Improving Noise Immunity

Consider above two waveform, where there is a noisy waveform at the input of the system or a simple circuit. The functionality of the circuit is to analyze the waveform and respond accordingly. Like for example, if the signal input voltage is beyond a specific voltage the signal need to be treated HIGH and if it is below specific voltage, it needs to treat as LOW. In the first figure, in reality we know that the signal has gone down because of noise and not that in reality the signal is going LOW. Because of this scenario, we see that there is a false triggering happening on the circuit side which makes us believe that signal has actually gone low. When many such variations are present in the signal, then the system tend to oscillate sometimes. One of the best ways to eliminate such scenario is to have to threshold levels one called upper threshold and second one called lower threshold. The region between upper and lower threshold is to be treated as void region and this helps to eliminate false triggering. This way any noise on the input signal has been ignored and this will certainly improve the performance of the system under noisy conditions. Technically, we say that the system noise immunity has increased. The device which has such characteristics is called a Schmitt trigger. Schmitt trigger employs a hysteresis a loop, nothing but the loop between the upper and lower threshold.so, most of the digital circuits which need noise immunity have schmitt trigger inputs rather than TTL inputs. When we are selecting a micro controller, we see that the specifications of the input pins have schmitt trigger inputs. So, from the next time when you are designing a circuit and it has be operated under noisy conditions, schmitt trigger is the one to use at the input.