difference DAQmx Channel - Analog Output - AO.Max and AO.DAC.Rng.High properties

[GabrielaK]

Hi,

what is the difference between the DAQmx Channel properties for Analog Output "AO.Max" and "AO.DAC.Rng.High"? I would like to make sure that my DAC card outputs only voltages within a certain range. For instance, if the "AO.Voltage.Rngs" property of DAQmx Device reports possible Analog Ouput Ranges of -5..5 Volts and -10..10 Volts for my card, I would like to set the maximum output voltage to 5 Volts, the minimum to -5 Volts and the offset ("AO.DAC.Offset.Value") to 5 Volts to make sure that only voltages between 0..10 Volts can be outputted even if the user sends -2 or 12 Volts to the task. Which property is the right one for that?

Best regards,

Gabs

[devchander]

By defination, AO.Max specifies the maximum value you expect to generate. The value is in the units you specify with a units property. If you try to write a value larger than the maximum value, NI-DAQmx generates an error. NI-DAQmx might coerce this value to a smaller value if other task settings restrict the device from generating the desired maximum.

So, this property will generate an error when voltages user sends -2 or 12 Volts and max and min are set between 0 to 10 Volts, rather than coerce it to range

[GabrielaK]

Thanks for your answer, devchander, so this property seems to be the correct solution for me.

Just for my understanding: what kind of behavior does the "AO.DAC.Rng.High" property cause in comparison?

Gabs

[devchander]

That property sets the effective DAC range, or in other words it assigns the voltage range for the resolution of your card.

Consider a card which has 16 bit resolution, and default input range as +/- 10 Volts.

If your signal to be generated is in the range +/-5 Volts, You will use only half the availible resolution

Now, if you set DAQ Range max and min as  +5 and - 5 Volts, you can use this entire 16 bit resolution to generate a signal of better quality and resolution.

This property does just that

Note: Not all NI AO DAQ cards support setting this Range and offset property. Which Card are you using??

Hope you understood it clearly

Regards,

Dev

Message Edited by devchander on 10-13-2006 06:11 AM

[GabrielaK]

Hi Dev,

oh - that is exactly what I also want to achieve. I want to set the output range to the range which gives me the best resolution. But also, I want to prevent that higher voltages can be outputted. So in your example with a 16 bit resolution board, if that board reports that +/-10Volts and +/-5 Volts both offer the full resolution, I would use the AO.DAC.Rng property to work with +/-5 Volts and 16 bit resolution, and an offset of 5 Volts which gives me the wanted output range of 0..10Volts with the full 16 bit resolution. Then I would set the AO.max/min properties to 10 and 0 Volts to make sure that an error is generated if higher/lower voltages are commanded. Is that right how I understand it??

Gabs

Oops - I have overseen your edit. Because we develop driver VIs and not applications for a special system, we do not know which board our customer is using. The only requirement is that it is DAQmx compatible. Do you know of any DAQmx compatible NI DAC card which does not support these properties??

Message Edited by Gabs on 10-13-2006 06:14 AM

[devchander]

That's right.

But, which DAQ card are you using??

[GabrielaK]

See my edit above 🙂

[devchander]

Oh, that's cool!

Some NI 67xx cards have a +/- 10 Volts range that can be set using and external reference ( not sure which all )

E-Series devices offer two analog output ranges- bipolar (+/- 10 V) and unipolar (0 to 10 V).

Some M-Series DAQ devices, have programmable output ranges that distribute the bits of resolution over a smaller voltage range and also support programmable DC offsets. ( I request you to check out the specs of those cards to know more)

Hoep this Helps,

Regards,

Dev

[GabrielaK]

Dev,

thanks a lot. I have added some error checking which verifies that the properties could be set. I hope that this will work under all circumstances with all boards...

Again, thanks a lot for your help!

Best regards,

Gabs