2016/2/1 下午 12:03:36        Administrator        General   0 Comments
Schmitt Trigger in Noise Suppression

Noises are typically found in digital circuits. And they frequently cause errors in their applications, affecting the accuracy of the applicable data. This article will be elaborating more on how Schmitt Triggers can be used to suppress these noises.

In common logic gates, when the input signal is greater than Vih, the output will be treated as 1; when it is smaller than Vil, the output will be treated as 0;
As the strength (voltage) between Vih and Vil may also be treated as 1s or 0s, it is generally not recommended to operate at this range of voltages. In addition, the rise time, fall time, and glitch that exist in an actual signal may also be treated as 1s and 0s. Please refer to the graph below.

In observation of the above mentioned discrepancy, a Schmitt trigger can be applied in suppressing this noise, as the output will be treated as one only when the input signal is VT+ (Positive-going input threshold voltage); zero when it is smaller than VT- (Negative-going input threshold voltage). Though the range between VT+ and VT- can be vague, it can additionally be used as a noise suppression zone as shown in the graph below.

Schmitt triggers are best applied in applications with the Vcc range of 1.65 – 5.5 V, albeit careful attention is needed when applying the operating voltage. If the signal is at 3.3 V, the same voltage should be used. If 5 V is used, the signal will never attain VT+, rendering the output permanently as 0. 

The above mentioned method of noise suppression with Schmitt triggers has already seen usage in many applications. However, true signal accuracy can only be achieved in a real life scenario if the properties of the signal are known and the appropriate solution applied.

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