Use External Excitation with the NI 9237 Module

Updated Jul 20, 2020

Reported In

Hardware

  • NI-9237
  • NI-9942

Issue Details

  • How do I use external excitation with an NI 9237?
  • What is the purpose of the NI 9942 accessory?
  • Can I use a load cell of 700 Ohms in full bridge configuration with external excitation?

Solution

For a full bridge measurement, the 9237 is only measuring differential voltage and is independent of the nominal resistance of the strain gauge. The input impedance of the NI 9237 is essentially an open circuit and should not affect the bridge based measurement.
 

For the external excitation voltage, there are two versions of the 9237: One with a Dsub connector and one with RJ-50 connectors. You can find the pinout of both connector variants of the device in the NI 9237 Getting Started Guide. The excitation pins for each channel are also noted below:
 

Dsub

Excitation Voltage Source: V_EX+/V_EX- on pins 10/29
  • Ch 0: EX0+/EX0- on pins 2/21
  • Ch 1: EX1+/EX1- on pins 6/25
  • Ch 2: EX2+/EX2- on pins 12/31
  • Ch 3: EX3+/EX3- on pins 16/35
 

RJ-50

Excitation Voltage Source: EX+/EX- on NI 9942 connector. It will be used if you are hooking up the external excitation to the NI 9237 module
  • Ch 0: EX+/EX- on pins 6/7 on CH0
  • Ch 1: EX+/EX- on pins 6/7 on CH1
  • Ch 2: EX+/EX- on pins 6/7 on CH2
  • Ch 3: EX+/EX- on pins 6/7 on CH3

Additional Information

The 9237 can provide 10 volts of excitation. If an external power supply must be used, because it must be connected through the NI 9237, do not exceed 10 V. Doing otherwise can damage the module.
 
When you enable external excitation, the excitation ± terminals (e.g. pins 6 and 7 on the RJ-50 version) disconnect from the internal cDAQ backplane source (used for internal excitation) and connect to the excitation voltage source pins on the front of the device. With external excitation enabled, a 2-10 volt external power supply needs to be connected to those pins in order to provide excitation. The externally supplied voltage is passed directly through to each channel and is not affected by setting a different excitation value in software.
 
However, if external excitation is selected but an external excitation source is not connected, there will be a voltage present on the EX ± because these channels are not completely decoupled from the internal excitation source. The voltage here is relative to any previous external excitation source that may have been connected.
 
When purchasing an external power supply for use with the 9237, the accuracy and precision of the power supply are not particularly important because the 9237 uses ratiometric measurements, meaning that most of these inaccuracies will be accounted for by the device itself.