how do I control analoge output offset

[Timmar]

Dear Labview Gurus

I have inherited some code that enables an analogue output signal to be generated with an offset.

I have tweaked it and it appears to work, much to my surprise.  I am unsure of exactly how it is doing it though.

I am wiring apfi 0 to a 5V reference, my initial expectations were to see a 5v offset.

What I want to know is why does it work. Does anyone know where I can find a user guide on this functionality.

I am using a PXI6289 and am trying to generate sub milivolt resolution signals with a 2.5v offset. (in this example, I am using larger voltages so I can measure it with a multimeter)

Here is my code

Thanks in advance

[edit]I think that his may be a labview questionNot sure how to change.

[Brian_P · Accepted solution]

Timmar,

Thank you for using the Forums.  I was able to replicate your system and I believe that I know what the problem is.  When wiring in the the +5 V to the PFI line this will bump the central voltage up to 5 V.  Putting the 2.5 V into the offset value actually subracts 2.5 V from the 5 V and so it looks like 2.5 V.  If you were to put -2.5 V it would actually increase to 7.5 V.  That being said, when choosing 2.5 volts to move the voltage down you are actually increasing your range to the next largest range which would be 5 V so you really aren't getting more resolution.  The best way to get resolution would be to input a known 2.5 voltage and then put 0 for your offset value.

I hope that this helps,

Brian P. 

[Timmar]

Brian,

Thanks for the response,

Now I know even less than I did before.

I think I may need to know more about the hardware architecture.

In my initial experiment I thought it would be preferable to use the internal reference, and offset from there.

Alas it only gave me options of 0, 5 or 10. C'est la vie.

So In experiment #2 I was doing a proof of concept and wired the closest voltage source I had lying around (5v) and used the external reference.

To my surprise I got an error message demanding I put in an offset value. This made no sense to me, I thought I was supplying the offset.

So just for fun I plugged in 2.5 and hey presto: 1. The value was accepted and 2: the voltage appeared on the pin.

What I later discovered was that I may have had my mutimeter leads on the wrong terminals, and my 5V source was not wired reliably.

Have put in a 2.5V ref in now and all is well.

Thanks for your insight!.

From your description, it seems that the software is adding the offset, and not the hardware.

Why couldn't it do this for internal reference, this would have saved me some wire?

[Brian_P]

Timmar,

I believe the reason that you can't do this for internal is because they are set voltages.  If you add an external and then add the offset value that actually increases the range to the range the set range that is above that output, so it would be just like outputting a regular voltage and the offset is kind of moot.  In your case though increasing the range is definetly a bad thing because with increased range the resolution decreases which is opposite of what you want.  So to get the best resolution use a know 2.5 V external and the a 0 offset.  I hope that makes sense.

Regards,

Brian P.

[Timmar]

Makes Sense, used your advice works well.

I still would like to find some literature on how to apply this.

My NI Data Acquisition Systems training course didn't cover it.

I cant find anything in this website either.

I have other staff that I woulf like to train in these methods and would prefer not to start from scratch

------------------------------------------------.

Would be cool if NI put a 2.5V Option in future devices. That way it will aid 5V based analogue circuits

I am working with strain gauges and 2,5V is the centre of the bridge.

A couple of 10k resistors across the 5V supply no worries mate!

I say in jest. I understand that a precision reference in not something to be taken lightly