1 Introduction

Chopping technique enables improved DC and drift performance. As a result, parameters associated with input offset are enhanced. Low frequency noise is also removed in the modulation and filtering process. Parameters such as V_os, CMRR, PSRR are expected to be excellent in CSA based on chopping techniques.

A chopper amplifier is typically made up of two parallel signal paths, a high-gain, low-bandwidth path and a low-gain high-speed path. Figure 1-1 shows a simplified block diagram of a chopping amplifier. Chopping is accomplished in the top low-bandwidth path, while the bottom high-speed path is feed-through, shaping the amplifier gain roll off at high frequencies. At low frequencies, the low-bandwidth path dominates, providing the desired DC precision.

For a multi-stage amplifier, input offset is dominated by that of the input stage. Chopping amplifiers achieve offset reduction by employing two pairs of synchronized switches in the signal path.

The working principle on input signal is demonstrated in frequency domain in Figure 1-2. As input signal goes through the chopping operation, a spectrum is plotted for locations A, B and C. The first switch modulates input signal to the chopping frequency and its odd harmonics. The second switch at the output reconstructs the input signal and passes on to the output stage.

The offset voltage of the input amplifier, which is modulated to high frequency by the reconstruction switch, is filtered out by a subsequent low-pass filter which is not shown. To contrast with that of input signal, the chopping operation on input offset is illustrated in Figure 1-3. Because offset voltage and 1/f noise are effectively removed, chopping can potentially achieve excellent DC accuracy.