Abstract: Apparatus for producing output signal changes corresponding to fluid pressure changes includes:(a) a transducer responsive to applied input pressure changes to produce capacitance changes at a variable capacitor,(b) a first circuit to produce a cyclic wave, and connected with the variable capacitor,(c) a second circuit including a switch to connect the capacitor to ground when the cyclic wave is at a first level, thereby to charge the variable capacitor, and to connect the variable capacitor to a second capacitor when the cyclic wave is at a second level, whereby the charge becomes distributed on the variable and second capacitors, and whereby a sawtooth signal is produced at a junction associated with the capacitors,(d) an operational amplifier electrically coupled with that junction for integrating the sawtooth signal to produce a first output voltage,(e) the first circuit including a voltage controlled oscillator having an input side coupled with the output side of the operational amplifier thereby to prod