A compact 10-b SAR ADC with unit-length capacitors and a passive FIR filter

This paper presents a compact 10-b successive approximation register analog-to-digital converter (SAR ADC) in 65-nm CMOS with an integrated passive finite impulse response (FIR) filter for anti-aliasing. Conventional switched-capacitor digital-to-analog converters (DACs) are usually implemented with unit elements for the best matching performance, at the cost of increased chip area. Instead, this paper proposes a unit-length capacitor implementation, which minimizes the number of components and thus minimizes area, while also achieving good linearity (integral non-linearity of 0.39 LSB, differential non-linearity of 0.55 LSB, and SFDR of 75 dB) despite using a small LSB capacitor of 125 aF. The 10-b SAR ADC occupies only 36 × 36 µm, thanks to the small-size DAC, and by placing the ADC circuits directly under the capacitors. The ADC was tested at 10 and 30 MS/s and achieves an effective number of bits of 9.18/9.10 bit with an figure-of-merit of 4.1/4.4 fJ per conversion-step, respectively. Besides the ADC, a passive analog FIR filter is added to implement an anti-aliasing filter. The topology is based on a passive charge-sharing network and thus only consumes power for the clock phase generation and switch drivers. A 4× time-interleaved 15-tap passive FIR filter is implemented, which can realize >42 dB out-of-band rejection and 4× decimation while occupying only 53 × 90 µm. The filter and the ADC together consume 39.2µW for an output rate of 10 MS/s.