Measurement Principle

• Principle
• Background

The capacitor is charged to the supply voltage and then discharged through one of the SG resistors. The discharge time down to an arbitrary trigger level is measured with ultra-high precision using a TDC (Time-to-Digital Converter). The discharge time is in the range 100 µs. The TDC unit used have a typical single-shot resolution of less than 20 ps.

This measuring process is repeated in time-multiplex with both resistors of a half-bridge, using the same capacitor and the same comparator. Calculating the ratio of the results will turn out the absolute values and temperature dependencies of the capacitor and the comparator.

Additional patented circuits and algorithms inside the PICOSTRAIN products compensate for further error sources like the switch-on resistance of the output drivers (R_dson) and the propagation delay of the comparator. The result is very precise, nearly free of gain errors and very stable with temperature. In total each single measurement is made of 8 discharge/charge cycles to solve this compensation task.

Due to the measuring principle, PICOSTRAIN does not need a full-bridge but a half-bridge is sufficient. The supply of the half-bridge is provided directly by the PICOSTRAIN circuits. There is no need for a separate supply of the SG. Also the reference voltage is not required.

Thanks to the pulsed drive PICOSTRAIN easily controls the current through the whole system and, even more important, reduces the current consumption to re-markably less than comparable ADC systems.