Chapter 3 Theory of Operation
SCXI-1121 User Manual 3-12 www.natinst.com
• You also need a 120 Ω 1/4 W precision resistor with tempco less than
or equal to 5 ppm, or an ammeter with the following specifications:
– Accuracy: ±0.004% standard
±0.016% sufficient
– Range: 0.5 mA
– Resolution: 6 1/2 digits
• If you use the resistor to calibrate the current excitation, you also need
an ohmmeter with four-wire measurement and the following
specifications:
– Accuracy: ±0.004% standard
±0.016% sufficient
– Range: 200 Ω
– Resolution: 5 1/2 digits
A multiranging 5 1/2-digit digital multimeter can provide you with most of
the necessary functions described previously. We will refer to the
measuring instrument as a digital multimeter (DMM).
Each channel on the SCXI-1121 has two potentiometers dedicated for
calibration. For the amplifier channels, one potentiometer is used to null the
output offset; the other is used to null the input offset. On the excitation
channels, one potentiometer is used to adjust the voltage reference, while
the other is used to adjust the current source.
Offset Null Adjust
Follow these steps to null the offset of the amplifier channels:
1. Short the inputs of the DMM together and then connect them to chassis
ground.
2. Record the measurement indicated by the DMM display. This is the
DMM inherent offset and should be subtracted from subsequent
measurements.
3. Short the channel inputs of interest together and then to chassis
ground.
4. Set the amplifier gain to 1.
5. Connect the amplifier output to the DMM. Make sure that the DMM
can achieve the accuracy and resolution you need.
6. Adjust the output potentiometer of the channel of interest until the
output is 0 ± 3 mV.
7. Set the amplifier gain to 1,000.