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Simultaneous A2D


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#1 Jeff_Birt

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Posted 10 December 2012 - 08:29 AM

I have an application where I need to read 2-3 A2D channels simultaneously, with external clocking, as well as keep track of a quadrature encoder. It looks like the U3 might be able to do this but I can't seem to find anything in the manual of specifications that says if the A2D inputs are multiplexed or not, or if multipled stating the actual number of A2Ds on board, i.e. two A2Ds with eight input muliplexors. This information could be in the docs but I could not find it. Any help would be appreciated. Thanks!

#2 LabJack Support

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Posted 10 December 2012 - 02:15 PM

The U3 has 1 ADC. All of our devices are multiplexed, so there is always interchannel delay. With the U3 in stream mode this can be as small as 12.5 microseconds:

http://labjack.com/s...users-guide/3.2

Also, the U3 does not support external triggering in stream mode, but you can stream the trigger signal in with the analog data and associate it in your software as desired.

#3 Jeff_Birt

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Posted 10 December 2012 - 05:46 PM

Thanks. Is the inter-channel delay fixed or at least deterministic, i.e. bounded something like 42us +- 2us (or something, for Res Index 2) during a scan? As long as I know what this range might be I think it will work as the samples can still be synced on the PC. After talking with the customer this afternoon there are a few other modes of operation this machine will need to support so my initial idea of having an encoder clock in the A2D would not work. I think being able to set the scan rate and knowing the inter-channel delay will let me take care of the rest of it in software. Thanks, Jeff

#4 LabJack Support

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Posted 11 December 2012 - 09:18 AM

Yes, the interchannel delay is pretty constant and the numbers in that table are actual measured numbers. We will do some testing and follow up here with re-measured numbers and the jitter we observe in those numbers.

#5 Jeff_Birt

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Posted 11 December 2012 - 03:11 PM

Thanks, that is very helpful. I greatly appreciate it. Jeff

#6 LabJack Support

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Posted 14 December 2012 - 11:25 AM

The variance is affected by other operations the device might be performing. Especially counter-timer functions. I think the easiest thing would be to output pulses to show the start of sampling. Would that work for you?


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