A high-performance microflowmeter with built-in self test

Abstract

This paper reports the development of a high-performance readout scheme based on a switched-capacitor circuit and intended for use with an ultrasensitive microflowmeter. The microflow transducer improves significantly on the resolution of current flow devices and uses a differential capacitive pressure sensor to measure the flow. The readout electronics feature a clocking speed of 100 kHz and can drive loads as high as 35 pF. The high d.c. gain of the circuit topology (75 dB) is relatively insensitive to stray input capacitance and is ideally suited for a multichip sensor realization. The uncompensated linearity of the overall readout circuit is 10 bits and the pressure/flow resolution is 12 bits. Since ultrasensitive membranes respond to electrostatic forces, the output is characterized as a function of the duty cycle and pulse width of the readout clocking waveforms. The membrane does not respond to these waveforms for high frequencies ( > 50 kHz), but for lower frequencies the diaphragm deflects in response to the time-average voltage applied across the device. A self-test mode can therefore be implemented simply by changing the duty cycle of this pulse. By modifying the amplitude of the waveform, the device can be autocalibrated over a limited pressure range. The transducer and circuitry have been integrated into a flow package, and the multichip device has been tested versus a calibrated gas flow.