Simple question on DaqMx sampling rate with PCI6251

[ZLiu]

Hi, All
I just learnt how to use DAQs with DAQMx from the LabVIEW 7.1 measurement menu. So I chained the following vi's in series: physical channels->create virtual channels(AI Volt)->read analog 1D DBL NChan 1Samp->erase task. There are 3 channels to be sampled. I noticed the reading from the 2nd channel is always less than a Keithley meter by 4%. I put a sample clock vi before the read channels and set 5000/s rate, then the difference is gone. It seems to me the simple example from the LabVIEW mannual needs this sample clock vi to work properly. But does anyone know what sampling rate is being used if it's not set by the sample clock vi? Max rate? Thanks for reading.

John

[AnujD]

I ran a quick test and the default sample rate without a DAQmx Timing vi is 1000 samples a second. This is going to be 1000 samples per channel.

It is always "good practice" to use the DAQmx Timing vi.

What are the voltages you are are reading across each of the channels?

Regards,
Anuj D.

[ZLiu]

Channel 1 is about 1mV, Channel 2 and 3 are several volts. I have trouble with channel 2's reading always several percents less than true value, not in chn 3. I was guessing the default rate was too high to allow the ADC to settle so I didn't get the true value. But with 1000/s as default I shall not see this problem happening. Now I'm more confused. Thanks for replying.

[AnujD]

What you are seeing is a phenomena called ghosting. You are exactly right -- it has to do with the settling time of your ADC.

Check out this KB for more information:

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

It is puzzling though, because the default of 1000 Hz you should not see any ghosting though. To check the sampling rate in your application, you can put down a DAQmx Timing Property node & choose Sample Rate as your output.

Regards,
Anuj D.