A tool for the assisted design of charge redistribution SAR ADCs

The optimal design of SAR ADCs requires the accurate estimate of nonlinearity and parasitic effects in the feedback charge-redistribution DAC. Since the effects of both mismatch and stray capacitances depend on the specific array topology, complex calculations, custom modeling and heavy simulations in common circuit design environments are often required. This paper presents a MATLAB-based numerical tool (CSAtool) to assist the design of the charge redistribution DACs adopted in SAR ADCs. The tool performs both parametric and statistical simulations taking into account capacitive mismatch and parasitic capacitances thus computing both differential and integral nonlinearity (DNL, INL). SNDR and ENoB degradation due to static non-linear effects is also estimated. An excellent agreement is obtained with the results of circuit simulators (e.g. Cadence Spectre) featuring up to 10⁴ shorter simulation time, allowing a large number statistical simulations which would be otherwise impracticable. Measurements on two fabricated SAR ADCs confirm that the proposed tool can be used as a valid instrument to assist the design of a charge redistribution SAR ADC and predict its static and dynamic metrics.