Design Considerations for Filtering Delta Sigma Converters

Abstract

Many signal chains need to digitize signals in the presence of interferers. The usual way of accomplishing this is to use an analog anti-alias filter followed by a Nyquist ADC. Continuous-time \(\varDelta \varSigma\) modulators (CT\(\varDelta \varSigma\) M) are compelling alternatives to Nyquist rate ADCs. While oversampling relaxes the requirements of the anti-alias filter, it is natural to wonder if the built-in filtering of a CT\(\varDelta \varSigma\) M, characterized by its Signal Transfer Function (STF), can be used to attenuate interferers so that an explicit filter up front can be dispensed with altogether. It turns out that the Signal Transfer Function (STF) of a conventional CTΔ ΣM is a by-product of NTF synthesis, with the designer having little control over it. The STF can be independently controlled by using a filter up front—however, this increases power dissipation and degrades linearity of the signal chain. Embedding the filter in the modulator achieves the same objective in a power efficient manner, while improving out-of-band linearity and reducing active area. This work reviews filtering \(\varDelta \varSigma\) conversion and circuit techniques for such converters.