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How does NI-USB 6009 actually operate when taking data from multiple channels?

 The question might seem pretty weird, and sorry for my English.

 

I know that NI-USB 6009 can not measure data from multiple channels simultaneously because there is only one set of MUX and ADC. But when in the labview I set the DAQ assistant (or build virtual channels) taking data from more than 2 channels, how does the NI-USB actually operate? Is it like, taking 1 data from channel 1 and then switch to channel 2 to take 1 data..... finally switch to channel n to take 1 data, and the total time of taking a pair of data is 1ms (say the rate per channel is 1KS/s)?

If so, why there's no swiching time?

And my professor said that there's capacitor in the MUX, which is several mF, and when the voltage signal source has relatively low resistence, the charging time might be quite long. For example if I measure 2 voltage source, one is around 9V and another is around -9V, is the capacitor going to have a great effect on data (or the time of data taking) when NI-USB is swiching between 2 channels too frequently?

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Hi JudeLi,

 

When taking analog input measurements on DAQmx device that uses a MUX, there are two clocks used to take the measurement: the Sample Clock and the Convert Clock.  The Sample Clock sets the time between data points on the same channel and the Convert Clock sets the rate that the MUX switches between each channel.  Below is a link to an article that describes the difference between these two clocks.

 

Difference Between the Sample Clock (Scan Clock) and the Convert Clock (Channel Clock)

http://digital.ni.com/public.nsf/allkb/4D1435DF82EF494186256D8A006DD6D4

 

 

On some DAQmx devices you can query and/or set the rate of both the Sample Clock and the Convert Clock.  On the USB-6009 however, you can only set the Sample Clock and DAQmx will automatically determine an appropriate rate for the Convert Clock.  You can still query the rate of the convert clock as described in the article below.

 

NI USB-6008/6009 Convert Clock

http://digital.ni.com/public.nsf/allkb/9DE07B9EF029C2D5862570A400784B37?OpenDocument

 

 

I'm not aware of capacitance in the MUX (there may be some), but there is definitely a small capacitance in the ADC.  I would expect this capacitance to be on the order of pF rather than mF though.  However, you are correct that this capacitance can cause behavior known as "ghosting".  I'm including a link to an article on this phenomenon below.

 

How Do I Eliminate Ghosting From My Measurements?

http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument

Josh B
Applications Engineer
National Instruments
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Hi Josh,

 

Thanks for replying! Those aritcles have been great help!

After reading them, I wrote an VI taking data from 2 channels, channel 0 is 5V DC in series with a resistance R and channel 1 is a triangluar shape AC signal with amplitudd around 2V. I've set R to be 50kOhm, 60KOhm,300KOhm and even 1.5MOhm, but the wave chart seems pretty normal even when the rate per channel is 20KS/s. Is the ghosting easy to happen for USB-6009?

 

Regards,

Jude

 

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Hi Jude,

 

It's good that you don't see any ghosting.  I don't think I would say that it is easy for ghosting to happen for the USB-6009, but it does happen from time to time.  With your setup, if ghosting were to happen it would probably manifest itself as a DC offset on channel 1 reading the triangle wave.

Josh B
Applications Engineer
National Instruments
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the 6009 is really not fast enough or accurate enough for ghosting to be apparent.  Get up around a few MHz and get 16bits of resolution it gets easier to spot.


"Should be" isn't "Is" -Jay
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Hi Josh,

 

Thanks for replying! 

Just one more small question. You've said that one can not set convert clock on usb-6009. Is it true under all circumstances, like, it's hardwared in this way, or is it true just under labview? Can I change it using other softwares by writing lower-level commands?

 

Regards,

Jude

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LabVIEW has exactly the same access to the DAQmx functions as any other language. If you really need the capability, you need more sophisticated hardware.
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