how to set input voltage range in daq?

[robertorodio]

How can I set the input range of a DAQ input channel? (NI DAQ USB 6259)

I am using Labview 7.1.

My purpose is to get the best AD conversion accuracy;
I am not sure that the inputs" max value" and "min Value " of Daqmx create virtual channel.vi (instance = AI voltage) is what I want; i want to select the input range of my daq so to act on the AD conversion minimum step.

is it possible to set different range for different channels?


Thanks,
Roberto

[Abhinav T]

Hi Roberto,

The Input range can be set in a number of ways. If you are doing this through the DAQ Assistant in LabVIEW, the Singal Input Range Max and Min will define your input range. The range you input here will automatically determine the most effective gain settings for the amplifier before the ADC - thus maximizing the range of your ADC. If you are programming using standard NI-DAQmx VIs please use the maximum and minimum value inputs in the DAQmx Create Channel.vi.

Please let me know if this helps.

[robertorodio]

Yes, actually this is what I did. I was not sure about the action of these inputs on the selection of the daq input range.

I still have two doubt:

1) I don't see any difference in the output file, regarding the input range selected. I mean, each data is stored with 6 decimal digits, and nothing changes if  I change input range.
I use the express VI to write the file.


2) I set +5V/-5V as a range and I observe as the reseult of a measurement values that are actually limited by my selection but that exceed a bit. Example, I see some +-5.23 V. Why?

Roberto

[Dennis_Knutson]

The number of digits in the file or even the numbre of digits on a front panel display have nothing to do with the range. The DAQ driver returns results as DBL at all times. The numeric representation does not change with the range. It is not like a DMM where you have 2 1/2, 3 1/2, number of digits. The Write Measurement File takes that double and then converts it to a string with x number of digits and writes that to a file.

You have to understand how ranges work and what happens when you specify a range. A DAQ board will have one or more physical range settings. They might be +/- .1, +/- 1, +/- 10, etc. There is a fixed number of ranges and they are set in hardware. I have not looked up the exact physical ranges that are supported by your board. That information will be in your manual. When you provide a range to the DAQ driver, the driver software determines the best physical range of the board that will accomodate your signal. So, when you specify +/- 5, my guess is that your board does not have that as a supported range and is using the next highest (i.e. +/- 10).

[robertorodio]

Tank you very much for your help, really striking.

Best regards,
Roberto