PCB News - Precautions for current loop in PCB design

Precautions for current loop in PCB design
For the current loop, you need to pay attention to the following basic matters:

1. If you use wiring, you should make it as thick as possible
If wiring is to be considered for the ground connection on the PCB board, the design should make the wiring as thick as possible. This is a good rule of thumb, but you should know that the minimum width of the ground wire is the effective width from this point to the end, where "end" refers to the point farthest from the power connection.
2. Ground loops should be avoided
3. If the ground plane cannot be used, a star connection strategy should be adopted (see Figure 6)
In this way, the ground current returns to the power connection end independently. In Figure 6, notice that not all devices have their own loops, U1 and U2 share loops. This can be done if you follow the guidelines in Article 4 and Article 5 below.
4. Digital current should not flow through analog devices
When the digital device is switched, the digital current in the loop is quite large, but it is only instantaneous. This phenomenon is caused by the effective inductance and impedance of the ground wire. For the inductance part of the ground plane or ground trace, the calculation formula is V = Ldi/dt, where V is the voltage generated, L is the inductance of the ground plane or ground trace, di is the current change of the digital device, and dt is duration. The calculation formula for the influence on the ground wire impedance is V= RI, where V is the voltage generated, R is the impedance of the ground plane or ground trace, and I is the current change caused by the digital device. These voltage changes on the ground plane or ground trace of the analog device will change the relationship between the signal and the ground in the signal chain (that is, the signal's ground voltage).
5. High-speed current should not flow through low-speed devices
Similar to the above, the ground return signal of the high-speed circuit will also cause the voltage on the ground plane to change. The calculation formula for this interference is the same as above. For the inductance of the ground plane or grounding trace, V = Ldi/dt; for the impedance of the ground plane or grounding trace, V = RI. Like digital current, when the ground plane or ground trace of a high-speed circuit passes through an analog device, the voltage change on the ground wire will change the relationship between the signal and the ground in the signal chain.