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 (SC PGSA), as a basic component of an integrated ultra-low power AFE. AFE resolution, sample rate and signal gain are configurable between 6 to 13 bit, 1 to 10 kS/s and -6 to 12 dB, respectively. The circuit draws 10.5 µW from a 1.8 V standard supply voltage, achieving an effective number of bits of 12.6 bit and a Walden figure of merit of 169.1 fJ/st. and 30.6 fJ/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 SC PGSA. 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 180 nm silicon-on-insulator technology was fabricated.

Successive Approximation Register Analog-to-Digital Converter (SAR ADC) for Biomedical Applications

“…Where the charge injection is defned as the charge that exists between the source and drain terminals when the sample switch is turned of, and the clock feed through is defned as the charge injected due to the overlapping coupling capacitor between the gate and drain terminals. Consequently, it is preferable to implement the sample switch by using CMOS transmission gates [8,64,103] or the bootstrap switch in Figure 4 [2,5,6,11,17,22,24,31,34,45,53,58,63,65 Te variations in conductivity are reduced by utilizing the CMOS transmission gates, but the problem of input dependence still exists. Additionally, a large parasitic capacitor that limits the resolution of SAR appears.…”

“…Due to their lowest number of transistors, this scheme saves both power and area. Te basic structure of the single-stage dynamic latch comparator [3,4,6,16,63,86,94] is presented in Figure 5. It consists of three sections, including the diferential amplifcation section (M1 and M2), the cross-coupled inverter section (M3:M6), and the reswitched section (S1:S4).…”

“…Considering these nodes are portable battery-operated electronic devices, energy-efcient integrated circuit designs are required. It can be claimed that low power consumption is one of the most important bottlenecks for wireless biomedical applications because of limited battery lifetime [6]. Te analog-to-digital converter (ADC) is a key building block as it can be considered the interface between the real analog domain and the digital processing domain.…”

Low-Power Modular Multi-Sensor Node with ZeSCIP Analog Frontend

“…With the advent of the Internet of Things (IoT), the usage of sensor networks for various applications has seen an explosive growth. As a starting point on the road to autonomous, harvester-powered (zero power) sensor nodes, the integrated ZeSCIP zero power analog frontend (AFE) was designed to readout typical environmental sensors with microwatt power consumption [1]. It aims at the dynamic IoT market, where short market times and low production costs are an important factor.…”

“…It aims at the dynamic IoT market, where short market times and low production costs are an important factor. The sensor data, digitized and pre-processed by ZeSCIP, is available for further processing and transmitting by a microcontroller, which was not taken into account in [1]. In this paper, ZeSCIP is brought into a realistic application scenario by attaching it in a test setup to a microntroller and to commercial sensors.…”