Common Mode Signals vs. Differential Mode Signals
This post will discuss how to solve EMI issues with common mode chokes and the advantages of using them over other solutions.
Understanding the difference between common mode signals and differential mode signals is extremely important, as we need to know what the circuit requires to reduce noise.
How do we know if the noise is differential or common mode? One way to test whether the problem is common mode or differential mode would be to attach a Würth Elektronik snap cable ferrite to the cable in question.
By attaching the snap cable ferrite, we have to ensure that if there is an improvement in the reduction of noise, the problem is common mode. If not, then there is a differential problem. But keep in mind that in the cable are both wires included, towards and backwards the load.
Common Mode Issues
Next we will discuss the common mode issues and focus on our surface mount product line. Common mode chokes are used to reduce a specific type of electrical noise known as Common Mode Noise. They are also referred to as Current Compensated chokes or Current Cancellation chokes.
Common mode chokes work well in applications like AC/DC power supplies (lines with large current flow) and signal lines, where distortion of the signal will create problems. When it comes to AC power, common mode noise is the noise signal between the ground and neutral/phase.
These tend to have higher frequency ranges than differential mode noise due to the fact that these originate from differential mode signals that are inductively or capacitive coupled.
Using this example of a flyback converter, we can see the formation of differential mode interference current on the supply lines. We can see that the interference current is travelling in two different directions.
For the differential signal we can see that the current flows into the circuit (V1), through the load and back out (V2). The voltages on each wire are relative to ground potential. They do not travel through ground though.
For systems that are cable based, we are looking for this differential signal. This is the signal that will carry the data.
For differential mode signals, the voltage on V1 minus V2 equals the Differential Voltage. Through a choke, this signal will create opposite, but equal, magnetic fields which will cancel each other out. The signal will pass through the choke unattenuated as it sees zero impedance created.
In this situation, we see that the common mode interference current is travelling in the same direction into both paths of the circuit. There has to be a return path and this is usually through a ground plane or wire.
Here we see that there is a parasitic capacitance, creating a path for the common mode current to flow to ground. Current will not flow through the load, as there is no potential difference across it.
The magnitudes of the signals are equal. These signals will add together instead of cancelling each other out. The result will be high impedance to the common mode signal, which will become heavily attenuated. This potential will vary with respect to ground. As this varies from ground, radiation or noise will become present.
These signals are unwanted, as they do not carry any data. Using independent grounds can also create a difference in potential resulting in common mode noise. This is very common as these can create ground loops.
This also occurs in ungrounded sources, such as remote ones. Making sure that your cable for the analog signal is grounded well and having a properly grounded system can alleviate a lot of headaches. These can act as antennas if designed wrong.
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