A second-order sigma-delta ADC using MOS capacitors for smart sensor applications [smart temperature sensor]

This paper presents a second-order sigma-delta ADC designed for use in a smart temperature sensor. It is operated in a ´one-shot´ mode, i.e. the converter is powered up, produces a single conversion result, and powers down again. This paper discusses the implications of this mode of operation for the design of the modulator and the decimation filter. A sinc² decimation filter is used, which is shown to provide a higher resolution then a more complex sinc³ with the same conversion time. Through continuous-time integration of the input and reference voltages, the use of a linear sampling capacitor at the input is avoided. The modulator was implemented in a 0.5 μm digital CMOS process using MOS capacitors. An effective resolution of 15.5 bits was measured with a conversion time of 25 ms.