CompactDAQ Advisor Sampling Rates

[HowAli]

Hi, Can anyone help with this question: CompactDAQ system with two 9205 modules. The web advisor tells me that if I select 10 channels from the first module and 5 from the second module, my sampling rate is 50 kS/s ... that's not right. If I then reverse them, I get 25 kS/s. Surely I should get the same answer from both situations ?

What I really want to know is: If I have 10 channels from the first module (25 kS/s per channel ?), but only one channel from the second module, what is the maximum sampling rate I can get on this single channel on the second module ? I would have thought 250 kS/s as I'm not sharing the ADC with other channels on this module.

An explanation would be appreciated as I really need to specify a system soon.

[HowAli]

OK, I'm talking to myself now, but ...

Problems with the Advisor notwithstanding, after further reading, the sample clock (which is common to all modules) is limited by the most heavily loaded module. In my case, the module with 10 channels can only be sampled at 25 kS/s (to enable all 10 channels to be captured between one sample and the next in a quais-simultaneous manner). My other module with only one channel has to use the same clock, so it's stuck at 25 kS/s as well.

However, if I shared the load between the two modules, say 5 channels per module, I'd be able to get 50 kS/s ?

Does this sound like I'm on the right track now ?

[KyleBryson]

Hi HowAli,

First off, the Advisor behavior is indeed incorrect, and I will report through the right channels; feel free to submit feedback off of that webpage also if you would like.

As for the actual behavior, I think you're on to it.  There's only one analog input sample clock in the cDAQ-9172, and you're limited by the slowest module in your task.  With 10 channels on a 9205, that is 25 kS/s.  5 channels on a 9205 would yield 50 kS/s.  With several modules with an identical number of channels, the rate is not necessarily identical because of some bottlenecks in the chassis; that's what the advisor is supposed to help with, when it's working.

Fortunately, there are Three Methods to Calculate Your NI CompactDAQ Analog Input Sampling Rate.  You can try things out using simulated devices in Measurement and Automation Explorer if you have NI-DAQmx installed; if you try to run/test a task with a rate that's too high, the error message will tell you the correct rate.

I went ahead and ran the case you asked about.  2 9205 modules with 5 channels each will run at 49.627 kS/s (or a sampling period of 20.15 microseconds).  You're definitely on the right track balancing the number of channels between modules if you want to maximize your sampling rate.

Apologies for the advisor problems; hope this is sufficient, or that you can use MAX with simulated devices and characterize your system.  Please post any more questions if you can't find the right info out there.

Regards,
Kyle

[HowAli]

Kyle,

Thanks for confirmation.

Alasdair