What is special about the capacitance measurement circuitry needed for ECT?
The capacitance measuring system must be able to measure very small inter-electrode capacitances, of the order of 10^-15 Farads (1 fF), in the presence of much larger capacitances to earth of the order of 200,000 fF (mainly due to the screened connecting cables). The capacitance measurement technique used in PTL equipment is based on the use of an excitation signal in the form of a 1.25MHz square waveform. The excitation signal is applied to one electrode (the SOURCE electrode) and the currents which flow into the remaining (DETECTOR) electrodes (which are held at virtual ground potential) are measured using a synchronous demodulator. These measured currents are proportional to the capacitance between the SOURCE electrode and the DETECTOR electrodes.
