfied.

Now suppose the differential-to-common gain ratio is $10^4$, or $80\mathrm{dB}$. The common-mode gain is then $100/10^4=0.01$. A $1\mathrm{V}$ common-input change contributes $10\mathrm{mV}$ at the output, five percent of the desired $0.20\mathrm{V}$ signal contribution. A high rejection ratio therefore does not mean zero error.

A source that changes both inputs together tests rejection; a source that changes their difference tests differential gain. Both tests must respect input common-mode limits, amplifier output range, load and frequency response. A plausible ideal output value alone does not prove that every internal stage remains linear.