A major problem of mixing analog and digital designs is noise coupling between the different domains. The noise generated in the digital sections of the circuit is coupled into the analog circuits through the power supplies and over the substrate. Further, routing an analog signal parallel to a digital line will result in a capacitive coupling between the two lines.
Noise generated in a digital circuit is particularity unpleasant in precision analog applications. The reason for this is that the `digital' noise is not purely random. E.g., if the digital circuit contains counters or synchronous logic, discrete spectral components are generated corresponding to the clock frequency and its harmonics. The sub-harmonics are also present. Such discrete spectral lines are often more disturbing than random (white) noise sources. Switched capacitor circuits are particularly sensitive to such noise. The original spectral line may lie outside the used frequency band, but due to the sampling performed in the switched capacitor stages, the spectral lines may be folded into the pass-band. An anti-aliasing filter in front of the switched capacitor only prevents aliasing of the signal at the input of the filter, however the noise generated on-chip may be coupled into any stage of the structure.
In this Chapter, some general techniques to reduce noise coupling will
be discussed. The techniques used to minimise noise coupling can also
be used for circuits, in which very low levels of cross-talk are
required
.