Bridge-output-to-frequency converter for smart thermal air-flow sensors

A fully integrated bridge-output-to-frequency converter has been realized as signal-conditioning circuitry for a one-chip smart thermal mass-flow sensor. The frequency converter reads out four separate resistor bridges of the air-flow sensor in order to realize high sensitivity and directional flow measurement over the full angle range of 360°. The converter selects the sensor bridges individually by means of a microprocessor-controlled analog multiplexer. The flow velocity and direction are also calculated by the microprocessor. The relative measurement cancels all first-order errors. The center frequency is 10 kHz and the sensitivity 1 Hz/(μV/V). A bridge-offset of 10 mV/V is allowed, and the linearity error is 0.1%