CASCADE PSEUDOMULTIBIT NOISE SHAPING MODULATORS

Cascade and multibit quantisation 1-bit feedback (NA/Dl) noise shaping modulators are presently considered the most attractive topologies for implementing high resolution A/D convertors. This is because the cascade concept allows stable high order noise shaping modulators to be implemented easily, and because the NA/D1 architecture allows the amount of quantisation noise handled to be reduced while avoiding the disadvantages of using N-bit D/A convertors (N greater-than-or-equal-to 2). The work presented here proposes that cascade and multibit modulators should be seen as particular cases in a wider modulator architecture, here referred to as GaMeS. The design methodology proposed also permits several new modulators to be derived. All of them allow us to explore the three parameters in the design of an A/D convertor, which are the oversampling ratio, M, the noise shaping order, L, and the quantiser resolution, N. An analytical study is made of integrator gain and pole error effects in the SNR performance, as well as of nonidealities present in N-bit quantisers. Simulations made with a user-friendly and behavioural simulator we had developed, prove the correctness of the analyses made, and indicate that high resolution wideband A/D convertors can take advantage of these new topologies.