When the 14B-12B pair opens, 12B is connected to Al's summing point via 13B. The sensor now discharges into the summing point through the 1 Âµ capacitor. Since the charge voltage is fixed, the average current into the summing point is determined by the sensor's humidity related value. The 1 Âµ value ac couples the sensor to the charge-discharge path, maintaining the required zero average voltage across the device. The 22M resistor prevents accumulation of charge, which would stop current flow.
The average current into Al's summing point is balanced by packets of charge delivered by the switched-capacitor gives Al an integrator-like response, and its output is dc.