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Sizing Up Ground Loops

On Feb 28, 2018

The term Ground Loop is often thrown into design literature as a placeholder for any circuit that causes ground noise. 

In truth, a Ground Loop is one of many electrical design issues that may induce or reinforce existing ground noise. In both printed circuit boards and wired systems, a Ground Loop is, by definition, any complete circuit of low impedance grounding. This can be obvious, as with a ground plane with an air gap inside:

Figure 1: Ground Loop in a Copper Plane.

This phenomenon can also be more abstract, as in communication circuitry:

Figure 2: Communication Circuit (left) with its Ground Loop highlighted (right)

Ground Loops are typically problematic due to one of three events:

  1. AC magnetic fields pass through the loop (or DC magnetic fields, if the Ground Loops move relative to them). The changing flux induces a current in the loop. This is the only way a Ground Loop by itself can produce a noisy current.

    Figure 3: Magnetic Flux-Induced Current.

  2. Two or more circuits joining a ground loop induce different voltage levels at their respective grounds.

    Figure 4: Multiple Ground Reference Current.

  3. Two or more circuits join a ground loop at significant distances apart, producing RF noise. High frequency signals can induce noise at significantly long circuits (circuits of a length greater than 1/20 wavelength)

It’s important to remember that ground noise can exist without a Ground Loop present. For example, an External Circuit can produce ground noise on its own:

Figure 5: Ground Noise from an External Circuit.

At Ball Systems, we tend to utilize the following three methods for eliminating noise due to Ground Loops. These methods are an industry standard for cost-effectiveness and reliability.

  1. Decrease the area of the Loop: This helps to reduce the effective of AC magnetic flux. If the area of the air gap in a copper plane is reduced, or the area between wires in a system, less flux will pass through the Ground Loop and the induced current will be smaller.

    Figure 6: Reducing the Area of the Loop.

  2. Isolate the ground path: This method prevents current from travelling through the Ground Loop by employing transformers or optoisolators, which will maintain voltage levels between the signal and ground paths. Complications such as parasitic capacitance and DC attenuation will occur.



    Figure 7: Transformer and Optoisolator Placement

  3. Utilize Common-mode Choke: This is a great method for attenuating high-frequency common-mode noise.

    Figure 8: Common-Mode Choke.

There are multiple ways to eliminate Ground Loops, but attention must be paid not to sacrifice safety by breaking any necessary grounded connections. Multiple options are available to balance space availability, cost, and effectiveness, and it’s important to settle where on this triangle your interests are focused.

If you are interested in working with us on your next project, contact us at (317) 804-2330.

 

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