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Abstract

A 'true' 16-bit second order Sigma-Delta based converter that has superior analog components and has a programmable comb filter which is coupled to the digital signal processor. This converter comprises a second order Sigma-Delta modulator and a programmable comb filter. The second order Sigma-Delta modulator dramatically attenuates the baseband quantization noise energy (which in turn increases the resolution of the converter), since its superior amplifiers and comparators enable it to oversample and coarsely quantize the analog input signal at a very high sampling frequency of 12 MHz. The amplifiers are class AB OTAs, which have cross coupled NMOS driven input stages, and cascoded output stages. Also, the common mode voltages are the optimal biasing points, and these voltages are kept constant by a differential input stage, by a PV independent temperature dependent current generator, by optimal device size, and by a common mode feedback circuitry. The programmable comb filter receives the coarsely digitized 1-bit output of the modulator at oversampling frequency F.sub.S, and provides a more accurate representation of the input signal to the DSP at slower sampling rate of F.sub.S /N. In addition, the comb filter uses a 20-bit data path, in order to enable the decimator (which is formed by the comb filter and by the FIR filter) to provide 16 bits of resolution to the DSP. The output of the programmable comb filter is then supplied to an FIR filter which is realized in the DSP, and this filter removes the remaining out-of-baseband noise.

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