Chopping amplifiers are one type of amplifier categorized as zero-drift because of their ability to eliminate DC errors over time and temperature. This is done internally by modulating the DC errors and low frequency noise up to a high frequency which can later be filtered out. If left unfiltered, these errors would show up as a ripple at the amplifier’s output at the frequency of modulation or chopping. Modern chopping amplifier outputs are stabilized however by internally filtering most of this content out. Depending on how the internal filtering is implemented, additional noise around the chopping frequency may still be observed and can be further filtered externally.
Noise can also manifest itself in the form of charge injection from the chopping switches on the inputs. This charge injection can interact with the source impedance to create noise at the chopping frequency. One way to measure the charge injection performance of a zero-drift amplifier is to insert a large source resistance, then measure the input voltage with a high impedance, wide-band probe. Note that if the noise is monitored at the output of the amplifier during this test, it will be filtered by the response of the amplifier. Lower source impedance and selection of more modern amplifiers like LTC2063 and ADA4523 will help minimize this affect.