National Instruments SCXI-1121 - Manual (Page 76)

Chapter 3 Theory of Operation
SCXI-1121 User Manual 3-14 www.natinst.com
This procedure calibrates the 10 V and 450 µA levels at the same time but
the accuracy achieved is limited to ±0.2%. To achieve better accuracies at
these levels, follow the procedure indicated above but set the excitation
levels to 10 V and to 450 µA instead of 3.333 V and 150 µA. If you do so,
the lower excitation levels of this channel will then be calibrated to ±0.2%
instead of to 0.04%. In the factory, the module is calibrated for 3.333 V and
150 µA.
You can seal the potentiometers after calibration with antisabotage lacquer
to avoid tampering with the calibration.
Table 3-2 lists the potentiometer reference designators that correspond to
each channel.
The resistor used to calibrate the current level must be a precision type with
a tempco of 5 ppm or less. You should measure and record the resistor value
before each calibration procedure. The DMM you are using should provide
you with the required resolution and accuracy to achieve the calibration
levels indicated in the paragraphs above. Annual or semi-annual calibration
is recommended to maintain the accuracy level.
Table 3-2. Calibration Potentiometer Reference Designators
Input Channel
Number
Amplifier Channel Excitation Channel
Input Null Output Null Voltage Mode Current Mode
0 R2 R3 R10 R7
1 R16 R4 R20 R17
2 R26 R5 R30 R27
3 R36 R6 R40 R37