analog input crosstalk - ni mydaq

[uglyoldbob]

I have noticed a problem with my ni MyDAQ when measuring two analog signals.

I have grounded the - terminal of both signals. On channel ai0 I apply the output signal from an LM35 (at about 23 degrees C). On the other input I attach a 5V square wave.

Using the oscilloscope VI to measure both channels, the second channel bleeds over very heavily to the first channel (at about 0.25V). When I tell the oscilloscope to stop looking at channel 1, the problem disappears.

If I reduce the frequency of the square wave to about 1kHz, the problem disappears until I increase the sampling rate. The crosstalk is nowhere near as severe in this case, but it is still present at high sampling frequencies.

[Beutlich]

Since the operation of the device is fine except when the oscilloscope is attached, it sounds like the oscilloscope is just not grounded to the same ground as the MyDAQ, which can cause noise. There is a good article that talks about noise and crosstalk considerations:

Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels.

[uglyoldbob]

I don't think that is the problem. Take a look at these three screenshots. The hardware configuration is the same for each of them. The only thing changed is the virtual instrument settings.

[T-REX$]

Hi Bob,

Have you checked the Field Wiring Guide to make sure that your devices are being connected appropriately to avoid noise? Also, how long are the wires to and from your devices? Are you sure that the impedences are properly matched?

[hs713]

I know this message was from a long time ago, but did you ever resolve the issue? I'm experiencing similar issues and wondering what you eventually did to fix it. 

[uglyoldbob]

I dont't really remember. I still have the mydaq. I'll grab it when I get a chance and recreate it. Message me here or at uglyoldbob.com/forum in case i forget.

[hs713]

Goodness I didn't think you would reply! Thanks very much for the response. If you do get a a chance, I'd be very interested to hear what you did. Out of curiosity is your MyDAQ a multiplexer (i.e. scans across channels with the same A/D converter) or a simultaneous DAQ with multiple A/D converters? (My issue is with simultaneous).

[byron12]

220 pF between all 4 inputs and AGND helped me.  There is still 1/20 as much crosstalk and the frequency response is limited.  The myDAQ AI seems to need low impedance sources to supply charge to the sample and hold cap after multiplex switching.