Jitter-resistant Capacitor Based Sine-Shaped DAC for Continuous-Time Sigma-Delta modulators

A novel current mode Capacitor Based Sine-Shaped (CBSS) Digital-to-Analog Converter (DAC) for Continuous-Time (CT) Sigma-Delta (ΣΔ) modulators, enabling smooth zero current transitions and greatly reducing the clock jitter noise, is described. The circuit is switched capacitor based and uses a cosine-wave voltage reference instead of constant current. The output is a sine-shaped waveform with precise zero transitions. Locking the reference cosine-wave voltage and the clock employed for data sampling ensures a virtual immunity to clock jitter. The use of a cosine-wave as reference and a constant capacitance loading the reference generator facilitates impedance matching thus making the scheme suitable for very high sampling rate. Moreover, the good matching between capacitors determines the accuracy of the integrators´ time constant and multi-bit linearity of the proposed DAC. The feature of the method has been verified with behavioral simulations and tested in a second order ΣΔ modulator.