Analysis of interference rejection in period-to-code converters

Period-to-code converters (such as universal counters and timers embedded in microcontrollers) are susceptible to interfering voltages present in either the input signal to be measured or in the internal voltage threshold of the counter. The interference effects are specially important when measuring signals with slow slew rate. The resulting standard deviation of the measured period depends on the amplitude and frequency of the interfering voltage. This paper provides a model that explains why an interfering frequency equal to that of the signal being measured or a multiple of it, has a minimal effect, but interference whose frequency is k+0.5 times that of the input (k being any integer) has a maximal effect. Experimental results quantitatively validate the model.