A 2.5nJ duty-cycled bridge-to-digital converter integrated in a 13mm3 pressure-sensing system

Abstract

Small form-factor piezoresistive MEMS sensors, often configured in a Wheatstone bridge, are widely used to measure physical signals such as pressure [1-3], temperature [4], force [1], and gas concentration. A common method to realize a digital output from the bridge involves biasing the bridge with a DC voltage source and using a low-noise amplifier followed by an ADC. While a bridge measurement can achieve high resolution and linearity, it is very power hungry [3] because the bridge resistance is low (typically 1–10kΩ). Both the high power and high instantaneous current make it unsuitable as a sensing interface in miniaturized microsystems with battery capacities of <10μAh and ∼15kΩ internal resistance [5]. Duty cycled excitation was proposed in [1] to reduce power in moderate dynamic range (DR) applications, lowering bridge excitation energy by up to 125x compared to static biasing. However, the excitation energy consumption (∼250nJ) is still much larger than the interface circuit conversion energy, and therefore limits overall sensor energy efficiency. To address this challenge, we propose an energy-efficient highly duty-cycled excitation bridge-sensor readout circuit for small battery-operated systems. Due to high battery resistances, the excitation voltage (VEX) is sourced from an on-chip decoupling capacitance that drops ∼100mV during excitation and then slowly recharges from the battery. To avoid accuracy degradation from this voltage fluctuation, the design samples not only the inputs (VIN+/−) but also VEX, from which it generates a DAC reference voltage (VDAC). We also propose an offset calibration and input-range matching method. We demonstrate operation of the bridge-to-digital converter (BDC) integrated with a complete and fully functional pressure-sensing system, including a processor, battery, power management unit, RF transmitter, and optical receiver.