Offset and Calibration (DAQ USB-6251)

[An Engineering Student]

Opened Measurement and Automation Explorer, right-clicked on the DAQ and selected Test Panels. Sent a 4 V DC signal to output AO0 and input that signal at input A19. The device read -7.9 V. Increased to 5 V, device read -6.9 V.

How do I calibrate the device for no offset?

[NI-Bongo]

Hello Engineering Student,

You may want to make sure you are connecting things properly.  This will depend on which terminal configuration you are using to do the reading.  In differential mode, you need to connect both the AO0 and AOGND terminals to the + and - of the AI channel you are using.  For single-ended mode, you need to use the AI channel and AIGND.  Here is a document that discusses the different configurations and when to use them:

Field Wiring and Noise Considerations for Analog Signals
http://zone.ni.com/devzone/cda/tut/p/id/3344

For the USB-6251, there are only 16 AI channels, 0-15.  In differential mode, AI0 and AI8 work together as AI0+ and AI0-, so you could connect AO0 (pin 15) to AI0 (pin 1) and AOGND (pin 16) to AI8 (pin 2).  You should then be able to read the voltage correctly.


Message Edited by NI-Bongo on 02-26-2008 11:21 AM