National Instruments 321906B-01 用户手册

Chapter 1

Routing Signals with Your NI 2501/2503

©

 National Instruments Corporation

1-3

Refer to Chapter 2, 

, for more information on 

triggers.

For low-voltage measurements, such as those provided by thermocouples, 
the NI 2503 has very low thermal offset—less than 2 

µ

V—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.

The NI 2501 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 
high impedance isolates the FET from the resistance, capacitance, 
and inductance 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 NI 2501 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.