Schmidt NAND gate is also called Schmidt trigger. The device can work like a normal NAND gate, and can also be connected as a Schmitt trigger. It will be used as pulse shaping circuit in digital display circuit.

Compared with the logic symbol of NAND gate, the logic symbol in ic is slightly different, and a box shape is added, which represents an important hysteresis characteristic of Schmidt trigger. When the input terminals are not connected in parallel, its input-output characteristics are as follows: when the input voltage V 1 rises to VT+ level, the flip-flop flips and outputs a negative jump; After a period of time, when the input voltage drops back to the VT+ level, the output will not return to the initial state, but when the input V 1 continues to drop to the VT- level, the output will flip to the high level (positive jump). This phenomenon is called hysteresis characteristic, VT+-VT-=△VT. △VT is called the hysteresis voltage of Schmidt trigger. △VT is related to the supply voltage of IC. When the power supply voltage rises, △VT increases slightly, and the value of △VT is generally around 3V. Schmidt trigger is often used to shape the pulse waveform and make the rising or falling edge of the waveform steep. It can also be used for voltage amplitude discrimination. It is also a very common device in digital circuits.