Analog blocks for high-speed oversampled A/D converters

The design of high performance sigma-delta converters involves high frequency operation. This constraint means analog blocks must be carefully designed to meet these requirements. BiCMOS technology offers advantages with respect to CMOS in realising high-speed analog blocks. This paper describes an operational amplifier and a comparator designed in BiCMOS technology. Suitable circuit solutions have been used to obtain the required values for DC gain, gain-bandwidth product and slew-rate. High-level simulations of both low-pass and band-pass oversampled converters indicate that the blocks designed allow a faster clock rate than CMOS and are suitable for operation in switched-capacitor circuits with sampling rate of up to 40 MHz.