Is there a way to quantify or graphically represent noise immunity? Is noise immunity different than noise margin? If yes, how?

Question

Generally, there is a talk about noise margin and noise immunity in digital circuits. Both terms seem interrelated but some texts differ from this. My query is whether these two are entirely different terms? If so, then is there a way to quantify noise immunity?

Answer 1

In my experience, the two refer to the same thing. However, "noise immunity" is a concept, and "noise margin" is a quantity. A salesman might say how the product has exceptional noise immunity, but the spec sheet would list the noise margin in volts, for example.

In the context of digital circuits, the noise margin is the difference between the nominal voltage level of a signal, and the nearest voltage at which it could be interpreted to be in a different state. For example, let's say a digital input has a logic low threshold of 1.2 V and a logic high threshold of 2.7 V. With 3.3 V in, the noise margin is 600 mV. If the 3.3 V signal dips by 600 mV, it is no longer guaranteed to be interpreted as a logic high. A 0 V signal has 1.2 V of noise margin in this example.