Interface Circuits Design for piezo- resistive Sensors

Abstract

In the recent years, there has been a growing demand on high resolution pressure sensors for new applications such as mobile altimeter, air speed measurements, control systems, medical in- struments and accuracy enhancement of the GPS receivers. Many of these applications are hand-held or battery operated, therefore the power dissipation of the pressure sensors remains as a main concern. In piezo-resistive sensors, the pressure to be measured changes the resistance value of piezo-resistive element. Typically, piezo-resistive sensors are realized in one of two configurations, either as a single resistor with current excitation, or in the form of

iii

Wheatstone bridge. Although a single resistor configuration can potentially achieve high signal-to-noise ratio (SNR), the Wheat- stone bridge is the most widely used readout configuration, due to its implementation simplicity and its immunity to common mode noise and interference from supply and environmental dis- turbance such as temperature, which in turn relax the interface circuit design. Compared to capacitive pressure sensors, piezo-resistive pres- sure sensors consume dc current in the sensor itself, making re- sistive sensors less attractive for low-power applications. The ob- jective of this work is to extend the applications of piezo-resistive sensors to the low power domain. The proposed interface cir- cuit is different than conventional interface circuits by periodi- cally switch the resistive bridge to save power. To reduce the interface circuit power even further, the turn ON time of differ- ent system blocks is optimized. The proposed interface is com- posed of a sample-and-hold circuit, a front-end-amplifier and a ∆Σ analog-to-digital-converter. The proposed interface is imple- mented in 0.13µm standard CMOS technology with 1.2V supply voltage. The interface circuit achieves a high resolution of 13.82