Dual bandwidth autozero loop for a voice frequency CODEC

摘要

An autozero loop for eliminating offsets in the analog to digital converter section of a voice frequency coder-decoder (CODEC) utilizing an array of capacitors and a linear resistor string. The autozero loop functions with a relatively small time constant to null offsets quickly during the power-up phase of CODEC operation and with a higher time constant after the power-up phase. A dual bandwidth sub-circuit in the loop is connected to a voltage generator and controlled by signals from a logic circuit to operate at different bandwidths and thus provide different offset cancelling feedback signals during the power-up and normal operating phases.