Measurements

[Kmar]

I have a DAQ.mx vi taking an Analog Input voltage. I'm getting 'unscaled amplitude', which I believe are A/D counts, but I was wondering a) How to translate these counts into voltages and b) How to change the unscaled amplitude to Volts and if this would change the resolution of the measurement. 

[Kmar]

I should add that I'm reading somewhere around 15,700 for a 2.5V signal, which doesn't match the formula: Voltage Reading = (binary reading/2^bit)*(Vmax-Vmin), give that my Vmax is 2 and my Vmin is -2 and there are 16 bits on the PCI-6221.

[crossrulz]

Why not just have DAQmx give you the actual voltage?  That would be a whole lot easier.

[Dennis_Knutson]

I agree that it would be simpler to read the scaled voltage in the first place. To use the raw data, you would need to read the calibration data and apply that. It is not the simple formula you have. It is incorrect anyway. You use a 4 volt range for some reason even though the device does not have such a range and obviously, your 2.5 volt signal would be outside that +/- 2 volt range.

[Kmar]

I agree too, but for all intents and purposes right now I'm under a limit to read A/D counts. How do I read the calibration data? Also, my signal is +/- 1.25 V, 2.5 Vpp, so it is within the 4V range.

[Dennis_Knutson · Accepted solution]

Reading the raw A/D only makes sense to me if that is how you are going to save the data in a file. In any case, see this.

You seem to be under the common misunderstanding that setting a range with the DAQmx Create Channel is the same as setting the actual range of the device. DAQmx uses the range data you provide to select the best physical range to acquire the data. As I already mentioned, the device does not have a +/-2V range. Your device is actually using the +/-5V range so the raw A/D count is correct. You need to read the specs or do a property read to get the actual range used.

[Kmar]

That is in fact how we're saving the data. That makes complete sense, I did do the math with +/5 V and it was much closer to expected and I changed the range to +/- 5 V and it didn't change the number, so I would assume that's the range the device is working with. Thank you so much.