Will NI USB 4431 can generate minimum output voltage of 0.4 microvolt beceuse the resolution is 24bit & output voltage is given as +/- 3.5V?

[Ravi_Shankar]

"Codewidth" is the technical term used to describe the smallest change the card can detect.

For your device codewidth = (voltage range)/(2^dac resolution) = 7V/(2^24) = 0.417microvolts.

There is one caveat of this concept. I haven't worked with this USB device but an example of pxi might help you.

PXI-6221 card is acapable of either bipolar (+10V to -10V) or unipolar (+10V to 0V) operation. These settings are made when selecting minimum and maximum output values when creating DAQmx output channels. If you do not require negative voltages you can improve your accuracy by using a 10V (i.e. unipolar) output range rather than 20V (i.e. bipolar) output range.

So the codewidth for unipolar output range is half of that for bipolar. If you can make this setting for your USB card you can actually generate lower voltage levels i.e below 0.417microvolts.

thanks

Ravi.

[Ravi_Shankar]

Corretion to the previous post.

Code width is not the only variable that affects the absolute accuracy of a DAQ card. Code width is a measure of how precisely the Analog to Digital Convertor (ADC) can translate an analog signal into its digital form before it can be passed on to the computer.  

Also consider the absolute accuracy of the channel.

For USB 4431, the gain accruacy is +/-0.35%max and the offset is 2mV max. Also considering distortion noise of around -82db max you can calculate the absolute accuracy as:

Absolute Accuracy = ±((Input Voltage * % of reading) + Offset + System Noise + Temperature Drift

thanks

Ravi.

Message Edited by Cji on 08-18-2009 09:12 AM

[Mark_SAI]

Hello,

If you look at page 4 of the USB-4431 datasheet, it specifies a dynamic range of 89db at 51.2 KS/s. So whilst the card will give you 24 bits of resolution happily, the lower 9 bits will be noisy/gibberish:

log(base2) [10^ (89db / 20) ] = 14.78258 effective bits.

I hope this helps!