Separating Multiple Analog Input Channels

[Pindur]

Hi,

I searched alot for an answer to this question and found nothing. I'm a little new, but I have a good understanding of labview, except for the DAQmx details.

Im using Labview 7.1 and a S series DAQ - PCI 6251.

This is to measure the DC bias on two AC singals. I am trying to aquire the signal from Ai0 and Ai1 at the same time. I believe the data is coming into the software correctly, but it is not being spilt into two signals correctly. If I set my input channel to only ai0, I get a reading around 5V. If ai1, i get 0.6V. These are correct. If my input is ai0:1 so I can do both measurements at the same time, then my values 4.5V and 1V. (for ai0 and ai1), which is obivously NOT the same. The grounding is correct, it is not an issue of drifting or noise. It is very constant.

To separate the signals I am using Spilt Signals. I also tried the divider.vi from another posting I found, althought I am not changing the sampling rate. It gives the same results as the Spilt Signals.

http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=BCB8746152612330E034080020E74861

Another attempt was to create two parallel tasks, one for each channel. Each having their own create, timing, read etc. I receive an error about a resource being reserve.

Please help!

Thank you,

Pindur

[Odd_Modem]

PCI 6251 is an M series, so there is only one ADC. When two adjacent channels in the scan start to 'bleed through' that usually means that the inter-channel delay is too short. In other words, you are sampling at a rate faster than the setting time of the ADC and hold cap.

[Pindur]

Thank you for getting back to me so fast. I was wrong about the series, the 6251 is a M. I don't understand how I am getting bleed through. The card supports 1Gs/s across multiple channels. So if I am measuring two channels I should be able to measure at 500 ks/s per channel, correct? The sample rate I am using is only 10 k/s. I tested it, as I change the sample rate the values do change. How can I get around this?

Thanks

[Ryan_V[DE]]

Pindur,

    What version of the DAQmx driver do you have? Version 7.3 and before uses round-robin sampling, where interchannel delay is maximized. Versions 7.4 and later have a small delay between channels, but try to make the samples as simultaneous as possible. If you have one of these later versions, you can increase the interchannel delay with a property node. Please refer to the following KnowledgeBase for more information on interchannel delay:

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

Hope this helps,
Ryan V.
National Instruments

[Steini]

Sorry for hitchhiking this (old) thread, but this seems to be the exact same problem we’re experiencing.
DAQmx version is 8.0.0f0, using LabView 7.1, DAQ card is 6221 PCI.
We’re updating an old lab experiment to run with LabView and wanted to test our connections by directly applying some voltage (1 - 5V) from a power supply to ai0 .. ai1 (more channels to be used later on). Readout is fine when only one channel is used, but reading multiple channels at once results in all kinds of interference (voltage dropping in one channel when the other is connected to ground or some lower voltage, and so on). Changing the sampling rate to something very low (e.g. 1Hz) does change the readout a bit, but still gives incorrect values.
Judging from other threads, we're maybe saturating the amplifier, which then gives that ghosting/cross talk. So... how would we go about testing the card with a power supply? Any recommended circuit to put in between?
Or should this "just work", so our hardware is faulty?

[Ryan_V[DE]]

Steini,

Have you attempted to change the convert clock rate as mentioned in the above KnowledgeBase? A slower convert clock rate will increase the time between samples, giving the instrumentation amplifier on your DAQ card longer to settle. Even at low sampling rates, the convert clock rate is still quite fast. Depending on the magnitude of your "interference," this may or may not be the cause. Another possible cause could be the specified voltage range of the input channel. Are both channels set for the same input ranges? Also, what happens when you read a grounded channel in between your first and second channels (Connect ai2 to ground and set your scan list to ai0, ai2, ai1)? Does this fix the second channel? Is the first channel ever inaccurate? Please let me know the answers to these questions, and I would be more than happy to help you continue troubleshooting.

Hope this helps,

Message Edited by Voltage Viper [DE] on 05-30-2006 11:24 PM

[Steini]

Ryan, thanks for your reply.
We came back to this problem today, and then we noticed it was our power supplies that were causing the problem. We had used piezo crystal drivers, which give a highly stabilized voltage (and happened to lie around...). Unfortunately, they are also designed to run with very high resistances. Even a 1kOhm resistance across the terminals dropped the output voltage to zero.
As soon as we replaced those drivers with a "real" laboratory power supply, it worked: no more crosstalking/ghosting.
So we are now able to test our program, and hopefully the problems won't reappear when we connect the measuring equipment.