AC-coupling instrumentation amplifier improves rejection range of differential dc input voltage

The need for conditioning low-level ac signals in the presence of both common-mode noise and differential dc voltage prevails in many applications. In such situations, ac-coupling to instrumentation and difference amplifiers is mandatory to extract the ac signal and reject common-mode noise and differential dc voltage. This situation typically occurs in bioelectric-signal acquisition, in which metallic-electrode polarization produces a large random differential dc voltage. Input ac-coupling is one approach to removing the differential dc content. The principle of this ac-coupled instrumentation amplifier is to maintain the mean output voltage at 0V. The only drawback of this topology is apparent in the expression for the highpass cutoff frequency. Therefore, to maintain a given cutoff frequency, you must multiply the time constant by a factor equal to the active-feedback stage gain.