How to use common-mode chokes on signal lines to eliminate common-mode noise

　　1. Offset improvement function for the signal line common-mode choke

　　The purpose of using common-mode chokes in signal lines is to eliminate common-mode noise. Since common-mode chokes are components used in transformers, they can be expected to improve offset in differential transmission circuits. In differential transmission circuits, designing two lines in a balanced state is ideal, but line imbalance events caused by manufacturing imbalance still occur from time to time. In this case, the arrival times of the signals on the two lines will differ, causing the transmission signal to shift accordingly.

　　Inserting a common-mode choke here can reduce offset.

　　Figure 2 shows the structure of offset improvement using a common-mode choke

　　The common-mode choke has the same structure as a transformer, so when the timing of the two signal lines rises and falls unevenly, the common-mode choke generates an electromotive force on the opposite side to ensure current balance. This approach ensures consistent timing of differential signals and improves offset.

　　This is a ripple detected in differential transmission lines with intentionally set different line lengths. It can be seen that without a filter (common-mode choke), the timing of the DOUT+ and DOUT- rise/fall is off.

　　The sum of DOUT+ and DOUT- should be fixed values when the two lines are balanced; at this point, due to the balance being broken, some offset exists.

　　When the common-mode choke is inserted, the timing of the rise and fall of both lines is consistent, showing that the sum of DOUT+ and DOUT- is basically fixed, and offset is improved.

　　2. Equivalent circuit diagram of the common-mode choke coil

　　I think insiders are already familiar with the following content, but since I am often asked these questions, I would like to take this opportunity to introduce the meaning of the black dots recorded in the equivalent circuit diagrams of common-mode chokes.

　　The equivalent circuit diagram of the common-mode choke is shown in Figure 4.

　　The shape is basically the same as the transformer. On one side of the coil, there are two spots marked with black dots.

　　People often ask: "Does this mean the coil is starting to rotate?" Actually, this doesn't mean there's something at the black spot, but rather the direction of magnetic force combining the two coils. Previous reports have already explained the structure of the common-mode choke. When the common-mode choke is operating, it requires the magnetic flux generated in both coils to be mutually reinforced by the common-mode current, while offsetting the differential mode current.

　　Therefore, when the rotation directions of the two coils are out of order, the opposite effect often occurs.

　　As shown in the upper half of Figure 5, when the black spot on the equivalent circuit is on the same side as the coil, the magnetic coupling acts as a common-mode choke. As shown in the lower half, when the black spot on the equivalent circuit is on the other side of the coil, the magnetic coupling no longer functions as a common-mode choke.

　　As you can see, the position of the black dot indicates the magnetic binding direction of each coil, and does not mean there is anything on the side with the black dot.

　　Additionally, this black dot originally indicated the voltage polarity when using a transformer.