acquisition synchronization-2 channels

[iturriamiren]

Dear all!

I'm working with the following vi. to adquire data from different channels. I have a USB-6009 device from which I'm reading data and independently, I'm generating a channel containing data from a boolean control I myself have built.

I'm saving the data coming from both channels in a single tdms file. What I have realized looking through the collected data using DIAdem is that the storage of data from both channels doesn't start at the same time (the difference is very small).

Is there any possibility to avoid this? In that case, I would really appreciate your help.

Thank you very much in advance,

Miren

[AlexNCSU · Accepted solution]

  Simultaneous (exact simultaneous) sampling from analog inputs is not possible with the USB 6009.  There are basically two types of analog sampling that can be done with any DAQ board/card/device; 1) simultaneous (or independent)  sampling, or 2) aggregate sampling.  The difference has to do with the type of Analog to Digital converter(s) on the board.  

  Simultaneous (or independent) sampling (which usually lists the rate as samples/sec/channel) has multiple Analog to Digital converters for each channel and can thus acquire each channel at the same time.

  Aggregate Sampling (which usually lists the rate as just samples/sec) typically has one analog to digital resource, and each channel must "share" this resource meaning only one channel can acquire a sample of data at any given time.  So the "samples/sec" rate is shared among all the channels being used.  If you sample 2 channels, the resource scans over those two channels as fast as it can to acquire both samples (one from each channel) but there will always be a delay between each sample (that delay should be equal to the inverse of the devices sampling rate in sec/sample).

 I hope this helps.

 --Alex--

[iturriamiren]

Thank you very much for your answer, Alex.

Could you just recommend me a board/card/device that allows simultaneous sampling instead of aggregate sampling? Does the NI-USB 6210 have this property?

[JesusG]

Hi iturriamiren,

E series (60xx) and M series boards (62xx) do not have this funcionality of simultaneous sampling. M and E series devices are designed for multiplexed sampling, so, your 6210 board is not very useful as this point is concerned. Simultaneous sampling devices are DSA boards, S and X series boards, and high speed digitizers. You can see all of this in the following document:

http://zone.ni.com/devzone/cda/tut/p/id/4105

I suppose you are interested in a multifunction acquisition device, so, your best choice would be an X or S series device. Feel free to contact your local NI branch office (I think it is NI Spain, 91 640 00 85) so as to get more information about pricing and whatever you need.

I hope this help!

Jesus.

[AlexNCSU]

  The DAQ systems I use for independent sampling are FPGA based (7933R, 7952R) which are fairly expensive.  The alternative to purchasing a system with independent sampling is to use TWO DAQ boards with analog input channels.  Each one will have its own analog to digital converter and will be able to operate independently.  You will have to configure the advanced triggering/timing for both boards in order to get them to sample at the same time.  I have done this before with two M-series boards.  There are various ways to achieve the synchronous timing depending on the boards used and the type of signals you are trying to measure.

  --Alex--