DAQ Assistant and Split Signal Results in Erroneous Values on Second Channel Input

[johannap]

I have used DAQ Assistant to acquire two analog inputs (ai0 and ai2, differential, 1k samples at 1kHz), then separated the signal with the Split Signals. The resulting value coming through ai2 is erroneous. If I run the same program only acquiring data through ai2 (ie. change the DAQ Assistant and remove the Split Signal), I have the correct value - I also have a multimeter connected to the power supply output. I have run similar programs with no issues before. And I have been debugging this problem for awhile; at this point in time, I have written a basic VI that simply sets a voltage on the power supply, reads in two (or one) differential analog input channels, and displays the mean, standard deviation and variance. Any ideas, suggestions or comments on how to fix this would be greatly appreciated. (I am running LabVIEW 8.6)

[johannap]

I wanted to add a few more details. It appears that DAQ Assistant is having trouble acquiring any two signals from two separate channels. Also I saved the signals to an lvm file before splitting them to determine if there was a problem with Split Signal. The error comes from/the problem is in the DAQ Assistant. Whichever combination of analog inputs I use, the second channel is always wrong; however, if the inputs are obtained in separate programs with the DAQ Assistant acquiring only a single channel (with all other factors remaining the same), they are correct. Ideas please!?

[RavensFan]

Do you see a "ghosting" of the first analog input into the data of the second?

If the input impedances of the signals going into the analog inputs are too high, then the amplifier doesn't have an opportunity to discharge from the first analog input when the second analog input is multiplexed in.

What is the source of the analog input signals?  Which DAQ device are you using?

For grins, try reading channels 0, 1, 2, 3.  Hook up your real signals to 0 and 2.  Hook up a wire from the analog input to ground for channels 1, and 3.  See if this affects the readings or improves them at all.

[johannap]

Yes, there is ghosting of the first analog input into the data of the second. As I change the input on ai0, keeping ai2 constant, the displayed value for ai2 changes. 

I ran the test you suggested. The signals of interest were on ai0 and ai2; ai1 and ai3 had both differential inputs connected to ground. The displayed value for ai2 improved; but is still slightly lower than it should be, that is slightly lower than it is when I am only reading ai2. 

I am using a PCI-6229 and SCB-68 screw terminals. The signals open circuit voltage output from two power supplies (Agilent 6645A and Sorensen DLM 150-4); eventually, these will be connected to hardware inside a vacuum chamber to provide an electric field. The signal goes through a voltage divider to protect the data acquisition hardware. 

(Thinking there might be something wrong with the PCI-6229, I also tried using a different one and saw the same results.)

[Brandon_B]

Hello Johannap,

You mentioned that you have the DAQ assistant setup for differential measurements, but then you also mentioned that you are measuring signals between the analog inputs and ground.  I just wanted to make sure that you had your connections setup for a differential measurement.  When measuring with a differential configuration you should connect your signals to two analog inputs.  The idea is to measure the difference between AI+ and AI- (hence the name differential).  This is ideal for floating signal sources such as batteries or sensors.  You mentioned that you were using AI0 and AI2 for the positive inputs, so the negative inputs should be connected to AI8 and AI10.

Also, you mentioned that you were seeing ghosting on your AI channels.   There is an article on our website titled "How Do I Eliminate Ghosting From My Measurements?" that you should take a look at.  Try this out and let me know how it goes.

Regards,

[johannap · Accepted solution]

I have setup the analog inputs properly. There are several reasons for using differential analog inputs, even when one of the inputs is tied to ground. 

The problem, which was the impedance, has been fixed in my setup. Thanks for the advice Ravens Fan. What confused me for a long time was the spec on the board that stated the input impedance of 10GΩ. It wasn't until I eventually found "Decreasing the Source Impedance of an Analog Input Signal", which states "Source impedances greater than 1 kΩ are not recommended with multiplexed data acquisition devices." I was able to redesign my system over the weekend; but I find it annoying that I wasted so much time because this spec was not included in the board documentation. I probably would have purchased a couple simultaneous sampling boards instead.