wrong readings when there is more than one channel with LabView Signal Express

[icarus-iii]

Hello I have DAQ 6251 and BNC2120, I want to collect two voltage readings from a very simple circuit which has  two resistances in series and a DC voltage. I am using LabVIew Signal Express to collect and save the data. The issue I am facing is, when I colllect the data one channel at a time, the voltage readings are right; however, when I try to collect data from two channels at the same time, one of the channel reading becomes wrong. After building the circuit on breadboard, I am getting two wires from two nodes in the circuit and inserting them into the two channels on BNC-2120 and also connecting the ground to AI-GND on BNC2120 and I also switch to GS mode in the BNC2120 for those two channels. Has anyone any idea what is going wrong when I try to collect data from two channels at the same time? I did check the waveforms from these channels in TestPanel of Measurement and Automation Explorer and the waveforms look okay there individually (but in TestPanel I can examine the channels one at a time and not more than one channel at the same time). Please note that I am choosing RSE as the terminal configuration in both two channels in LabView Signal Express.

Thanks

[Kyle_M]

Icarus-iii

Can you try taking data from a pair of batteries at one time? this will check to see if the issue is in the 6251 or the circuitry/wiring. If it doesnt work as expected, try a couple other channel to make sure you dont just have a problem with a specific channel. 

Regards,

Kyle Mozdzyn

Applications Engineering

National Instruments

[icarus-iii]

Hello Kyle,

It looks like the problem I am facing is ghosting effect since with lower sampling rate, the DAQ readings start to become more accurate; however, I do need to make the data collection at higher sampling rates and the channels of interest will all have high impedances; besides we may not use voltage follower type of circuits between DAQ and those nodes of interest (to reduce impedance); are there any other solutions to have accurate readings for multiple channels under these circumstances with this DAQ device (6251)?

Thanks

icarus-iii

[Kyle_M]

Icarus,

Did you see similar issues when running a simple battery test, or does it only happen when on your circuitry? Lets make sure this isnt a device issue so we can then troubleshoot the circuitry. Also, do you see the issue if you hook it up in differential mode with the ground tied to the com line? 

Regards,

Kyle Mozdzyn

Applications Engineering

National Instruments

[icarus-iii]

Hi Kyle,

I did the testing with the battery and first used a voltage divider circuit (two resistances, 100 ohm each) and collected the voltage data around the two resistances separately in differential mode; by differential mode, I mean in the software part (LabView Signal Express), I choose the "differential mode setting" in the acquire setup; and in the hardware part, I connect the two alligator terminals (red-black) of a "BNC-to-alligator cable" to the two nodes of the resistance and the BNC connector side of the cable to the BNC-2120 analog input (repeat this for the other resistance as well); this way (using 100ohm resistances in the voltage divider circuit) I do read almost equal value voltage readings from the two differential analog voltage inputs via the DAQ which seems to be okay; however, when I use 1 Mohm resistances in the voltage divider circuit, then I do not read correct readings from the two channels; additionally, the waveforms of the two channels differ from each other considearbly. When I first posted this question, I was using resistances with high impedances like 10Mohm and 1Mohm; I suppose this must be indicating a ghosting issue. Please note that for ground connection, I am using "AI-GND" on BNC 2120, and selecting GS on the BNC2120. So, in order to collect data from multiple channels but each channel having high impedances, what could be the best way to do in order not to be affected from the ghosting issue with the PCI-6251 DAQ card?

Thanks,

Icarus 

[Kyle_M]

Icarus,

Im somewhat confused as to why youre using a voltage divider to begin with. We should step back a minute and discuss what the end goal is youre trying to achieve. What is this DC voltage were measuring, and what is the purpose of the circuitry? Is there any reason we cant simply hook up the signal into the 2120 and read it off that way? 

Regards,

Kyle Mozdzyn

Applications Engineering

National Instruments