A chopper stabilized biasing circuit suitable for cascaded wheatstone-bridge-like sensors

Abstract

Many sensitive devices are based on Wheatstone bridge structures or can be modeled as Wheatstone bridges like Hall effect magnetic sensors. These sensors require a biasing circuit, and many solutions were proposed. However, up to now, none of them gives the opportunity to cascade several sensors, while such a cascade can help in improving the signal-to-noise-ratio (SNR) or in removing some parasitic effects through the direct summing/subtraction of sensing/parasitic effects. The circuit this paper presents is based on an operational transconductance amplifier with n output stages, and allows to cascade n Wheatstone-bridge-like sensors. It is shown that the maximal number of bridges which can be efficiently cascaded is limited by the output resistance of the output stages. Nevertheless, this number remains sufficient in practical cases, easily up to n=10. To remove the 1/f noise coming from the output stages, a chopper stabilization is used. We also establish formulas which allow quick hand calculation of the main parameters of the circuit. A prototype where 10 Hall effect sensors are cascaded is presented as well as experimental results.