PCB design for noise budget, voltage tolerance, do you understand? (I)

In high-speed PCB design, a large part of the work is to make noise budgets, planning the noise level of various noise sources in the system. This involves a very basic but very important concept: voltage tolerance.

Voltage margin is the difference between the output of the driver and the sensitivity of the receiver input in the worst case. Many devices are input voltage sensitive.

For the driver terminal output high level is not lower than VOH min, the output low level is not higher than VOL max. For the receiver input, as long as it is higher than VIH min, it can ensure reliable reception of logic 1, as long as it is lower than VIL max to ensure that logic 0 is received. However, if the input voltage is in the region between VIH min and VIL max, it may be judged as 1 by the receiving circuit, or may be judged as 0, so the input voltage cannot be in this unstable region for the receiving circuit. In terms of high-level output and input relationship, there is a difference between the minimum output value and the minimum allowable input value, which is the high-level voltage tolerance.

That is: high level voltage tolerance = VOH min - VIH min . The same low level voltage tolerance = VIL max - VOL max .

The voltage margin provides a buffer zone for handling various undesirable factors in the processing circuitry, allowing the system to tolerate signal distortion during transmission and reception to some extent. The voltage tolerance plays an important role in the system noise budget design. The total noise of the system cannot exceed the voltage tolerance. Otherwise, the system will not work normally when the signal enters the unstable region of the receiving end.