Change PXI-6115 channel to DC coupling

[John_Swierk]

I have a PXI-6115 and it appears that the channels are set to AC coupling.  For my system, I need to have the channels set to DC coupling.  I have a signal coming off a chopper at 5Hz, about 1V, and a signal coming off of a photodiode at 10Hz, also around 1V.  The should be square waves and are when I look at them in a DC coupling oscilloscope, but it looks like the PXI-6115 is AC coupling them.  I'm using a SCB-68 breakout box with a 50ohm resistor on each channel.

When I collect the signal through DAQ Assist, I still see the AC signal even when I change the coupling there.  Which makes me think the AC coupling is happening at a more fundamental level.

Any ideas on what I can do?

[Kevin_F]

Hey John,

Take a look at this example for configuring the AC/DC coupling of a particular channel in software: http://digital.ni.com/public.nsf/allkb/0B9BB4D7696CDA9F86256FA30063ECFC.  This method should be the same as setting AC/DC coupling in DAQ assistant but sometimes there are issues with using the Express VI.  I am a little confused by what you mean when you see that you still see the AC signal when changing the coupling.  Figure 4-16 on this DevZone (http://zone.ni.com/devzone/cda/ph/p/id/9) shows what your signal may look like if you are using AC coupling on a sqaure wave.  If this is the type of result you are seeing then this may be the issue you are having.  It would be useful for us to have a screenshot of your code and the signal that you are seeing as an output of DAQ Assistant.  Please post back with your results and any additional questions.

[John_Swierk]

Hi Kevin-

I've attached code and signal screen shots.  Its definitely AC coupling on a square wave.  I've tested this with an oscilloscope and switching the scope from DC to AC coupling gives an identical signal to what I'm seeing through the PXI-6115.  You'll see in the code that I've tried adding the channel property node to set the coupling to DC, but it didn't help, maybe I put it in the wrong place.  FYI, I'm using 3 of the 4 analog channels in the PXI-6115.

Thanks,
John

[Beau_H]

Hey John 

Look at the attachement, it looks like you need to change the Property Node to write, (Right click on the Node and the first option is "change all to write") then you should be able to connect the DC constant to the node.

Beau H

National Instruments

[John_Swierk]

Hi Beau-

I made the suggested change and that did not correct the problem.  Any other thoughts?

John

[Beau_H]

ok well could you post that VI so ppl can see what they get when they attempt to run the program with a simulated card?

thanks

BeauH

[John_Swierk]

Hi Beau-

This is a copy of the VI.

Thank you for your help,
John

[Beau_H]

John 

The DAQ_TEST1_generate.vi is not in that file, you mind posting that vi

thx

BeauH

[John_Swierk]

Hi Beau-

I'm sorry I forgot to add that.

Thanks,
John

[Beau_H]

Hey John

I am not sure what code this is since it is nothing like the code that was posted in you screen shots, did you get the output you were expecting when you ran the sample vi that you modified? 

Could you post the code from your screen shots so we can look at that?

thanks

BeauH