Concept of Hysteresis and Threshold point in Schmitt Trigger

Question

I understand the concept of Schmitt Trigger where there is 2 transition threshold points so that input changing between the hysteresis would not create output transitions . The very first observation that I made in this design requirement is the overlap/ hysteresis is between +1 and -1, so shouldn't the threshold be +1 and -1. So can threshold be +2 and -2 as in the options ? Are there more points to consider while deciding threshold ? Can I say as a thumbrule that the overlapping segment of inputs should always be included in the hysterisis that is here between +1 and -1, anything more is always good but that should always be less than the Maximum and minimum that is here +7 and -7?

Answer 1

In this case, the noise goes to -1V when the pulse is in a positive phase and to +1V when the pulse is in the negative phase.

Choosing +/- 1V as thresholds could cause the noise to change the output state when it should not because the noise is reaching the switching threshold. For the +/- 0.5V option, the noise would definitely cause unintended switching.

Therefore +/- 0.5V is not a good option.

As the total signal varies between +7V and -7V, the +/- 8V option would never change the output state.

The +/-2V option gives an extra 1V of noise immunity and (for the given task) would be suitable thresholds.

For the general case, the thresholds should also be below the noise free input signal; as we usually protect against noise up to a certain level, a noise free input should be part of the calculation.