4-7
Each input channel is provided with a switchable response filter. This is a
single-pole filter with a time constant of about 5ms. You must use this
filter when handling AC inputs, as these inputs periodically pass through
zero at the voltage crossings, and the logic output is not sustained at these
instants, as shown in Figure 4-7.
Figure 4-7. Unfiltered Opto-Isolator Output with AC Input
The filter “holds” the output long enough to prevent discontinuous logic
readings from occurring at the zero crossings of an AC signal. With DC
signals, the use of the filter is optional and can provide a useful amount of
noise rejection (for spikes or contact debouncing) at the expense of
response speed. Expect a 5 to 10ms delay when the relay turns on and off.
If the filter is not used, the channel has a response controlled mainly by
the delays in the opto-isolator; expect a “fast” response with a turn on/off
delay of around 50µs.
Volt
+5 V
0
Time
