A Fill-In Technique for Robust IMD Suppression in Chopper Amplifiers

In chopper amplifiers, the interaction between the input signal and the chopper clock can give rise to intermodulation distortion (IMD). This chopper-induced IMD is mainly due to amplifier delay, which causes large pulses at the output of the amplifier’s output chopper. This article proposes the use of a so-called fill-in technique to eliminate these pulses, and thus the resulting IMD, by multiplexing the outputs of two identical amplifiers that are chopped in quadrature. A prototype chopper-stabilized amplifier was implemented in a 180-nm CMOS process. Measurements show that the fill-in technique suppresses chopper-induced IMD by 28 dB, resulting in an IMD of −126 dB for input frequencies near 4
$F_{\mathrm {CH}}$
(=80 kHz). It also improves the amplifier’s two-tone IMD (with 79 and 80 kHz inputs) from −97 to −107 dB, which is the same as that obtained without chopping.