Schmitt Trigger - Applications - Noise Immunity

Noise Immunity

One application of a Schmitt trigger is to increase the noise immunity in a circuit with only a single input threshold. With only one input threshold, a noisy input signal near that threshold could cause the output to switch rapidly back and forth from noise alone. A noisy Schmitt Trigger input signal near one threshold can cause only one switch in output value, after which it would have to move beyond the other threshold in order to cause another switch.

For example, in Fairchild Semiconductor's QSE15x family of infrared photosensors, an amplified infrared photodiode generates an electric signal that switches frequently between its absolute lowest value and its absolute highest value. This signal is then low-pass filtered to form a smooth signal that rises and falls corresponding to the relative amount of time the switching signal is on and off. That filtered output passes to the input of a Schmitt trigger. The net effect is that the output of the Schmitt trigger only passes from low to high after a received infrared signal excites the photodiode for longer than some known delay, and once the Schmitt trigger is high, it only moves low after the infrared signal ceases to excite the photodiode for longer than a similar known delay. Whereas the photodiode is prone to spurious switching due to noise from the environment, the delay added by the filter and Schmitt trigger ensures that the output only switches when there is certainly an input stimulating the device.

As discussed in the example above, the Fairchild Semiconductor QSE15x family of photosensors use a Schmitt trigger internally for noise immunity. Schmitt triggers are common in many switching circuits for similar reasons (e.g., for switch debouncing).

The following 7400 series devices include a Schmitt trigger on their input or on each of their inputs:

• 7413: Dual Schmitt trigger 4-input NAND Gate
• 7414: Hex Schmitt trigger Inverter
• 7418: Dual Schmitt trigger 4-input NAND Gate
• 7419: Hex Schmitt trigger Inverter
• 74121: Monostable Multivibrator with Schmitt Trigger Inputs
• 74132: Quad 2-input NAND Schmitt Trigger
• 74221: Dual Monostable Multivibrator with Schmitt Trigger Input
• 74232: Quad NOR Schmitt Trigger
• 74310: Octal Buffer with Schmitt Trigger Inputs
• 74340: Octal Buffer with Schmitt Trigger Inputs and three-state inverted outputs
• 74341: Octal Buffer with Schmitt Trigger Inputs and three-state noninverted outputs
• 74344: Octal Buffer with Schmitt Trigger Inputs and three-state noninverted outputs
• 74(HC/HCT)7541 Octal Buffer with Schmitt Trigger Inputs and Three-State Noninverted Outputs
• SN74LV8151 is a 10-bit universal Schmitt-trigger buffer with 3-state outputs

A number of 4000 series devices include a Schmitt trigger on inputs, for example:

• 4017: Decade Counter with Decoded Outputs
• 4020: 14-Stage Binary Ripple Counter
• 4022: Octal Counter with Decoded Outputs
• 4024: 7-Stage Binary Ripple Counter
• 4040: 12-Stage Binary Ripple Counter
• 4093: Quad 2-Input NAND
• 40106: Hex Inverter
• 14538: Dual Monostable Multivibrator

Dual Schmitt input configurable single-gate CMOS logic, AND, OR, XOR, NAND, NOR, XNOR

• NC7SZ57 Fairchild
• NC7SZ58 Fairchild
• SN74LVC1G57 Texas Instruments
• SN74LVC1G58 Texas Instruments