How Can PCB Effectively Suppress Electromagnetic Interference
Strictly speaking, all voltage signals are "differential", because one voltage is always relative to another.But most of the time, we use ground as a voltage reference point to measure another voltage, which is called a single-ended signal.Because it is compared with "ground", a single-ended signal usually behaves on a PCB with only one Track.
So what is a differential signal?Different from the traditional method of one signal wire and one ground wire, differential transmission transmits signals on both wires. The amplitudes of the two signals are equal, the phase difference is 180 degrees, and the polarity is opposite.The signal transmitted over these two wires is a differential signal.
Advantages and disadvantages of differential signals
advantages
Strong anti-interference ability.The interference noise is generally loaded onto two signal lines at the same time and the difference is 0, that is, the noise has no influence on the logical meaning of the signal.
It can effectively suppress electromagnetic interference (EMI).Since the two lines are close together and the signal amplitude is equal, the coupled electromagnetic fields between the two lines and the ground are equal in amplitude, and their signals are opposite in polarity, and their electromagnetic fields cancel each other out.Therefore, there is less electromagnetic interference to the outside world.
Accurate timing positioning.The receiving end of differential signal is the point where the difference of signal amplitude on two lines has positive and negative jump, which is used as the point of logic 0/1 jump.However, the threshold voltage is used as the jump point of signal logic 0/1 in the common single-terminal signal, which is greatly affected by the ratio of threshold voltage and signal amplitude voltage, so it is not suitable for the signal with low amplitude.
disadvantages
If the area of the circuit board is very tight, the single-terminal signal can only have one signal line, the ground line goes to the ground plane, and the difference signal must go to two lines of equal length, equal width, close together, and at the same level.This often happens when the pins of the chip are so closely spaced that they can only pass through a single thread.
Differential signal wiring requirements
On a PCB board, the differential routing must be two lines of equal length, equal width, close together, and on the same plane.
Equal length: Equal length means that the length of two lines should be as long as possible, in order to ensure that the two differential signals remain opposite polarity at all times.Reduce the common-mode component.
Equal width and equal distance: Equal width means that the line width of two signals should be kept the same, and equal distance means that the space between two lines should be kept constant and keep parallel.
The difference lines are close to each other, and the closer they are, the smaller the loop area is, and the smaller the loop area of induced current under the line is. EMI control is also good.
Differential wiring is required on the same plate layer, and common mode noise is introduced because the difference between different layers, such as impedance and through hole, will reduce the effect of differential mode transmission.
Reminder: In THE PCB routing rules, there is a "key signal line priority" principle, that is, power supply, simulation signal, high-speed signal, clock signal, differential signal and synchronization signal and other key signal priority routing.
