Oversampling Sigma-Delta A/D conversion for instrumentation and measurement

Oversampling sigma-delta ADCs can achieve high resolution and accuracy without the need for precise component matching and costly, complex analog circuitry. Although they have been in use in communication systems for many years, only recently have they been used in the instrumentation and measurement field. However, the sampling rate of conventional sigma-delta ADCs can only be varied by selecting a limited number of output rates of the filters; users have no control over the sampling rate. This limits the application of sigma-delta modulators in the measurement field.;The dissertation describes the application of sigma-delta modulators (without built-in filters) as ADCs in measurement systems. A novel variable sampling rate sigma-delta modulator architecture is described. A second-order, variable sampling rate sigma-delta modulator was designed, built and tested. At the maximum clock rate the modulator behaves as a conventional sigma-delta modulator. At lower clock rates the user can control the ratio between sampling rate and signal frequency, this is a standard feature of conventional ADCs.;Data processing techniques for measurement systems using sigma-delta modulators as ADCs are described. The effect of filtering on phase measurement and the effect of various filters on the measurement accuracy of both amplitude and phase were investigated. A fast three-parameter fit and fast curvefitting-filtering algorithm were developed. The speed-up factor of the algorithms can be significant when a long data sequence is used in the data processing.;A/D conversion in a sigma-delta ADC is the process of modulation and demodulation; this makes it impossible for sigma-delta ADCs to measure instantaneous values of an input signal. However, the advantages of the sigma-delta ADCs make them ideal for use in a digital lock-in amplifier (DLA) where the instantaneous measurement is not required. A prototype DLA using variable sampling rate sigma-delta modulators as ADCs was developed. The experimental results show that the system can achieve satisfactory accuracy for both amplitude and phase measurements.