A robust CMOS compander

A robust CMOS compander circuit meeting all of the requirements for analog cellular telephony and using an improved sigma-delta compander topology is presented. Rather than digitizing and reconstructing the input signal using a sigma-delta modulator as has been done previously, only the amplitude path is digitized while the voice path remains analog. The amplitude information is obtained digitally, and is reduced to a single bit using a first-order sigma-delta modulator. Performing this function digitally eliminates problems due to analog offsets and in implementing the long time constant required. The output signal is formed by gating the analog input signal under control of the amplitude signal. The expander and compressor circuits each consist of a single op amp and 2000 gates of digital logic, and have been implemented on 0.8-μm CMOS processes. The ADC for the amplitude path uses a compact switched-capacitor second-order sigma-delta modulator implemented using a single amplifier. No external components are required. Tracking error for the compressor was measured to be less than 0.3 dB over a 60-dB input range when operating on a 3.0-V supply. The test time, when compared to conventional compander implementations, is considerably reduced