The 
measuring principle shows a new approach to capacitance measurement. For this purpose the sense capacitor and a reference capacitor are connected to a resistor, forming a low-pass filter. Sense and reference capacitor should be in the same range to minimize gain drift. Practically there is no limit for the capacitor value. Sensors from nearly 0 fF up to tens of nF can be measured. The devices also support differential sensors with inside linearization.
The capacitors are charged to the supply voltage and then discharged through the resistor. The discharge time down to an arbitrary trigger level is measured with ultra-high precision using a TDC (Time-to-Digital Converter).
The recommended discharge time is in the range of 2 to 10 µs. The TDC unit used in the PSŲ21 has a single-shot resolution of typ. 15 ps which can be increased up to the Femtosecond range using averaging.
This measuring process is repeated in time-multiplex with both Capacitors, using the same resistor and the same comparator. Calculating the ratio of the results will turn out the absolute values and temperature dependencies of the resistor and the comparator.
