Frequency shift dither for analogue to digital converters

Abstract

This paper discusses a new dither technique to linearise an analogue to digital converter (ADC). This typically results in a more than 20 dB reduction in harmonic distortion. Additive dither has been demonstrated to reduce the spurii to a significant extent. In this paper frequency shift (FS) dither is described. FS dither applies a frequency shift to the signals to be digitised. After analogue to digital conversion the signal is demodulated by removing this frequency shift using digital signal processing (DSP) techniques. FS dither has the advantage that the demodulated output has very low levels of harmonic distortion compared to a system without FS dither. By applying both additive and FS dither at the same time, input signals with amplitudes far less than a quantum level can be digitised and detected accurately.