Dithering in Analogue-to-Digital Conversion

This study analyses properties of the analogue-to-digital converter, which is very central in modern measurement equipment. The work is mainly theoretical, and the theoretical models were verified partly with experimental study, partly with a virtual instrument developed for this purpose. Fundamental properties of analogue-to-digital conversion are presented as background of the study. Limitations coming from the time-discrete nature of sampling and from the amplitude-discrete nature of quantisation are introduced. The basic idea of dithering is presented. The condition for systematic correctness is developed. The random uncertainties of some practically interesting dithering schemes are developed. It is presented that dithering can be combined with suppression of network interference. Also the possibility to use unavoidable thermal noise of the measurand as dither signal is presented. A short analysis on information transfer of the dithered conversion is presented. It is shown that dithering gives more accuracy, but this reduces the bandwidth of the conversion. Indeed, dithering may be characterised as changing bandwidth against resolution. Two practical applications are briefly discussed. The first one shows how broad-range dithering can improve even the integral nonlinearity of the ADC. The second application is equipment for multichannel temperature measurement with very good resolution and speed.