The encoder fail detect senses illegal voltages on the encoder inputs (1V > illegal > 3V) using window comparators on both encoder channels. This is both a static as well as dynamic measurement so it applies from zero to any RPM. The reaction time is nearly instantaneous (< 2uS) so there is no delay from fault to shutdown.

The comparator levels are set to the minimum acceptable encoder input voltages. The secondary advantage of the circuit is it will reject sub-standard encoders from the get-go instead of allowing them to operate and produce faulty results.

Mariss