Do expected min/maxVal in DAQmxCreateAOVoltageChannel have meaning for 6723 card

[Tonu]

Dear Sir/Madam,

I am creating slow ramp waveforms using PCI 6723 32-channel 13-bit analog card in PC.  Ramp waveforms run from -0.5 to 0.5V, and I am measuring too large Moire pattern in current (details are irrelevant).

AO type is buffered infinite waveform; repetition frequencies are in the order of 0.1 to 0.001 Hz.  AO channels are created with DAQmxCreateAOVoltageChannels()  command, which

has parameters minVal and maxVal.   These parameters make sense if there is a variable gain analog amplifier in use - by setting these values close to expected min/max, one could improve the 'smoothness' of waveform that is all 8192 bits would be used per waveform. 

However I could not find anywhere in my 6723 manuals a statement that such is the case for 6723 card.

Thus, if no amplifier is present and minVal maxVal parameters are not used at all then the actual resolution I use is only 400 bits per waveform.  And then the observed Moire patterns make perfect sense. 

Q1. Are minVal and maxVal parameters are in use for 6723 card.

Q2. If yes, would you please explain how these parameters actually work e.g. is there a discreet set of amplifier gains, etc..

Sincerely,

Tonu 

[Brian F.]

Hello Tonu.

Thank you for contacting National Instruments.

Your 6723 card has one output range: +-10V. Thus, if you do have a small signal (.5V to -.5V), there isn't a way to configure the board for better resolution across this range. If your application requires better resolution across this range, I would suggest acquiring one of our 671x or 673x cards. These cards have another output range called +-AO EXT REF where you can actually set the voltage range to be anything within +-10V. For example, you could set your card to have the range of +-1V and then utilize the full resolution of the card across this range.  However, the noise on the card will still exist so if you try to set a very small voltage range, the noise will still be present in the same amount that it would have been on the +-10V range. So, even though you have increased resolution, your accuracy of measurement will still be unchanged because of the signal to noise ratio.

I hope this fully answers your question. If you have additional questions or if any of my statements require further clarification, please feel free to ask. Have a great weekend!

Brian F

Applications Engineer

National Instruments