National Instruments SCXI-1120 - Manual (Page 58)

Chapter 3 Theory of Operation
© National Instruments Corporation 3-11 SCXI-1120 User Manual
A multiranging 3 1/2-digit digital multimeter can provide you with the necessary function as
described previously. We will refer to the measuring instrument as a digital multimeter (DMM).
Each channel on the SCXI-1120 has two potentiometers dedicated for calibration. One
potentiometer is used to null the output offset; the other is used to null the input offset.
Offset Null Adjust
To null the offset of the amplifier channels, complete the following steps:
1. Set the DMM range to the smallest range that can measure
±
3 mV.
2. Short the inputs of the DMM together and then to chassis ground.
3. Record the measurement indicated by the DMM display. This is the DMM inherent offset
and it should be subtracted from subsequent measurements.
4. Short the desired channel inputs together and then to chassis ground.
5. Set the amplifier gain to 1.
6. Connect the amplifier output to the DMM. Make sure that the DMM can achieve the
accuracy and resolution you need.
7. Adjust the output potentiometer of the desired channel until the output is 0
±
3 mV.
8. Set the DMM range to the smallest range that can measure
±
6 mV.
9. Set the amplifier gain to 1,000.
10. Adjust the input potentiometer of the desired channel until the output is 0
±
6 mV.
11. Go to the next channel.
To avoid erroneous results when nulling the amplifier, follow these steps in the order indicated.
You may 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.