ni daq PCI-6221 same data on all channels.

[bh3560]

I have a DAQ PCI-6221 with an SCB-68 connection box. I have 9 channels configured to acquire varying dc voltage. I have it configure for analog input with a RSE ground. The problem is that with channels 2-9 unconnected the data from channel 1 is appearing on all the channels. I have tested the individual channels with max and it seems to work correctly but when you use the DAQmx or DAQ assistant the problems occurs. I am using LV 8.5.1 XP SP3. I have used the USB 6009 in the same configuration RSE etc and it worked fine. I hope someone can help me.

Thanks,

Brett

[Christian Mergl]

Hey Brett,

What is your Signal Source?

Christian

[bh3560]

Christian,

The signals are coming from 9 different 8mm vcrs. I am taking the RF signal and sending it through a rectifier to convert the RF envelope into a DC voltage. The signals are currently being recorded onto chart recorders. I am trying to upgrade the test from an old basic program to LabView and use LabView to record the signals and generate the graphs. Today I am going to try setting up the USB-6009 to see if the makes a difference. The reason I am not using this model is there are not enough analog inputs but I can use enough to see if there is a difference. I will post my findings.

Thanks for your reply,

Brett

[Christian Mergl]

Hey Brett,

Is your Signal Source also ground referenced, or does it have a very high impedance?

Because maybe you have a kind of a ground loop which causes the problem (which could not show up for USB DAQ) or the Multiplexer is not able to get rid of the voltage potential on his inputs because of the high impedance of the Signal Source which could cause the wrong measurements.

Another thing you could try is to leave one channel after the first one open and connect it to the cards GND, so that a possible voltage on the MUX could go away.

Christian

[bh3560]

Christian,

The signal has a low resistance with power turned off. It is about 15k ohms. I disconnected all of the inputs and ran test panels in Max and all the channels have the same voltage. They should be at zero at least I think they should be with no input. I did ground the next channel and it did break the chain but then I don't have enough inputs. One thing that was tried is I disconnected all inputs except one and it is hooked to a straight dc source and all channels still showed the same data. The dc source we use to calibrate the chart recorders it is a  Datel voltage calibrator. One other note is when you take a multimeter and actually measure channel 1 the voltage on all the other channels goes down. Maybe I need to put a pull down resister on each channel. 

Thanks,

Brett

[Christian Mergl]

Hey,

You could use NRSE as terminal configuration instead of RSE and then connect the AI Sense with a resistor (~10-100k) to AI GND. Can you test this?

Thanks,

Christian

[minnellac]

In my experience, this is typical for this hardware. If you terminate the other channels, you won't see the crosstalk. Also, if you actually connect the other channels up to the daq card you shouldn't see a problem unless the source impedance is too high. If the source impedance is too high, and you are seeing signals from one channel influence the next channel, you can do a couple things to reduce the problem. One thing, that I believe was already alluded to, is terminate every other channel (including the last channel if your sampling rate is high enough that there is little time between sampling the last channel in one scan, and the first channel in the next scan). Alternatively, depending on your sample rate, you can increase the interchannel delay. This will give each channel a greater settling time, and let the excess charge bleed off; you may need to decrease the sampling rate to get adequate performance. Third, you can install unity gain buffers on each channel between the signal source and the DAQ card. I'm sure there are more possible solutions (including using higher quality DAQ equipment), but there are few to get you started.

good luck

Chris

[bh3560]

Chris,

What would you recommend on how to terminate each channel. I am not sure how to do this. Would I place a resister across each channel?

Thanks,

Brett

[bh3560]

Chris,

You are right about having all the channels hooked up to the signal source.  I had failed to hook up all the channels that I had mapped out in the DAQmx. This was a problem I will never forget. I want to thank you and Christian for your time in helping me.

Thanks,

Brett

[Brooks_C]

Hello Brett,

Just for completeness (and other forum users) I believe that this behavior called ghosting as described here:  Incorrect Readings when Scanning Unconnected or Open Channels

I'm glad you got everything working!

Cheers!