Digital measurement of impedance ratios

Claim 1

A circuit arrangement for measuring the ratio of the values of first and second electrical capacitances, and arrangement comprising: (a) means for switching a supply voltage to provide an alternating supply voltage to each of said first and second capacitances, (b) synchronous demodulator means, (c) amplifying means coupling said capacitances to said synchronous demodulator means, (d) said synchronous demodulator means being connected to demodulate the alternating supply signal transmitted from said capacitors, (e) integrator means connected to receive the demodulated signal to provide an integral signal representative of the time-integral thereof, (f) control means providing a control signal to control said switch means, said control signal having first and second values whereby to provide a cycle of operation having two stages, namely a first stage wherein said alternating supply voltage is applied to said first capacitance for a first predetermined interval of time and the value of said integral signal increases from a threshold value to a value dependent on the value of said first capacitance during said first time interval and a second stage in which said alternating supply voltage is applied to said second capacitance and the value of said integral signal returns to said threshold value in a second interval of time, the value of said second interval being dependent on the value of said second capacitance, and (g) means for determining the value of said second time interval relative to said first time interval to provide an indication of the ratio on the values of the first and second capacitances.