Relationship Between Channel-To-Earth Isolation and Crosstalk Parameters

Updated Sep 10, 2018

Reported In

Hardware

  • PXI Analog Input Module
  • C Series Universal Analog Input Device
  • Voltage Input Module for SCXI

Driver

  • NI-DAQmx

Issue Details

I'm using a data acquisition devices and I've seen the parameters Channel-to-earth isolation and Crosstalk. How these parameters are related?   

Solution

Many instruments offer input-to-output isolation and tout that as a great feature. Some isolation is better than none, but it’s important to understand exactly what you get with an instrument that offers only input-to-output isolation: All channels share a common ground that is isolated from power. There are no consequences to this if you connect only one channel to a multi-channel instrument. That single channel’s ground reference is free to float to any value within the isolation spec of the instrument without damage. However, connect a second channel at the same time and you could be headed for major problems.
 

In particular, when using  Channel-to-Earth Ground Isolation, channels of the device and the device's Earth ground are electrically isolated from one another.  Channel-to-Earth isolation is represented in the Figure 1. Voltages of the isolated front end (Va-c ) are on the same bus; these voltages are not isolated from one another. Ve,d are on a separate bus and are isolated from the front end.  This is the most fundamental type of isolation and this protection is covered by bank and channel-to-channel isolation.  Channel-to-Earth Ground Isolation can also be thought of as Channel-to-Bus isolation with only one bank, as in the case of the NI PCI-6230. Normally, this isolation type is present on NI 9000 series modules.

Note: For the following diagrams, the diagonal hash marks indicate the isolation barrier, this separates circuitry. The transformer symbols represent electromagnetic isolation used to couple a signal across the isolation barrier by generating an electromagnetic field proportional to the electrical signal.
 



Measured signals will almost inevitably include some amount of noise or unwanted signal from the surrounding environment (crosstalk). Proper shielding and connections can reduce the effect of crosstalk between channels and other noise from the environment.

As example, measuring a 1 V signal using the NI-9215 (BNC) which has channel-to-earth ground isolation and crosstalk of -80 dB means that the maximum unwanted singal due to surrounded channels is 0.1 mV. 

Additional Information

Learn about isolation topologies used in instruments and the positive benefits that isolation can provide. Article topics include ground loops, common-mode voltage, isolation topologies, analog isolation, digital isolation, and isolation types see the Isolation Types and Considerations when Taking a Measurement white paper.

See the document Unexpected Voltages, Floating or Crosstalk in Analog Input Channels for additional details about floating measurements and crosstalk in data acquisition devices.

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