A simple and highly efficient successive-approximation registers (SAR) ADC realisation is proposed. It requires an opamp, a comparator and only three equal-valued capacitors. The proposed scheme is insensitive to the capacitor parasitics, and has a small capacitance spread. With an oversampled scheme, it also allows noise-shaping the remaining quantisation error. It is well-suited for low-frequency instrumentation and measurement applications, and for use in extended-counting ADCs.