How can I measure DC offset with the PCI 6110?

[Tstin1227]

Hello All,

I am having a frustrating problem with the PCI-6110. I have the BNC-2110 breakout interface, and I have a signal on a BNC that I am connecting to one of the two analog inputs. The BNC-2110 input switch is set to floating source, so the ground of the BNC should be tied to the AI GND. However, the MAX test panel (and any VI I write) cannot detect a change in the DC offset of an analog waveform. It registers a sine wave and changes in that wave's amplitude, but no change in the DC offset.  I guess I am confused as to how I can use the BNC-2110 to measure a DC signal with the PCI-6110. Any help you have will be greatly appreciated.

Sincerely,

Ted

[JoshY]

Hi Ted,

How are you generating this DC offset? Are you able to measure a voltage from say a power supply or a battery? Here's an article that might be helpful in terms of wiring and measuring analog signals.

[Tstin1227]

Hi Josh,

Thanks for your reply and the link. I can generate a DC signal a number of ways. The first is a DC output through a BNC cable from a temperature measurement device I am using. This signal may be floating with respect to the DAQ card ground. The second is a function generator; it can generate a sine wave and add a DC offset the the waveform. Again, this signal may be a flaoting source. I am interfacing to the PCI 6111 with a BNC 2110. Supposedly this has an option for FS signals; it ties the signal ground to the DAQ ground. However, this switch on the BNC 2110 has no effect. The test panel in MAX registers the AC waveform, but does not register when I change the DC offset. Similarly, the MAX test panel doesn't see the DC signal from the temperature sensor at all.

Thanks,

Ted

[Dennis_Knutson]

FS mode does NOT connect the shield to gnd. GS mode does. Look at figures 2 and 3 of the manual.

The card also has configurable AC or DC coupling. What have you chosen?

[Tstin1227]

Ah THAT is what I need to know. I do not know how to configure the card for DC coupling. Cna someone direct me to this information?

Thanks,

Ted

[Tstin1227]

Also, maybe I am reading this diagram incorrectly, but that looks to me like the sheild is conencted via a parallel resistor-capacitor to AI GND

[Dennis_Knutson]

And in GS mode, the shield is directly connected to gnd - not through a cap.

In LabVIEW, you can select coupling with a DAQmx Channel AI.Coupling property node. In MAX, if you create an analog input task, the coupling is on the Device tab of the Voltage Input Setup. I don't see where it is selectable on the test panel.