National Instruments SCXI-1121 - Manual (Page 75)

Chapter 3 Theory of Operation
© National Instruments Corporation 3-13 SCXI-1121 User Manual
8. Adjust the input potentiometer of the channel of interest until the
output is 0 ± 6 mV.
9. Go to the next channel.
To avoid erroneous results when nulling the amplifier, follow these steps in
the order indicated.
Excitation Adjust
When calibrating the excitation channels, you should always start with the
voltage excitation and then proceed to the current excitation, because the
voltage excitation reference is used as a voltage reference for the current
excitation. The following procedure will show you how to recalibrate your
module excitation channel to the factory-calibration setting.
1. Connect a 120
Ω
precision resistor to the output of your excitation
channel. Before connecting this resistor, measure it with a four-wire
ohmmeter and record the exact value you have measured.
2. Set up the excitation channel of interest to 3.333 V excitation level.
3. Connect your DMM leads to the excitation output as close as possible
to the resistor body.
4. Adjust the excitation voltage potentiometer until you read
3.333 V ± 0.04%.
5. Set up your channel for 150 µA excitation level.
6. Adjust the excitation current potentiometer until you read
(150 µA x R120) V ± 0.04%, where R
120
is the measured value of the
precision resistor.
7. Go to the next channel.
If you are using an ammeter to calibrate the current excitation level, you do
not need a low-tempco resistor a simple 120
Ω
, 1%, 1/4 W, 100 ppm,
metal-film resistor will do, and you do not need to measure the resistor.
After following the previous procedure through step 4, follow these steps:
1. Remove the resistor from the excitation channel.
2. Set up your channel for 150 µA excitation level.
3. Connect the ammeter leads to the excitation channel output.
4. Adjust the excitation current potentiometer until you read
150 µA ± 0.04%.
5. Go to the next channel.