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

Abstract

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F_{\mathrm {CH}}$ </tex-math></inline-formula> (=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.