Incorrect analog values from NI 9205 in multichannel mode

[Socki]

Hello everybody

I'm actually measuring 10 battery cell voltages (which are connected in series) with a NI 9205 (slot 1) in a NI cDAQ-9172 chassis.

Every cell voltage is measured in differential mode with channels 0 to 7, 16 and 17. I have no connections to the COM pin of the NI 9205.

The other modules in the chassis are NI 9217 (2 modules,slot 2 and 3), NI 9481 (slot 4) and NI 9401 (slot 5).

In my LabView program, I'm putting all channels from the NI 9205 and the NI 9217 modules in one task, because just one task can run at a time for analog input measurement.

My problem is, that some of the values measured by the NI 9205 do not match the reality. Every of the ten cell voltages is around 2.60 V (measured with DMM). But for example, channel 2 shows 2.67. Some channels are right and others are not.

I tried to vary the sampling rate, but this does not really solve the problem.

For example:

Sampling rate = 1000 => Channel 2 = 2.72 V

Sampling rate = 100 => Channel 2 = 2.67 V

Sampling rate = 10 => Channel 2 = 2.67 V

Then I tried to put every channel in a single task and let run one task after each other. And then it worked!

But the data acqusition is so slow, when creating a single task for every channel and that's why I prefer the way with multiple channels in one task.

I'm not sure if my wiring should consider any connections to ground!?

Thanks for any help!

Regards

Socki

[Henrik_Volkers]

As statet in the manual, you should provide a path for the bias current. Propably a 1M from battery low to AGND is worth a try.

[Socki]

Thanks Henrik

I connected the resistor between battery low and AGND but the results got even worse.

Some values are now around 0 V, others over 3 V.

Any other ideas that could help the issue?

Greetings

Socki

[pallen]

Hello Socki,

While I can't say for sure, it looks like you might be getting some crossover between your channels.  I had a very similar situation last year.  Although my problem stemmed from the combination of both differential and single ended devices connected to the same card. 

This page describes some wiring options.  Possibly not suitable in your particular case.  But good reference info. 

http://zone.ni.com/devzone/cda/tut/p/id/3344

If you are seeing channel crossover, there is something called an Interchannel delay that you can adjust in your code. 

Another option that was suggested to me when I was still troubleshooting this was to include a "blank" channel in my Task.  Scanning the grounded channel between "live" channels can help to eliminate channel crosstalk in some cases. 

There's a page for Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels here:  http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/b9bcdfd960c06b9186256a37007490cd?O...

Good luck. 

[Socki · Accepted solution]

Hey Patrick

Your hint really helped me out of trouble.

I had to reduce the convert clock rate of my AI module. Now it works great!

Sampling rate is 20 and convert clock rate is 200 (for 10 channels)

The vi from the following link shows how to change the convert clock rate (Set Convert Clock.vi):

http://digital.ni.com/public.nsf/websearch/42484E84DA98053686256D32006E0494?OpenDocument

For my module I additionally had to choose and connect the "active devices" setting on the  DAQmx-Timing property node.

Thank you!!

Regards

Socki

[MTx]

Hello Socki,

i have the same problem you had and i have done some alterations but i don't achieve correct values.

Could you show me how to change the clock converter and the sampling rate?

Best regards,

José

[Socki]

Hey José

Click the following link:

 http://digital.ni.com/public.nsf/websearch/42484E84DA98053686256D32006E0494?OpenDocument

At the bottom of the side, you can find some attachements. The "Set Convert Clock.vi" describes how to set the convert clock rate and the sampling rate.

Regards Socki

[MTx]

Hello Socki,

what have you connected to the ActiveDevs optinos?

The best

[Socki]

Hy José

I forgot to tell you this.

To the ActiveDevs I connected just a constant with the name of my module (cDAQ1Mod1).

Information:

For the sampling mode I chose continuous samples and the rate is 15.

My convert clock rate is 200.

Cheers

[MTx]

the convert clock i just change the convert clock but i don't know how to change the sampling rate.

When you say sampling rate is the ratiio betwwen samples and frequency?

The best