A Precise Capacitance-to-Pulse Width Converter for Integrated Sensors

Abstract

This work describes a novel approach for interfacing capacitive sensors in the sub-pF range. The system generates a PWM signal with a linear relationship between the pulse duration and the sensor capacitance. The circuit exhibits intrinsic low sensitivity to temperature and process variations and is therefore an interesting solution when extremely wide operating temperature ranges are required. A detailed analysis of the noise characteristics, aimed to give indications about the circuit optimisation, is presented. The interface has been designed using the 0.35 µm BCD6 process of STMicroelectronics and tested by means of electrical simulations.

A low power capacitance to pulse width converter for integrated sensors

“…In stationary conditions, the charge balance over one clock period must be zero. Summing up the contributions in the three phases as given by (3)(4)(5) and considering Eqn. (1) for I CA , it can be easily proven that:…”

“…Recently, a CMOS capacitance-to-PWM converter marked by very low temperature sensitivity over a wide temperature range has been presented [3]. The circuit proposed in this work reaches an even smaller temperature sensitivity with a power consumption more than two orders of magnitude lower.…”

“…The circuit proposed in this work reaches an even smaller temperature sensitivity with a power consumption more than two orders of magnitude lower. This result has been obtained using a fully differential current amplifier with local feedback loops providing a nearly ideal virtual ground at the input nodes, with more relaxed current and stability requirements than the OTA based architecture of [3]. The effectiveness of the proposed approach is illustrated by means of electrical simulations, performed on a prototype designed using 0.32 µm/ 3.3 V CMOS devices from the Bipolar-CMOS-DMOS process BCD6s of STMicroelectronics.…”