Characterization of the RFI Rectification Behavior of Instrumentation Amplifiers

In this paper, the rectification behavior of popular in-amps (instrumentation amplifiers) is measured in both common- and single-ended RF noise injection modes. It is recommended that AD8221 be used in common mode RF noise injection environment, and AD8220 and AD8429 be used in single-ended RF noise injection environment. The mechanism of RF noise rectification inside in-amps is also discussed. It is verified that rectification mainly happens at the non-inverting input of two op-amps in the first stage of an in-amp. The DC voltage difference between inverting input and non-inverting input of the in-amp is further amplified, which will cause a large DC offset at the output. It is shown that symmetry of the first stage in an instrumentation amplifier is very important. The asymmetry of the first stage will increase the DC offset at the output dramatically.