5.1.10. Simultaneous Switching Noise Guidelines
Simultaneous switching noise (SSN) is another important factor to consider when designing a PCB. Although SSN is dominant on the device package, the board layout can help reduce some of the noise.
Every current loop has an inductance value. The current loop in Figure 82 has the following inductance:
When a driver switches from high to low, a voltage develops in the GND plane, thus:
The noise that develops in the GND plane can become a problem for signal-integrity, especially when there are a lot of drivers switching simultaneously. The noise generated by SSNs can couple to adjacent structures. Proper layout and decoupling reduces noise coupling. The high number of simultaneously switching drivers can cause the power supply to collapse. Thus, the power supply voltage at a certain region loses some of its strength, depending on where the switching is concentrated.
For more information, refer to the following:
- Lloop = L1 (signal) + L2 (GND) – 2 LM (mutual inductance)
- V = Lloop (di/dt)
- AN 472: Stratix II GX SSN Design Guidelines (PDF)
- AN 508: Cyclone III Simultaneous Switching Noise (SSN) Design Guidelines (PDF)