Loops on the Ground

This week in one of my group meetings, a colleague presented her work on identifying and eliminating ground loops. I had never heard of the phenomenon, but it's an interesting pitfall in circuit design. When we talk about voltages, we always need some reference point – Voltage isn't an absolute value, but the difference between two points. When designing a circuit, it can be useful (and safer) to have a connection that can give or take current freely. This lets you get rid of excess positive or negative charges, like static electricity, that otherwise could damage components. This is why modern outlets have 3 connections: positive, negative, and ground.

When we connect devices together though, a problem can arise:

Here we have two components, both connected to AC outlets (+, –, GND). One of them sends a signal to the other. Since voltage is a difference, we need a second connection as reference, so we use the ground wire from the outlets. Now we have a loop of wire though – A loop is a type of antenna, which can pick up signals.

According to this article, the voltage produced by a small-loop antenna is

where N is the number of loops (1 in this case), A is the area, λ is the wavelength, E is the electric field strength, and θ is the angle between the loop and the signal. This article, about honeybee exposure to RF signals, gives the maximum electric field strength measured as 0.226 V/m. FM radio signals go up to about 200 MHz, or a wavelength of 1.5 m. If the area of the loop is 0.1 square meters, or about 32 cm per side, we can get as much as 95 mV of interference, easily enough to throw off a delicate measurement!

Marika's parents are visiting us right now, so I asked my engineer father-in-law Scott if he was familiar with ground loops. He related an interesting experience: He used to have a phone that he would plug into his car power, and an audio jack to play music through the speakers. The power and audio shared a ground connection, resulting in a loop that would add static noise on top of anything he played!