Instrumentation amplifiers 101

Instrumentation amplifiers are amplifiers that measure small signals in noisy environments. Also known as in amps, these devices employ a technique called common-mode rejection (CMR) to distinguish the signal of interest from the noise. In amps have very high input impedance—typically many gigaohms—since signal sources connected to these amplifiers often have output impedances of several kilohms or more. In amps operate between dc and about 1 MHz. Designers who specify these amplifiers are primarily concerned with power-supply current, 3-dB bandwidth, common-mode rejection ratio (CMRR), input voltage offset and offset voltage-drift with temperature, noise (referred to input), and input bias current. Most instrumentation amplifiers are comprised of three operational amplifiers (op amps) arranged in two stages. Two of the op amps are configured as a preamp, while the third is a difference amplifier. The preamp provides high input impedance, low noise, and gain. The difference amplifier rejects common-mode noise and can provide some additional gain if needed.