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Multi-channel sampling problem PCI-6221

Hi,

When sampling multiple analog input voltage channels in a single DAQ task, using the PCI-6221 (in RSE mode), my voltage readings are not accurate. As I change sample rate up and down my voltage readings move up and down as much as 2 volts. Is there a workaround/fix for this? I assume what must be happening is that the PCI-6221 has only one ADC and multiplexes the inputs for multi-channels causing inaccuracies in the voltage readings. Is this correct?

Thanks,

John Voltz
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John,

There a few things that could be causing your problems.

1. As you guessed, there is a chance you might see some "ghost" channels because there is only on ADC on the DAQ board. Normally, though, this discrepancy will NOT be 2 volts or anything nearly that large.

Check out this KnowledgeBase:

http://digital.ni.com/public.nsf/websearch/5B86A63ECC0D48828625629800571743?OpenDocument

2. The other problem could be with how you are wiring things up. Take a look at this link:

http://digital.ni.com/public.nsf/websearch/D509679FFAE2764386256297005D0C9D
http://digital.ni.com/public.nsf/websearch/1FFCBE3451ABEE09862562970059881D

Also, if these do not solve your problem, reply with exactly how you are wiring up your channels.

Regards,
Anuj D.
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The voltage readings are as large as 2 volts, because I forgot to mention that I am using a custom scale with my configuration, which obviously amplifies the problem. The scale has a slope of approximately 5 (calibrated to a specific resistive voltage divider) and Y intercept of 0. I'll check the links you posted to see if I can track down a solution.

Thanks,
John Voltz
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Here's a simple schematic of my configuration.
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Everything seems perfect in your schematic.
Are you still having troubles?


One easy way to see if your problem is related to the multiplexer is to slow down your acquisition speed (to maybe 1 Hz). This will ensure that the ADC has plenty of time to settle.

Regards,

Anuj
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In the attachment is a Measurement And Automation Explorer test I used to verify my problem. When applying 10 VDC to an input and setting the sample rate to the max of 62500, I read 7.73 VDC. Changing the sample rate to 100, I read 10.00 VDC. Accuracy appears to roll off dramatically at about 2000 samples/s. Does NI offer a DAQ card that would be capable of sampling 4-8 channels simultaneously at >50 ks/s without this settling problem? It looks like I may have chosen the wrong card for this application, but I'm sure the price of the right card will also be prohibitive.

John Voltz
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John,

NI offers the S Series boards (search for 6115 at ni.com). These have one ADC for each channel. Are all the channels you are reading 10 volts? Or are some channels 0 volts and some channels 10 volts?

Regards,
Anuj D.
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