Why Do I Have an Offset in My Absolute Measurements with My SCXI-1520 When Excitation Is Turned On?Hardware: SCXI>>Modules>>SCXI-1520
Problem: When I use external excitation (user defined) on my SCXI-1520, I see an offset error. How can I eliminate this (calibrate) without nulling my transducer? Solution: The instrumentation amplifier (IA) of the SCXI-1520 has an input for voltage reference. This input is fed from the output of the nulling compensation circuitry which in turn varies with respect to the on-board excitation voltage. (See page 1-2 of the SCXI-1520 User Manual for schematic). As a result, when an excitation voltage is applied, the reference voltage for the IA changes, creating an offset in the measurement. This causes the null compensation to be incorrect for the measurement. The offset is intended to be manipulated in order to move this offset to the null reference of your transducer. (Please see the SCXI-1520 User Manual and the Strain Null example in the related links bellow for details.) If you have NI-DAQ 6.9.2 or later, you can run the strain null routine on a calibration ground, (calgnd) making the offset from the nulling compensation circuitry 0 volts. From LabVIEW simply enter a string of the type,ob0!sc1!md3!calgnd2, in the SCXI Strain Null.vi to null a particular channel to calgnd. From CVI, call SCXI_Calibrate_Setup with Calop=1 before doing the Strain Null and then afterwards with Calop = 0 to disable the calgnd. Related Links: Product Manual: SCXI-1520 User Manual Attachments:
Report Date: 07/19/2001 Last Updated: 08/01/2003 Document ID: 2BIC1IRW |
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