edge counting inaccurate

[goldenrad]

I'm getting inaccurate results from my USB-6210 edge counting check from MAX. I have a TTL source that generates four pulses that is connect to PFI0 through a simple RC filter (390 ohm and 0.02uF). I start the edge counting from the test panel in MAX choosing PFI0 as the edge source. When I trigger my four pulses I receive a much larger count (sometime up to 20 counts) from the MAX test panel. Only when I remove the RC filter do I receive an accurate count (4 pulses exactly). I need to use the RC to filter out some 45KHz noise generated by another piece of equipment in the lab. I know I can set up the USB-6210 to use a time-based filter but I thought a simple RC filter would be a quick solution.

I have tried this with a USB-6211 with the same results. Only the USB-6009 preformed as expected with accurate counts. The USB-6009 edge count is not affected by the RC filter. 

Any thoughts?

Thanks,

[johnsold]

A filter like that produces a very slow (by logic standards) "edge." The time constant of the filter is about 8 us. Logic devices like risetimes in the nanosecond region. Logic devices have a linear region and while in that region very small amounts of noise, including internally generated noise in the logic IC itself, can generate false outputs. If the time-based filter will work for the frequencies involved, it is almost certainly the better choice.

Lynn

[goldenrad]

Agreed, but I'm assuming that the input should have a hysteresis mechanism (schottky) so that it would be unaffected by the noise in the linear region. It is interesting that the filter works perfectly on the USB-6009.

[John_P1]

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@goldenrad wrote:

... I'm assuming that the input should have a hysteresis mechanism ...

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It doesn't :(.  You should use the digital filter.

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@goldenrad wrote:

It is interesting that the filter works perfectly on the USB-6009.

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It's probably a bandwidth issue, there's no hysteresis on the 6009 inputs either.  The 6009 event counter can only count up to a 5 MHz signal according to its specificaitons.  The 6210 however supports an "external base clock frequency" of up to 20 MHz.  

Best Regards,

[goldenrad]

Okay, thanks for the information. I'll implement the digital filter.