Basics of Schmitt Trigger Circuits – Part 4

Schmitt triggers serve a purpose in any kind of high-speed data communication system with some form of digital signal processing. Actually, they serve a dual purpose: to clean up noise and interference on data lines while still maintaining a high data flow rate, and to convert a random analogue waveform into an ON-OFF digital waveform with fast, clean edge transitions.
This provides an advantage over filters, which can filter out noise, but slow the data rate down significantly because of their limited bandwidth. Also, standard filters are not able to provide a nice, clean digital output with fast edge transitions when a slow input waveform is applied. These two advantages of Schmitt triggers are explained in more detail as follows:

Noisy Signal Inputs

The effects of noise and interference are a major problem in digital systems as longer and longer cables are used and higher and higher data rates are required. Some of the more common ways to reduce noise include using shielded cables, using twisted wires, matching impedances and reducing output impedances. These techniques can be effective in reducing noise, but there will still be some noise left on an input line, and that could trigger unwanted signals within a circuit.

Most of the standard buffers, inverters, and comparators used in digital circuits have only one threshold value on the input. So, the output changes state as soon as the input waveform crosses this threshold in either direction. If a random noise signal crosses this threshold point on an input multiple times, it will be seen on the output as a series of pulses. Also, a waveform with slow edge transitions could appear on the output as a series of oscillating noise pulses.

Sometimes a filter is used to reduce this extra noise, such as in an RC network. But any time a filter like this is used on the data path, it slows down the maximum data rate significantly. Filters block out noise, but they also block out high-frequency digital signals.

A Schmitt trigger cleans this is up. After the output changes its state as its input crosses a threshold, the threshold itself also changes, so then the input has to move farther in the opposite direction to cause another change in output. Because of this hysteresis effect, using Schmitt triggers is probably the most effective way to reduce noise and interference problems in a digital circuit. Noise and interference problems can usually be solved, if not eliminated, by adding hysteresis on the input line in the form of a Schmitt trigger. As long as the amplitude of the noise or interference on the input is less than the width of the hysteresis gap of the Schmitt trigger, there will be no effects of noise on the output.  Even if the amplitude is slightly greater, it should not affect the output unless the input signal is centered on the hysteresis gap.

The threshold levels might have to be adjusted in order to achieve maximum noise elimination. This can easily be done by changing the values of a resistorin the positive feedback network, or by using a potentiometer. The main benefit that a Schmitt trigger provides over filters is that it doesn’t slow down the data rate, and actually speeds it up in some cases via conversion of slow waveforms into fast waveforms (faster edge transitions).

Almost any digital IC on the market today uses some form of Schmitt trigger action (hysteresis) on its digital inputs. These include MCUs, memory chips, logic gates and so on. Although these digital ICs might have hysteresis on their inputs, many of them also have limitations for their input rise and fall times displayed on their spec sheets, and these have to be taken into consideration. An ideal Schmitt trigger does not have any rise or fall time limitations on its input.

Slow Input Waveforms

Sometimes the hysteresis gap is too small, or there is only one threshold value (a non-Schmitt trigger device) where the output goes high if the input rises above the threshold, and the output goes low if the input signal falls below it. In cases like these, there is a marginal area around the threshold, and a slow input signal can easily cause oscillations or excess current to flow through the circuit, which could even damage the device.

These slow input signals can sometimes happen even in fast digital circuits under power up conditions or other conditions where a filter (such as an RC network) is used to feed signals to the inputs. Problems of this type often occur within the “de-bounce” circuitry of manual switches, long cables or wiring, and heavily loaded circuits.

For example, if a slow ramp signal (integrator) is applied to a buffer and it crosses the single threshold point on the input, the output will change its state (from low to high, for example). This triggering action could cause extra current to be drawn from the power supply momentarily, and also lower the VCC power level slightly. This change could be enough to cause the output to change its state again from high to low, as the buffer senses that the input crossed the threshold again (despite the input staying the same). This could repeat again in the opposite direction, so a series of oscillating pulse would appear on the output.

 Using a Schmitt trigger in this instance will not only eliminate the oscillations, but it will also translate the slow edge transitions into a clean series of ON-OFF pulses with nearly vertical edge transitions. The output of a Schmitt trigger can then be used to as an input to the following device according to its rise and fall time specs. (Although oscillations can be eliminated by using a Schmitt trigger, there could still be excess current flow in a transition, which may need to be corrected some other way.)

The Schmitt trigger is also found in cases where an analogue input, such as a sinusoidal waveform, audio waveform, or sawtooth waveform, needs to be converted into a square wave or some other type of ON-OFF digital signal with fast edge transitions.

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