How can I minimize counter interference on analog input channels.

[LoKey]

I am counting a 5MHz TTL signal as well as doing multiple analog measurements with E-series DAQ, SH68-68 EP cable, CB68, etc. I am experiencing some interference (tens of millivolts and up) on the analog input channels resulting from the TTL signal simply being connected to the counter source. I have 'rounded up all the usual suspects' and of course tried all 3 analog input modes. I have also tried using different return lines for the TTL signal etc. etc. All to no avail. I hav found no information that defines which return lines should be used for the counter sources.
The one thing I have not yet tried is to terminate all unused analog input lines as I have not yet determined the best way to terminat
e them. (short to AIGND, 100 ohm to AIGND, short to AISENSE if in NRSE mode, etc. etc.)
The best effect I have seen to date is to simply add 200 ohms series R at the counter source pin connection on the CB68 connector block. While this is a significant improvement, it still does not allow 'normal' analog input measurements.

[Russell_G.]

Hi Paul

You say that you have tried all three grounding methods. Does one method provide cleaner signals than any other method? Usually differential mode will provide better noise rejection on your analog input channels since they would not be sharing a ground with the counter.

Do you get the same noise levels in MAX as you do with LabVIEW?

Here is a link that may help with some of your noise issues.


Let me know if you have any other questions.

Russell G.
Applications Engineering
National Instruments