The noise margin is defined as:

Noise Margin High = VOH - VIH

Noise Margin Low = VIL - VOL

Is the noise margin calculated the same way for schmitt and non-schmitt buffers? If so then why?

This question is within context of input output buffers of ASICs.


1 Answer 1


The noise margin calculation is the same for all types of inputs.

When you have a constant voltage level (either high or low), the noise margin prevents noise from changing the logic level seen at the input.

When the signal is switching (high→low or low→high) and near the actual threshold, noise could make the voltage level seen at the input cross the threshold multiple times. A Schmitt trigger has two threshold levels (hysteresis), which prevents these oscillations. (See What is a Schmitt Trigger and its application exactly?)

  • \$\begingroup\$ If we are taking about asic input buffer, we may not know the voh and volof it. Similarly, for an asic output buffer, we may not know the vih and vil. What to do in that case? \$\endgroup\$
    – quantum231
    Jul 20, 2016 at 13:32
  • \$\begingroup\$ If an output and an input are not connected together, then there is no signal. \$\endgroup\$
    – CL.
    Jul 20, 2016 at 13:39

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