Integrated instrumentation amplifier for the phase readout of piezoresistive strain gauges

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 a relatively high bridge operating frequency (100 kHz) in order to actually exploit the advantages of this readout method. The filter should have a well-determined phase behavior to prevent a change in the frequency from affecting the output phase angle. Special emphasis has been placed on the compatibility between micromachining technologies in silicon and bipolar processing, so that a fully integrated smart silicon micromechanical sensor can be made