The concept of voltage tolerance in the noise budget of PCB design
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 VOHmin, the output low level is not higher than VOLmax. For the input of the receiving end, as long as it is higher than VIHmin, it can ensure reliable reception of logic 1, as long as it is lower than VILmax, it can guarantee the reception of logic 0. However, if the input voltage is in the region between VIHmin and VILmax, it may be judged as 1 by the receiving circuit, and may also 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.
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.
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