A flexible low-power high-resolution integrated interface for capacitive sensors

Abstract

This paper presents a low-power high-resolution CMOS interface for capacitive sensors. The circuit is based on the use of a switched-capacitor charge amplifier, which converts the input capacitance to a voltage, which modulates the period of a relaxation oscillator. Auto-calibration is used to eliminate the undesired effects of transfer-parameter drift. The interface is suited for capacitive sensors with capacitance values from 1 pF up to 220 pF. Moreover, the measurement time can be set from about100 µs up to 50 ms. For the 10 pF range, parasitic capacitances up to 680 pF can be handled while the settling accuracy is more than 14 bits. For a measurement time of 1s, the resolution can be as high as 20 bits. In the 10 pF range, for parasitic capacitance up to 680 pF, the measured nonlinearity error is less than 5×10 -5 . All these features have been achieved with a chip consuming only 3 mm 2 of silicon area and 5 mW of power.

Error analysis of a charge-balancing capacitive sensor interface with resistive reference

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An energy-efficient 15-bit capacitive sensor interface

“…Moreover, an auto-calibration technique is applied to enable the use of a low-power comparator. Experimental results of a prototype implemented in 0.35 μm standard CMOS technology show that with a supply current of only 64 μA and a figure-of-merit of 0.05 nJ/step, the new interface circuit is more than an order-of-magnitude more energy efficient than earlier PM-based interfaces [3,4].…”

“…Such interfaces are essentially relaxation oscillators that convert the sensor's capacitance into a time period, which can be easily digitized using a counter. However, with supply currents in the mA range, prior implementations of such interfaces are not suitable for use in energy-constrained applications [3,4].…”

Target voltage independent capacitance measurement circuit implemented by 0.18 µm CMOS for PWM-MEMS control

“…Our goal of research is to realize capacitance measurement circuit which satisfies these requirements. Recent papers introduced some capacitance measurement circuits and these are mainly divided into three types which are named pulse width modulation (PWM) [2][3], period modulation (PM) [4][5] and delta-sigma modulation (ΔΣ) [6]- [8]. PWM and PM utilize current sources to inject and draw charges of capacitor periodically and derive their capacitance from rise (or fall) time of capacitors.…”