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available sampling rates USB-6251

Hello,

 

We use the USB-6251 to sample between two and eight channels simultaeously.  I understand that the maximum aggregated sampling rate when using more than one channel is 1000 ks/s.  My question is: are we limited to 1000/number of channels (and integers) as possible sampling rates, i.e. 500, 333, 250, 200, 167, 143, 125 ks/s for 2-8 ch respectively?

 

What happens if we request intermediate sampling rate values from our interface software (e.g. 2 ch at 350 ks/s, or 8 ch at 75 ks/s) - will the 6251 use the nearest of the values listed above, or sample at the actual value requested?  We don't get any indications of errors  from our software, but i would like to understand what is going on with the DAQ.

 

Thank you,

 

Chris

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There are a couple distinct constraints that determine what sample rates are achievable.

 

One relates to the A/D and multiplexing circuitry.  Time is needed between conversions to settle to a value that's stable enough to meet the board's specs.  Your list of theoretical max sampling rates as a function of # channels addresses this consideration.

 

The second constraint is based on needing an integer divisor of the internal timebase clock used to drive acquisition timing.  The internal timebase used for M-series is likely 80 MHz, but possibly 20 MHz.   If you request a 350 kHz rate, DAQmx will find that the theoretical divisor of 80 MHz is 228.57

It will then be forced to use an adjacent integer, either 228 or 229, as the divisor.  (It will act consistently, I'm just not certain what algorithm is used for rounding and/or truncating).  So the actual sample rate will be several hundred Hz above or below your theoretical target.

 

In LabVIEW, after programming a desired sample rate you can query a DAQmx timing property node to retrieve the actual sample rate.

 

 

-Kevin P

CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).
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Hi Kevin,

 

Thank you very much for your reply.

 

So, as i understand it, we can use sampling rates below the max possible rate (constrained by number of channels).  But if the rate/time base is not an integer there will be a difference from our intended rate.  That's good to know.  I'll try to track down the time base value and thanks for the labview tip.

 

Chris

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