The comparator significantly influences the quality of the measurement. It is not integrated in the PSØ21. It is possible to get results of outstanding quality using a very simple bipolar comparator circuit as shown in the figure. The circuit is very insensitive to low supply voltage. When driving the chip with Vcc and Vio at the lower limit of 1.8 V, the absolute noise is still less than with 5 V - a proof of the power and robustness of this simple and low-cost measuring principle.

This comparator with its 5 transistors in parallel shows the lowest noise. In many applications 1 transistor might be sufficient. It has a quiescent current of about 180 µA at 3 V supply voltage.

The PSØ21 can fully control the comparator on time. The comparator current can be reduced at low resolution and low update rates to a few µa allowing even solar cell driven systems