31.4 A Chopper-Stabilized Amplifier with -107dB IMD and 28dB Suppression of Chopper-Induced IMD

Amplifiers often employ chopping to achieve low offset and low-frequency noise. However, the interaction between the input signal and the chopper clock can cause chopper-induced intermodulation distortion (IMD) [1] –[5]. This is especially problematic for input frequencies (Fin) near even multiples of the chopping frequency (FCH), as the resulting IMD tones fold-back to low frequencies and so cannot be filtered out. In [2] –[4], spread-spectrum clocks are used to convert such tones into noise-like signals. However, this increases the noise floor and does not address the underlying problem. This paper shows that chopper-induced IMD is mainly due to amplifier delay, which results in large chopping spikes. A novel fill-in technique is proposed that mitigates these spikes, and so reduces the chopper-induced IMD. In a prototype chopper-stabilized amplifier, it reduces the chopper-induced IMD by 28dB, resulting in an IMD of -126dB for input frequencies near 4FCH (=80kHz). Similarly, it improves the chopped amplifier’s two-tone IMD (79 and 80kHz) from -97dB to -107dB, thus maintaining the same IMD as the un-chopped amplifier.