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National Instruments Corporation
3
SCXI-1303 Installation Guide
whether your signal is ground-referenced or floating. The channels with
open thermocouples will saturate at all sample rates of the module. To
replace your resistor networks, refer to the Changing Resistor Networks
section.
SCXI-1100 Module
For the open thermocouple channel to saturate without disturbing the
measurements on any other channel, use an interchannel delay of 200
µ
s
at a gain of 100 or higher, which corresponds to a sample rate of 5 kHz.
With the 10
Ω
bias resistors installed in the SCXI-1303, you can measure
accurately at the module’s maximum sampling rate, but the open
thermocouple channel may not saturate if the interchannel delay is less
than 200
µ
s or if the sample rate is more than 5 kHz at a gain of 100 or
higher.
If you want fast open-thermocouple detection and you have a short
thermocouple or if high accuracy is not important, you can replace the
pullup resistors with a lower value resistor network. For example, you
could replace the pullup resistor network with a 1 M
Ω,
10-pin bused
configuration resistor network (not supplied) and have a sample rate of
20 kHz (interchannel delay of 50
µ
s typical).
With a 10
Ω
bias resistor
network, the current leakage would be 5
µ
A (5 V/1 M
Ω
), which may result
in a larger offset error because of thermocouple lead resistance. To replace
your resistor networks, refer to the Changing Resistor Networks section.
Errors Due to Open-Thermocouple Detection Circuitry
Open-thermocouple detection circuitry can cause two types of
measurement errors. These errors are the results of common-mode voltage
at the input of the SCXI module and current leakage into your signal leads.
Common-Mode Voltage at the Input of the
SCXI Module
With 10 M
Ω
pullup and bias resistors, a common-mode voltage of
2.5 VDC will develop if the thermocouple is floating. At a gain of 100,
the common-mode rejection of the SCXI-1102/B/C module is sufficiently
high that the resulting offset voltage is negligible.
If your application demands extremely high accuracy, you can eliminate
this offset error by calibrating your system. You can also remove the pullup
resistor, giving up the open-thermocouple detection feature in the process
or use the 10
Ω
bias resistor networks, which will bring the common-mode
voltage down to nearly 0 VDC.
Summary of Contents for SCXI-1303
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