Chapter 1 Routing Signals with Your NI2501/2503
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National Instruments Corporation 1-3 NI 2501/2503 User Manual
Refer to Chapter2, NI 2501/2503 Operation, for more information on
triggers.
Cold-Junction Sensor Channel
For low-voltage measurements, such as those provided by thermocouples,
the NI2503 has very low thermal offset—less than 2 µV—to ensure
accurate measurements. For cold-junction compensation when switching
thermocouples, the NI 2501/2503 has a dedicated channel for a
temperature sensor. The National Instruments TB-2605 and TBX-68S
terminal blocks both have onboard temperature sensors that connect to the
dedicated cold-junction sensor channel.
NI 2501 Amplifier
The NI25 01 has an amplifier with a gain of 1. This amplifier can be
switched in-line before the COM0 signal and the AB0 signal. The amplifier
helps to decrease the settling time of the FET switch. The amplifier’s
highimpedance isolates the FET from the resistance, capacitance,
andinductance in the external wiring. This isolation decreases the
resistance/capacitance time constant seen by the FET, which improves
settling time.
The amplifier has an offset that should be calibrated for more accurate
measurements. The calibration data can be stored into and retrieved from
the EEPROM on the board. The software you received with your switch
card includes API function calls to retrieve and store calibration data.
When the amplifier is used, the switch card becomes directional, where
CHx is for signal inputs and COM0 or AB0 are for signal outputs.
The NI2501 also has an amplifier in the CJS0+ to COM0+ signal path.
This amplifier in the cold-junction sensor signal path is used to improve
settling time performance. It prevents the impedance of the cold-junction
sensor from increasing the settling time when switching the CJS channel.
As a result, when scanning thermocouples, the settling time for the
cold-junction sensor channel should be about the same as for the channels
with the thermocouples.