- Schmitt Gates Defined: A Schmitt Gate is a type of logic gate that employs two separate threshold voltages to handle transitions, enhancing stability and reducing noise.
- Hysteresis Function: The unique feature of Schmitt Gates is hysteresis, a mechanism that allows them to maintain output stability against noisy signals.
- Circuit Symbols: Circuit symbols for Schmitt NAND and Schmitt inverter gates help differentiate them from conventional logic gates.
- Comparison to Conventional Gates: Unlike standard gates, Schmitt Gates operate on the principle of hysteresis, similar to the B-H loop in ferromagnetic materials.
- Practical Application: Schmitt Gates are crucial in electronic circuits where signal stability is essential, particularly in environments with high electrical noise.
A Schmitt Gates device is a logic gate whose input uses two trip points instead of one. Ordinary gates switch at a single threshold voltage for both rising and falling edges, somewhere between a valid low and a valid high. A slow or noisy edge that lingers near that one trip point can make the output chatter.
An ideal clean edge into an inverter gives the clean response in the graph below.
A practical noisy or slow input can produce the response in the next figure.
Noise on the slow input shows up as extra edges on the output. Split the rising and falling trip voltages so a small wiggle cannot recross the threshold.
Internal positive feedback creates that split, called hysteresis. Schmitt-input ICs already include the feedback; you do not add it by changing how a standard gate is made.
Catalogue examples are Schmitt NAND gates and Schmitt inverters. Those Schmitt gates read the input against two thresholds: one for low-to-high output change and one for high-to-low. Figures (a) and (b) below are the circuit symbols of a Schmitt NAND and a Schmitt inverter.
Schmitt gates differ from ordinary gates because their input transfer curve has a hysteresis loop. The loop looks similar to a B-H plot for a ferromagnetic material, as a sketch only, not as the same physics.
The figure above is a typical transfer characteristic. The gap between the two thresholds is the hysteresis voltage. The next figure shows a Schmitt inverter cleaning a slow, noisy input.
