A 10.5µW programmable SAR ADC Frontend with SC Preamplifier for Low-Power IoT Sensor Nodes

Abstract

Massive deployment of wireless autonomous sensor nodes requires their lifetime extension and cost reduction. The analog frontend (AFE) plays a key role in this context. This paper presents a successive approximation register analog-to-digital converter (SAR ADC) with a switched-capacitor programmable gain switched preamplifier (SCPGSA), as a basic component of an integrated ultra-low power AFE. AFE resolution, sample rate and signal gain are configurable between 6 to 13bit, 1 to 10kS/s and -6 to 12dB, respectively. The circuit draws 10.5µW from a 1.8V standard supply voltage, achieving an effective number of bits of 12.6bit and a Walden figure of merit of 169.1fJ/st. and 30.6fJ/st., computed with and without preamplifier, respectively. The circuit is employed in a modular internet of things sensor node, suitable to be solely powered from microenergy sources (energy harvesters). In order to feed charge-scaling SAR ADC inputs with the sensor voltages, typically a preamplifier stage is implemented, which can create energy overhead of magnitudes larger than the ADC power. This paper presents a duty-cycled preamplifier with programmable gain for SAR ADCs, utilizing switched-capacitor switched-opamp technique in the SCPGSA. No additional buffer circuitry is needed to charge the SAR ADC, and the preamplifier design is relaxed in power constraint. The circuit targets the low-cost internet of things market. Cost efficiency is achieved by technology choice, wide configurability and shortened ASIC design cycles. The latter results from partly generated layout, easing reuse of circuit parts from a different CMOS node. A testchip in a low-cost 180nm silicon-on-insulator technology was fabricated.