National Instruments SCXI-1121 - Manual (Page 73)

Chapter 3 Theory of Operation
© National Instruments Corporation 3-11 SCXI-1121 User Manual
After isolation, further filtering is available to increase the noise immunity
of the amplifier channel. It is important to note that the overall amplifier
bandwidth is determined by both filtering stages, so to achieve the required
bandwidth, both filtering sections should be set the same, as indicated in
Chapter 2,
Configuration and Installation .
Excitation Output Channels
In addition to the four input channels, the SCXI-1121 contains four fully
isolated excitation channels, each corresponding to an input channel. For
instance, input channel 0 corresponds to excitation channel 0. Each
excitation channel consists of a voltage/current source with overvoltage
protection and current limiting. Two levels of excitation are available for
each mode of operation. In the voltage mode you can set the level to
3.333 V or 10 V in the current mode you can set the level to 150 µA or
450 µA. You can choose one configuration out of the four available. To
configure the excitation channels refer to Chapter 2,
Configuration and
Installation . The excitation channels are isolated from each other and are
independently configurable for voltage or current excitation.
Calibration
Calibration Equipment Requirements
For best measurement results, calibrate the SCXI-1121 so that its offset is
adjusted to 0 ± 3 mV RTO and 0 ± 6 µV RTI and its excitation output is
adjusted to ±0.04%. According to standard practice, the equipment used to
calibrate the SCXI-1121 should be 10 times as accurate as the SCXI-1121,
that is, have 0.004% rated accuracy. Practically speaking, calibration
equipment with four times the accuracy of the item under calibration is
generally considered acceptable. Four times the SCXI-1121 accuracy is
0.016%. To calibrate the SCXI-1121 you need the following equipment:
• For the excitation channels, you need a voltmeter with the following
specifications:
– Accuracy: ±0.004% standard
±0.016% sufficient
– Range: 0 to +5 V for 3.333 V and greater than +10 V for 10 V
– Resolution: 5 1/2 digits