ADC Dither Methods comparison

caroline11

The object of this short document is to compare wideband and band-limited methods when using dither noise to reduce quantisation spurs in A/D converters. The basic problem is that, when applying a ‘clean’ sine-wave input to the A/D, spurs appear atfixed frequencies depending on the exact input and clock frequencies and at levels depending on the number of effective bits in the A/D. These spurs are caused by the regular interaction between the input sine-wave and the quantisation levels. Under normal conditions, with system noise present, these interactions are ‘dithered’ by the noise itself which tends to ‘smear’ out the spurs into the noise floor. With clean signals, however, such as might occur with large signals and AGC which pushes the system noise floor below that of the A/D, the spurs can become a problem.