Sampling Multiple Channels of Single cDAQ Module

[Pete.Dunc]

Hi all,

I am currently researching the use of cDAQ for an oil analysis system, in particular the NI 9201 module for AI voltage.

We will have 6 SE inputs which lie within the voltage/current ratings, although they all require differing sample rates.  It is my understanding that the module is capable of 500 kHz which will be divided between the inputs.

Does the sample rate have to be divided equally?  Or could I for instance allocate 250 kHz to one channel, and divide the rest between the other 5.  Synchronous measurement between devices is not required.

Many thanks in advance,

Peter

[DBerry]

Peter,

If you have the cDaq-9172 chassis there is only one analog input timing engine.  see the following KB http://digital.ni.com/public.nsf/allkb/E7036C1870F6605686257528007F7A72

If you have one of the newer chassis, they have multiple timing engines that could be used for multiple AI tasks with different sampling rates.

In case of the 9172, you can sample at the 500kHz/6ch and then re-sample the channels you desire to have at a lower rate.

Dan

[Pete.Dunc]

Hi Dan,

Thank you for that link, it was very useful.  I have read some of the associated DAQmx help files but still have a couple of questions.

I will be using the newer 9174 chassis so will be able to take advantage of the 3 timing engines.  My modules are NI 9201, NI 9211 and NI 9263 (AO).

• Am I correct in saying I could sample each module at a different sample rate?
• If I sample the NI 9201 at 500kHz/6ch, will I have an effective sample rate of ~83kHz on a given channel?

The reason for asking is that 4 of the 6 channels used on the NI 9201 require a sample rate much lower than 83kHz per channel (which is fine as I can resample as per your suggestion), although the other 2 channels have a higher frequency input.

• Would it be possible to configure one task which accesses, for example, 8 channels ( 4 x NI 9201 and 4 x NI 9211 ) in the 10 Hz range, and another task which accesses the remaining 2 channels (NI 9201) at a much higher sample rate?

I have some (limited!) experience of DAQ but I am keen to learn; any references would be appreciated.

Thanks again for your help,

Peter

[DBerry · Accepted solution]

Peter,

See my replies to each bullet:

• Am I correct in saying I could sample each module at a different sample rate? Yes
• If I sample the NI 9201 at 500kHz/6ch, will I have an effective sample rate of ~83kHz on a given channel? Yes

Would it be possible to configure one task which accesses, for example, 8 channels ( 4 x NI 9201 and 4 x NI 9211 ) in the 10 Hz range, and another task which accesses the remaining 2 channels (NI 9201) at a much higher sample rate?

Unfortunately, you can't specify different rates on a single module.  The following post might be misleading (http://forums.ni.com/t5/Multifunction-DAQ/Suggestions-for-multiple-thermocouples-sampling/m-p/998643...) but I believe they are suggesting that the multiple timing engines allow for multiple tasks at different sampling rates as long as the tasks are accessing unique modules.

The following KB suggests that only one rate per module can be used.

http://digital.ni.com/public.nsf/allkb/96FD2F4685065C7686256F25006EE8DE

Your best bet is to sample the 9201(all 6 channels you need) at the effective 83kHz and then resample the 4 channels of those that you need at a slower rate.  You can then specify a second task to sample the 9211 at the 10Hz..

[Pete.Dunc]

I didn't expect to be able to sample different channels on the same module at different rates, but it would have been handy!

I need a higher rate than 83 kHz for 2 of the channels, so will need to add another module.  Alternatively extracting the frequency information in circuitry before passing the signal to the DAQ would solve the issue.  Although that is a completely different challenge.

Thanks again for your advice,

Peter