Optimization of an Oversampling Feedback Coder for Signal Processing Chips

This paper describes the design of a special type of oversampling coder, a so called adaptive interpolative delta modulator. This type of coder incorporates most of the known tricks for improving the signal to noise ratio of an oversampling feedback coder. This system provides higher quality of the processable code than other coders. Due to that fact, the oversampling ratio is substantially reduced. The loop parameters are optimized by means of a software model, which results from a very accurate analytical model. The coder is used in a codec filter chip with digitaly programmable gain. In order to achieve a sufficient programmable gain range in the digital part, the coder and decoder must have a dynamic range of more than 90 dB which has to be achieved in the presence of large amounts of digital switching. Including the loop delay in the analytical model and modelling the circuit nonidealities correctly leads to a coder with a dynamic range of almost 94 dB. Results presented are based on both simulations and measurements.