The circuitry including the variable capacitance sensor is excited by an external DC power source. A portion of the direct current power is converted to a high-frequency alternating current which is in circuit with the capacitance sensor and individual rectifying means for each of the capacitances. The rectified currents from the capacitances are compared to a reference current and the resulting signal together with a measure of all the other direct currents used by the circuit provide a signal to control the total current supplied to the circuitry. The total current then bears a prescribed and dependent relation to the capacitance of the sensor. The rectifying means form separate rectifying loops each of which is driven by an AC excitation winding. These rectifying loops are used in a special circuit configuration with a capacitance pressure sensor to provide a highly linear output signal as a function of differential pressure.