Measurement Computing WBK17 - Manual (Page 6)

WBK17, pg. 6 987996 WBK17, Counter/Encoder Module
Comparator
Each channel has its own individually programmable comparator as shown in the figure below. The output
of the filter stage feeds the comparator circuit. The analog waveform that is present at the comparator is
gain adjusted and then multiplexed onto the WaveBook’s analog expansion port. This allows the analog
input waveform (as it appears at the comparator, post coupling, post filter) to be scanned like any other
analog waveform. The analog path going to the WaveBook is bandwidth-limited to 450 kHz. The analog
path between the input and the comparator is not bandwidth-limited unless a low-pass filter is used.
Comparator
The comparator has a programmable threshold, set by the DAC. The threshold can be set anywhere from
–12.5V to +12.5V in 100mV steps, referred to input. Since the counter output and analog waveform can
be scanned together, the effects of different comparator switching thresholds can be easily observed. This
allows easy adjustment of the comparator-switching threshold based on input waveform characteristics such
as noise and ringing.
The following diagram shows two common input waveforms: a square wave that has some ringing and a
sine wave. The comparator threshold should be set so that the ringing on the square wave does not cause
extraneous switching of the comparator, causing false counts to be measured. Ideally, the comparator
threshold should be set so that the comparator switches at the point of fastest slew rate on the input
waveform. This occurs in the grayed regions of the waveforms.
Amplitude modulated noise may also cause false switching of the comparator. The effects of amplitude
modulated noise can be minimized by setting the threshold at the point of fastest slew rate on the input
waveform. The sine wave shown below has its fastest slew rate within the gray region.
Set the comparator threshold within the gray regions to avoid the effects of ringing and noise.
Setting the Comparator Threshold