Excessive noise with PCI6143 and hi impedance input

[Jack K]

I have a PCI6143 installed in a Windows XP computer for evaluation right now. The test I'm evaluating for is reading up to 5 DC signals from battery cells at up to 1Ks/s. For calibration and verification purposes I need to use a voltage calibrator on the inputs. When I read the cells there is no problems. When I use a calibrator I get a significant noise in the signal. It only affects the channel the calibrator is hooked up to and not the rest and it's only at sample rates of 100s/s or higher. The noise is not AC. The pattern is usually 10 samples at the right voltage and 5 at 0.5V less. It doesn't seem to change this pattern for sample rates up to 100ks/s.
Is there a problem with the input impedance mismatch here? I've used different calibrators and power supplies with the same results. I've checked their outputs on desktop DAQ devices at 100ks/s and there is no appreciable noise.
Any thoughts or possible solutions?
Thanks

[Otis]

Hi Jack,

Some of your wording confused me a bit. When you were mentioning that you were using a 'calibrator' on the inputs. Do you mean that the PCI-6143 is the calibrator or is some other device doing the calibration?

One thing you mentioned is that the level seems to be high for 10 samples and low for 5 samples. So you are saying that the time of the cycle is independent of the sample speed?

One last thing to try is to take the measurements with different channels. Does the behavior remain the same? Is it the same for all of the channels that you are reading data?

Regards,

[Jack K]

Otis,
By 'calibrator' I mean a precision voltage source. Since we can't use the actual battery cells for the test run we normally use a source like this to check the voltage readings each day prior to testing. The output impedance of the calibrator is <10 milliohms, which is pretty close to what the cells are. When I use another power supply to check the voltages with a higher output impedance (1kohms) I still get the problem. The PCI-6143 is used to measure the voltages of the cells during the test. It measures the cells perfectly but we need to verify that once per day with a precision source during a test run (customer requirement).

Yes the cycle is pretty much independent of the sample speed above 100 samples per second. It always seems to be 10 samples correct with 5 samples low. Below 100 S/s it doesn't seem to be there at all.

It happens on any channel the calibrator is attached to. I have tried it alone on each channel and simultaneously with as many as 6 cells each on various channels and it still happens.

Another thing is that if I do the same test using a benchtop DAQ (i.e. Nicolet Vision Xe) it doesn't happen. We are trying to move toward simpler test setups and having a card in the computer is easier than hauling around an external device with its keyboard and mouse and something to set it on along with all the data collection headaches. Plus the internal card is much cheaper and easier to use for most of our testing.

Thanks,
Jack

[Marni]

Hello Jack,

When you read from your cells there aren't any problems, but when you read from the calibrator there is a problem?

The sample rate doesn't matter with the battery cells, correct?

Is the calibrator battery powered or wall powered?

Are you reading your signals in as differential, referenced single ended, or non-referenced single ended? Do you get the same results if it is read in as a differential signal?

Do you see these results in the Measurement and Automation test panels?

Please let me know if you have any questions. Have a great day!

Sincerely,
Marni S.

[Jack K]

Marni,
That is true, the cells give no problems but the supplies do.
The sample rate doesn't do anything as far as interference on the cells. Even with the supply on a channel and the cells on the rest there is no noise with the cells, just the supply.
The calibrator is wall powered. It is a standard (Datel DVC 8500) that we've been using for years and I've seen it used in many other labs too.
I'm only reading the signals in differential mode. I'm not too sure this board can do single ended as it's a simultaneous sample board with an A/D for each channel. We need to use differential in the readings because there is no common point in the cells. They are all being tested individually just at the same time.
Yes, the results show up every where I test it. I tried things like 1000 samples per trigger and 1 sample for 1000 triggers just to see if it was a triggering problem.
I've tried the computer and calibrator plugged into the same power strip and also completely different outlets (checking for grounding problems) and have even run a simple grounding strap between the cases of the two.
The board will run the actual test perfectly but it's the dummy run required by the customer that it won't do.
Thanks for all your help,
Jack

[Marni]

Hello Jack,

Do you get these results in the Measurement and Automation Explorer? Do you get the same results with 0 V from the calibrator? Have a great day!

Sincerely,
Marni S.

[Jack K]

Marni,
I get the same problem in Measurement and Automation Explorer also.
The same thing does happen at 0V from the calibrator but you really have to look to see it. It basically shows up as a one or two millivolt noise signal but with the same pattern.
I have thought about using a signal conditioner (an isolator) on the channel during the calibration run but the ones I have don't have fast enough response and the results will be a bit off. Since it's only a test run I could probably get away with it. It'll be a few days until I can try that though.

Thanks for all your help.

Jack