An instrumentation amplifier with low offset-error drift for radiometric applications

This paper presents a high-precision instrumentation amplifier for interfacing differential output of power sensor in microwave total power radiometer. This amplifier is based on commercially available chopper op-amps and comprises a differential amplifier, active low-pass filter, single-ended to differential output buffer. For improving CMRR and offset-error drift, precision foil-resistors are used in op-amps external feedback. The experimental hybrid circuit has been designed and its performance characteristics were measured in different temperature points. To verify offset-error drift in a long time period the measurements were repeated after 2 years of this circuit usage. The results of measurements of gain, CMRR, white noise, offset-errors and output voltage stability analysis, based on the Allan Variances calculations, are given. Proposed instrumentation amplifier provides 11 nV/sqrt(Hz) input-referred voltage noise and 100 V/V differential signal gain in the frequency range from DC to 10 kHz which lies in the first Nyquist zone of chopping frequency equal to 200 kHz. 12 nV/0C and 160 nV per 2 years offset voltage drift (referred to input) were achieved.