Introduction to Schmitt Trigger Circuits - Part 1

Submitted by Mr.Judith. 

Almost any digital circuit used in modern high-speed data communications needs some form of Schmitt trigger action on its inputs. The main purpose of a Schmitt trigger here is to eliminate noise and interference on data lines and provide a nice clean digital output with fast edge transitions.

The rise and fall times must be low enough in a digital output that it can be applied as inputs to the following stages in a circuit. (Many ICs have limitations of the type of edge transition that can appear on an input.) The main advantage of Schmitt triggers here is that they clean up noisy signals while still maintaining a high data flow rate, unlike filters, which can filter out noise, but slow the data rate down significantly.

Schmitt triggers are also commonly found in circuits that need a waveform with slow edge transitions to be translated into a digital waveform with fast, clean edge transitions. A Schmitt trigger can transform almost any analog waveform - such as a sine wave or sawtooth waveform - into an ON-OFF digital signal with fast edge transitions.

Schmitt triggers are active digital devices with one input and one output, like a buffer or inverter. Under operation, the digital output can either be high or low, and this output changes state only when its input voltage goes above or below two preset threshold voltage limits. If the output happens to be low, the output will not change to high unless the input signal goes above a certain upper threshold limit. Likewise, if the output happens to be high, the output will not change to low until the input signal goes below certain lower threshold limit. The lower threshold is somewhat lower than the upper threshold limit. Any kind of waveform can be applied to the input (sinusoidal waves, sawtooths, audio waveforms, pulses, etc.) as long as its amplitude is within the operating voltage range.

The diagram below shows the hysteresis resulting from the upper and lower input voltage threshold values.  Any time the input is above the upper threshold limit, the output is high. When the input is below the lower threshold, the output is low, and when the input signal voltage happens to be between the upper and lower threshold limits, the output retains its previous value, which can be either high or low. The distance between the lower threshold and the upper threshold is called the hysteresis gap. The output always retains its previous state until the input changes sufficiently to trigger it to change. This is the reason for the “trigger” designation in the name. The Schmitt trigger operates in much the same way as a bistable latch circuit or a bistable multivibrator, as it has an internal 1 bit memory, and changes its state depending on trigger conditions.

Texas Instruments provides Schmitt trigger functions in almost all of its technology families, from the old 74XX family to the latest AUP1T family. These ICs can be packaged with either an inverting or non-inverting Schmitt trigger. Most Schmitt trigger devices, such as the 74HC14, have threshold levels at a fixed ratio of Vcc. This might be adequate for most applications, but sometimes the threshold levels need to be changed depending on input signal conditions. For example, the input signal range might be smaller than the fixed hysteresis gap. The threshold levels can be changed in ICs like the 74HC14 by connecting a negative feedback resistor from output to input along with another resistor connecting the input signal to the device input. This provides the positive feedback needed for hysteresis, and the hysteresis gap can now be adjusted by changing the values of the two added resistors, or by using a potentiometer. The resistors should be of great enough value to keep the input impedance at a high level.

           

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