Integrated Instrumentation Amplifier for the Phase-Readout of a Piezoresistive Smart Sensor

An integrated instrumentation amplifier combined with a bandpass filter has been fabricated in a bipolar process for the phase-readout of piezoresistive sensors. Changes in the resistance can be measured directly using an AC-operated bridge configuration. A special technique has been employed to convert these changes into a phase angle relative to the excitation signal. The instrumentation amplifier should have a very high Common Mode Rejection Ratio (CMRR) at the bridge operating frequency in order to actually exploit the available high sensitivity of this readout method. The filter should have a well-determined phase behaviour, to prevent a change in the frequency from affecting the phase angle. Special emphasis has been put on the compatibility between micromachining technologies in silicon and bipolar processing, so that a fully integrated smart silicon sensor, such as a pressure sensor, can be made.