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Need a clarification in Counter measurements

Dear all,

 

Overview: 

I am using NI PXI 6722(Multifunction DAQ card) to count the no of pulses generated from source.

it has two counter I/P channel. I configure "Count edge" to count the no of pulses generated.

http://www.ni.com/pdf/manuals/370735f.pdf

 

My source signal Frequency ranges from 1HZ to 10 HZ. signal waveform is Square waveform. Low state is 0V and High State is 5V

 

Issue : Getting extra no of pulses than expected. (Ex :For 10HZ, Expected 600 pulse/min but actual 800+ pulse/min)

 

Observation/workaround : 

NI PXI 6722 has two Onboard clocks(100KHZ and 20MHZ). I am not using any external clock so far for this requirement.

I assume, By default the card is taking 100KHZ clock as ref clock. So it is looking for the change for every 10us. 

I tried with three different sources with the same frequency(10Hz). Two sources out of three sources rising time is less than 10us which counts(600/min) are perfect as per the frequency. The third source (Actual source)raise time is 50us so its giving extra no than expected.

 

I assume, this issue will get over if I use 50KHZ or less than 50KHZ as a clock ref.

 

Please suggest me whether my observation is making sense or not. 

 

Thanks,

Kumaran

 

 

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Why not just use a 74LS14 Schmitt trigger to clean up the pulses before presenting them to the 6722?

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