60hz signal coming from Analog Input

[GlennPiper]

Hello All!

I am using a PCI-6110 and noticed that if I connect a o-scope up to one of the analog inputs I can see a roughly sinusoidal 60hz >100mV signal on the scope screen. Once my application exits, however, the signal stops.

In my app, I am setting up the 4 analog channels to use an external 5Mhz sample clock and an external gate signal. These two signals are generated by another board and piped-in over the RTSI bus. Because the sample clock can not be pulled from the RTSI on this card, I send it out to the connector block, jump it to the proper PFI on the block, and then it is send back to the card. As you may expect, this signal appears to couple to the Analog Input a bit, but only as long as I leave the scan clock running. The signal I am trying to hunt down now is only present once I turn the clocks off.

When finished acquiring data, my app uses the AIClear VI to reset the Analog portion of the card, and uses the Counter Stop VI and Counter Control VI to stop the counters. After calling these VI's is when I see the signal in question.

I should also mention that I am using a BNC-2110 connecting block, and that the ground source switch is set to FS. If I switch it to GS, the signal on the scope gets MUCH larger and is a much cleaner sine wave.

Does anyone have any ideas what may be the cause of this signal and how I can get rid of it???

Thanks!

Glenn

[sergesl]

Hello Glen,

The first thing that comes to mind is that this may be noise from the power outlet (60 Hz). Now, the question is how are you getting it?
The first thing I would like you to try is perform your acquisition with everything internal. i.e. no external scan clock and check if you still see the 60 Hz signal. If not, then you are sending it in somehow with your wiring. I doubt though that this is the case. I am leaning more toward some kind of noise introduced by the device you are measuring from. You said that when you selected Floating Source from the BNC-2110, you had a smaller noise amplitude. I would recomment setting it back to GS and using bias resistors are described in the following tutorial..
Also, you said you connected the scope to an analog input line. During your acquisition, you did not see the 60 Hz noise you said. That means it is not really affecting your acquisition. If this is the case, then you should not worry about it.

[WM_John_Weeks]

Serges seems to have read the post the same way I did. It sounds like the scope is causing the problem. It might be that connecting the scope probe shield is creating a ground loop.

[GlennPiper]

After exploring this a bit more, I concur. If I use a virtual o-scope VI monotoring, say channel 1, and connect that channel to the physical o-scope, I can see the signal on both "scopes". If I disconnect the physical 0-scope, the signal on the virtual scope goes away.

The odd part is that if I keep the physical scope connected, and simply exit all of my VI's, the signal on the physical o-scope goes away!

At one point yesterday I measured the signal and saw that the noise was a 20mhz signal. I am wondering if this is some kind of coupling from the internal clock on the card? I would hope not. I do seem to remember that the frequency was lower when I measured it in the past, and since I was using a newer o-scope yesterday than in the past, it may just be a coincidence. That being said, if I enable one of the counters to run as an oscillator, I can see that it disturbs the analog input (as long as the physical scope was connected), so it may related after all.

As I was messing around yesterday, I felt like I was chasing a ghost, but got the strong sense that the issue is a ground-loop of some kind. Sadly, I was not documenting all of the different ways I altered the connections and software settings, making it difficult to "see" a pattern, so I plan to do that next time. We are planning on moving the system to it's "home" soon, however, so I think I will wait until then so that I don't get stuck doing this all again once it has been moved. For now I am just planning on keeping the o-scope away from it and going with the belief that it is clean when left alone 🙂


Thanks for the help guys!

Glenn

[GlennPiper]

Serges,

You mentioned using bias resistors and setting the ground switch to GS... Thanks for this tip!
I just re-read the manual for that connection block and discovered that I mis-understood these switches. I had thought the DAQ inputs were floating in the FS position. Oops...

Thanks!

Glenn

[sergesl]

You are welcome Glenn...