Grounding Problem?

[Water]

Hi,

I have a query.

Recently, I have gotten a NI USB-6212 in replacement of the old PCI-6133. I have intended to use both the analog output and input from the USB NI card. The analog output seems to work fine. But I have some problems working with the analog input. There seem to be some sort of square pulse noise readout from the analog input. The analog input is connected to an amplifier board which amplifies the signal from a PMT. The signal readout from the analog input is shown in Photodetector1 and PhotoDetector2 bmp. This is taken from the program that I have written. It seem that the cycle/square pulse repeat itself at every 50Hz. BTW, I am using differential input from AI5

At first, I believe there is some kind of interference from the analog output but the noise still occurs after shifting the analog input to some other physical channel.  We have kinda of also rule out the possibility of the square pulse coming from the detector or amplifier board as we have also connected the signal readout to an oscilloscope. It seems to be working fine.

Kinda rule out the problem with the software program. I have been previously using the same program for PCI-6133. It has been working fine. Have tried to reconnect the wires back to the old board PCI-6133. It seems to be still functioning as per normal without the above mentioned square pulse.

Another verification test that I have done is to connect the analog input of USB 6212 to a function generator. I have tried to read in a 5KHz sinusoidal signal. It seems to be working fine. No presence of the square pulse is shown. The readout shown in FunctionGenerator bmp

I am wondering whether this problem could be attributed to some grounding problem. Would like some additional guidance on what could be the problem and how could we solve for it?

A million thanks for your kind help! (All pictures found in Figure.zip)

Message Edited by Water on 06-02-2008 09:17 AM

[Steve_B]

Hi Water,

What is your expected signal supposed to look like? I am also interested to know if you are making a multi-channel measurement. A major difference between the 6133 and the 6212 is that there is a multiplexor that switches between the channels on the 6212 because there is only one A/D convertor, while the 6133 had an A/D convertor for each channel. There could be an issue of ghosting if you are making a multi-channel measurement. I have included a link to a KnowledgeBase article that addresses the issue. If you could provide a screenshot of this portion of your code it could make it easier to see how the Analog Input is configured.

How Do I Eliminate Ghosting From My Measurements?
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YHy6CAG&l=en-US

[Water]

Hi,

Thanks for the help.

THis is the configuration I am setting for the DAQ AI measurement. I am setting up the USB 6212 for 2 Analog Output and 1 Analog Input. The two task are separated and the setup diagram are arranged into sub vi. The diagram shown is the configuration sub vi for the the USB 6212 Analog Input

Thanks

[Water]

Hi,

Managed to solve for the problem... connected the AI+ and AI- with 100k resistor routing to AI GND. And the square pulse signal seems to be gone. Will observe for a few more trial. Guess my source impedance is pretty high henceby causing this wierd problem

Thanks for your help!