Where Is the Ground Reference on NI-9223?

Updated Jul 14, 2021

Reported In

Hardware

  • NI-9223
  • NI-9222
  • NI-9224
  • NI-9225
  • NI-9229
  • NI-9238
  • NI-9239
  • NI-9251

Issue Details

In the Field Wiring and Noise Considerations for Analog Signals it is suggested to use bias resistors between the signal lines (AI+ and AI-) and ground reference of the channel (AI GND or COM) when differential configuration is used with floating sources. However, the NI-9223 does not have a GND or COM connection available. Where to connect the bias resistors in this case?
 

Solution

The information provided in the document above is a general recommendation, a specific module's wiring recommendation for different configurations can be found in the Getting Started Guide of the module. For the NI-9223 we do not need to use bias resistors, the signal source can be connected directly to the AI channel. 
 
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Additional Information

The purpose of the bias resistors is to ensure that the common-mode voltage level of the signal with respect to the measurement system ground remains in the common-mode input range of the measurement device. If we check the datasheet of the NI-9223, we do not find a specification for common-mode voltage range, while for the other modules (those that require bias resistors in differential input with floating sources) we do. In case of NI-9223 the ground reference of the internal ADC is internally connected to the AI- input of the channel which eliminates the need for bias resistors. The same applies to the other modules with channel-to-channel isolation and the NI-9251 which has no isolation.

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