Dynamic Offset Cancellation Techniques for Operational Amplifiers

At low frequencies, offset, 1/f noise and drift are the dominant error sources of operational amplifiers. This is especially true in CMOS technology. This chapter reviews precision techniques that can be used to achieve low 1/f noise and low offset in operational amplifiers. There are three types of CMOS offset cancellation techniques: trimming, chopping, and auto-zeroing. Trimming is usually performed during production to eliminate offset. Auto-zeroing is a sampling technique in which the offset is measured and then subtracted in subsequent clock phases. Chopping, on the other hand, is a continuous-time modulation technique in which the signal and offset are modulated to different frequencies. Due to the modulated offset and 1/f noise, a chopper ripple appears at the amplifier output. Since chopping and auto-zeroing are dynamic techniques that continuously reduce offset, they also remove low frequency 1/f noise as well as offset drift over temperature or time. In auto-zeroing amplifiers, the residual offset is mainly caused by charge injection and clock feed-through. While in chopper amplifiers, the residual offset is mainly caused by demodulated clock feed-through spikes. Several techniques can be used to counteract these non-idealities. Later in this chapter, several dynamic-offset-compensation techniques used in operational amplifiers will be discussed, e.g. ping-pong auto-zeroing, offset stabilization, and specifically, chopper offset stabilization of a low-frequency path in a multi-path amplifier. To suppress chopper ripple, numerous ripple reduction techniques can be used. It will be shown that these all have significant drawbacks, and thus new techniques are required.