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How do I use Daqmx EXT REF

Hi all,

I need to improve analog voltage output of PXI 6733. From this post, I can see that I might to get it by using Ext Ref pin. And to get there, I have few things to ask.

1. PXI 6733 is 16-Bit Analog output with default internal voltage reference +/- 10V. Which means LSB = 20V / 65536 = 305 uV.

And I want to generate my analog output with very small Vpp (sinewave with +/- 100 mVpp), but I want to have a good LSB with it. What should I do? should I connect an external voltage source (AC or DC??) into ext ref pin?

or can I somehow mange to get this without using external voltage source?

2. How do I get that property node? I mean, I tried to get the property node from the pallete, I clicked search, but I can only see these properties, but I can't click it:

 

daqmx.png


Regards, 

Yan.


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Message 1 of 9
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Hi Mystogan,

 

LSB is the abbreviation for least significant bits. 1 LSB is the smallest change, which the output can make. For a 200mV peak-to-peak sinus-signal, your signal will have 200mV/305µV=655 steps which represent your whole signal.

 

You can find additional information for LSB and other measurement specific words at the following website:

 

Data Acquisition Specifications -- a Glossary - Developer Zone - National Instruments
http://www.ni.com/white-paper/3218/en

I attached a VI, which show you how to select between the internal and the external reference signal. If you don't own a very precise voltage source, you should use the internal reference.

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hi Stephan,

thanks for the link and the effort to make the VI. But I don't think your VI would help me to get me there, its like, I'm asking, "HOW to catch a fish? what should I do?", then you just give me the fish, instead telling me how.

And about the LSB thing, you're right, that I get 655 steps for my sinewave, and I want to have better (much more) than that. An I've seen that people can get better (more steps) by using EXT REF pin. But I'm not sure how to do that,

or what kind of voltage source should I use. That's why I'm asking how to use this EXT REF pin to work.

 

No offense or anything bro, just trying to explain what I want to achieve.

 

Regards,

Yan.

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Hi Yan,

 

sorry for the mistake. I made a little video, how I created the VI, which I atteched my last post. You can watch the video here. No you can catch the fish by yourself  Smiley Happy

 

http://screencast.com/t/SIPaYMn1Os

 

I will watch, if and maybee how to use the EXt Ref pin for your purpose, but for that I need a little bit more time.

 

Regards, Stephan

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Message 4 of 9
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Hi Yan,

 

i found the attached example VI, which shows how to use the EXT Ref Pin in a DAQmx programm.

 

The following manual describes on page3-1 the function of the EXT REF Pin and how you can use this pin to minimize the value of the LSB.

 

Analog Output Series User Manual - June 2007 - National Instruments
http://digital.ni.com/manuals.nsf/websearch/9EB2F4E475DF58928625730900584FF3

 

Stephan

 

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Message 5 of 9
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hi stephan, tyvm for the effort making the video.

But here (in my Labview) I can't get my property node expanded like yours. This is the screenshot. Maybe I should change something in the Option??

 

daqmx.png

And thanks again for the example. And what about the external voltage source? it should be bipolar between AO EXT REF and AO GND, and max. +/- 11 V.

What would it means, when I have external voltage source of +5 V (DC), then I wire my +5 to pin AO EXT REF, and then wire my 0 V (from voltage source) to my AO GND.

Is that means, I would have a reference from -2,5V to +2,5V now?

And then, do I still need to write 5  in "Reference Value"-property? or should I write 2,5?

 

Regards,

Yan.

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Good morning Yan,

 

the behavior of your property node is realy strange. I never seen such a problem bevor. I found a KB on the web, wich maybee could help you:

 

Missing Properties in DAQmx Property Nodes - National Instruments
http://digital.ni.com/public.nsf/allkb/A92F81190B730C6686256F3C00609461?OpenDocument

Your second question is also a little bit difficult for me to answer, because I don't own the 6733 and i never used the Ext Ref Pin. If would need the Ext Ref pin, I would simply connect +10V to the Ext Ref Pin and measure on LSB and change a little bit at the value of the property node. Not the best approach, but i don't know it better.

 

Stephan

Message 7 of 9
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Morning Stephan,

 

your KB link is very helpful to me, it shows me what I want, change the FILTER, its like teaching me how to catch the fish. Thanks a lot. And for the EXT REF pin, I think I should ask directly NI through email.

 

Regards,

Yan.

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Message 8 of 9
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hi Stephan,

 

I'm trying to improve the precision of my voltage generation. And I've found here and there, about how to use it. But I'm not sure how to check if its work (improved resolution) using my digitizer (pxi 5105).

This is what I do to test it, and got the same minimum LSB voltage on both channels.

 

test.png

 

 

 

this is the zoomed in signal picture, both of them have relative the same LSB, about 1,7mV

 

 

 

 

zoomed in.png

 

 

maybe I've missed something basic? or my digitizer cant display them properly?

 

regards,

Yan.

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