A novel noise efficient feedback DAC within a switched capacitor ΣΔ ADC

Efficient sampling of the reference noise within a bilinear switched capacitor ΣΔ analog-to-digital converter (ADC), resulting in improved thermal noise performance is presented. Bilinear integrators contain a zero at the Nyquist frequency, with the result that no charge is transferred from the reference when a transition occurs in the modulator output. The average noise power added by the reference digital-to-analog converter (DAC) can be reduced substantially if the reference DAC is sampled only when charge is to be transferred. For midscale inputs, the sampled noise from a single bit reference DAC is reduced by more than 5 dB. When multibit quantization and feedback is used the reference noise can be further suppressed, in the case of 5 bits of feedback the reference noise is reduced by more than 20 dB.