A 0.20-V to 0.25-V, Sub-nW, Rail-to-Rail, 10-bit SAR ADC for Self-Sustainable IoT Applications

Abstract

This paper presents a 10-bit SAR ADC operating at a supply voltage (VDD) of 0.225 V and down to 0.2 V for self-sustainable IoT applications. We propose an ultra-low VDD temperature-compensated bias current generator for biasing the comparator against temperature variation to address the temperature-dependent issue of the MOSFETs operating in the subthreshold region. In addition, the design fixes the positive input terminal of the comparator at VDD to bias the input transistor pair of the comparator with a sufficient voltage headroom at such a low VDD. The double-boosted with leakage reduction sampling switch is also proposed to address the severe leakage issue at low sampling rates. A test chip has been fabricated in 180-nm CMOS. The ADC core occupies only 0.024 mm(2). Measurement results show that at 0.225V, the DNL and INL are within +1.04/-0.66 and +0.97/-1.04 LSB in the rail-to-rail input range, respectively. The measured peak SNDR with the Nyquist input frequency is 49.8 dB at 450 S/s and 0.225V. The whole ADC totally consumes 0.85 nW at 0.225 V including the circuit leakages. It corresponds to an FoM of 8.0 fJ/conv.-step.