Precise biasing of analog-to-digital converters by means of auto-zero feedback

Auto-zero biasing refers to a technique whereby the DC bias in an analog-to-digital converter is controlled using negative feedback. A portion of the converter´s digital output, typically the sign bit, is decoded locally, and the resulting analog signal is then averaged and subtracted from the input signal. This feedback controls the converter´s bias with a precision that scales with the amplitude of the input. For small signals it is therefore possible to achieve almost arbitrarily precise biasing without precision circuit components. The properties of auto-zero control circuits are described, and the design of such circuits is illustrated by means of a μ-law PCM codec example.