Determining the Voltage already written to an Analog Output?

[anlangworthy]

Is there any way to easily determine (from within a LabView VI) what voltage is currently being sent out from an Analog Output? I suppose I could just send a copy of the output back into an Analog Input and monitor it that way, but it seems as if there must be an easier way to do it from within LabView. I have a PCI-6036E. Please edify my simple mind.

Thanks.

[Dennis_Knutson]

Since a LabVIEW VI set the analog out in the first place, the simplest way is pass that information through a wire to a different VI or even a global variable.

[dsb@NI]

I don't think it is that simple as LabVIEW is writing to the AO buffer (as opposed to directly driving the AO line). I think the tricky part of the original question is that he wants to know what last voltage value was "actually" output to the AO line.

I think the easiest way is to physically wrap the output back to an AI channel.

[rolfk]

Doug Bendele wrote:

> I don't think it is that simple as LabVIEW is writing to the AO buffer
> (as opposed to directly driving the AO line). I think the tricky part
> of the original question is that he wants to know what last voltage
> value was "actually" output to the AO line.
>
> I think the easiest way is to physically wrap the output back to an AI
> channel.

If he wants to know the real value and not the value supposedly output
that is the only solution. Although there are often calibration ADCs on
the Analog outputs which are used to calibrate the card, there are not
really functions to read those values back into LabVIEW. The calibration
routines use them internally but don't return them.

Rolf Kalbermatter

[Dennis_Knutson]

But what's the "real" value. There's some uncertainty in reading it back with an analog input channel. The floating point number used to program the device is converted to a 12/14/16 bit (or whatever resolution of the DAQ card is) number and then output. So the output may not be "exactly" what it is set to. But then reading back through the analog input, another conversion takes place. Is this reading any more accurate? If the board is correctly calibrated, I assume that the output voltage is what I set it to plus or minus the resolution of the D/A converter.

[rolfk]

Dennis Knutson wrote:

> But what's the "real" value. There's some uncertainty in reading it
> back with an analog input channel. The floating point number used to
> program the device is converted to a 12/14/16 bit (or whatever
> resolution of the DAQ card is) number and then output. So the output
> may not be "exactly" what it is set to. But then reading back through
> the analog input, another conversion takes place. Is this reading any
> more accurate? If the board is correctly calibrated, I assume that the
> output voltage is what I set it to plus or minus the resolution of the
> D/A converter.

Hmm, you are right of course, but if you want to take that further you
are going to have to use a 24 digits DMM and then there is s
till
Heisenberg and Quantum physics which will always prevent you from
getting absolutly accurate 😉

Rolf Kalbermatter

[dsb@NI]

I think I may have generate some confusion by putting quotes around actually. I was not meaning to stress the idea of obtaining the exact value but more the idea of obtaining the last value that was actually written to the line. When performing AO, you typically fill an output buffer with the AO values. If these values were all the same, I would go ahead and assume that the DAC put out exactly what I asked for. If you are outputting a dynamic signal, at any given instant, how do you know which output value was last output by the DAC? For this question, I think the easiest answer is to measure the output value with an input channel.