A high-performance microflowmeter with built-in self test

A high-performance readout scheme based on a switched-capacitor circuit and intended for use with an ultrasensitive microflowmeter was developed. The microflow transducer improves on the resolution of current flow devices and uses a differential capacitive sensor to measure the flow. The readout electronics features a clocking speed of 100 kHz and can drive loads as high as 35 pF. The high DC gain of the circuit (75 dB) nulls out stray input capacitance. The uncompensated linearity of the overall readout circuit is 10 b and the pressure/flow resolution is 12 b. The output was 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 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 by changing the duty cycle of this pulse. The transducer and circuitry have been integrated into a flow package, and the multichip device has been tested versus a calibrated gas flow.<
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