Schmitt Trigger Hysteresis Provides Noise-free Switching and Output

Key Takeaways

• A Schmitt trigger can be used as a comparator when it has no hysteresis to create clean digital pulses.

• A Schmitt trigger can have hysteresis implemented to provide noise immunity, making it essentially a comparator with added hysteresis.

• For a high-speed or high-frequency Schmitt trigger, you can create simulations using the right op-amp simulation models and circuit design tools.

Schmitt triggers have been around longer than some engineers have been alive, and they have long been a fundamental component for tracking switching between two voltage states, just like a comparator. These days, Schmitt triggers are integrated into IC designs, or they might be used in medium-voltage circuits as a simple analog comparator. With the range of op-amps available on the market, it’s easy to create a Schmitt trigger circuit for a range of analog applications.

Just like comparators, Schmitt triggers can be designed with hysteresis to provide some insensitivity to noise. To ensure noise immunity during switching, the Schmitt trigger hysteresis window needs to be carefully designed to accommodate any noise on the input trigger signal. Here’s how to design the hysteresis window in a Schmitt trigger circuit and why you might want to use a Schmitt trigger instead of a comparator circuit in your analog system.

Why Use a Schmitt Trigger Instead of a Comparator?

A Schmitt trigger is closely related to a comparator circuit. Schmitt triggers and comparators are largely the same circuits; any comparator becomes a Schmitt trigger when some positive feedback is added to the circuit, which then adds hysteresis. In other words, all Schmitt triggers are comparators, they are just configured to switch at different transition voltages thanks to the addition of hysteresis.

Comparators with hysteresis are sometimes distinguished from Schmitt triggers as entirely different circuits, but they are essentially the same type of circuit and provide very similar functions. All comparators have some built-in hysteresis, while Schmitt triggers include additional hysteresis thanks to the positive feedback loop in the circuit. The table below provides a brief comparison of Schmitt trigger and comparator circuits in terms of their output and hysteresis characteristics.

The hysteresis window size is the most important parameter determining noise immunity in Schmitt trigger circuits; this will be discussed in more detail below. To see how these characteristics arise, it helps to break down a common topology for Schmitt triggers, which are often implemented using operational amplifiers.

Schmitt Trigger Hysteresis with an Op-amp

Schmitt triggers will have some extra components compared to a typical op-amp-based comparator circuit without hysteresis. The level of noise immunity that can be achieved with a Schmitt trigger circuit is provided by adding some hysteresis through a positive feedback loop. An example method for adding hysteresis and controlling the threshold voltage levels is shown in the circuit diagram below. This diagram shows an inverting Schmitt trigger circuit with positive feedback applied to the non-inverting input.

Schmitt trigger circuit with hysteresis.

In this circuit diagram, the output voltage depends on the threshold voltage values V_hi and V_lo. The output voltage can be calculated simply using the values for the three resistors in the feedback loop. These resistors form a voltage divider that simultaneously sets the upper and lower threshold voltages by changing the reference voltage at the non-inverting input. The equations for the upper and lower threshold voltages are shown below.

Voltage dividers in the feedback loop set the Schmitt trigger hysteresis.

In the above voltage divider circuits, the voltage seen at the non-inverting input is the threshold voltage; this is equivalent to the reference voltage in a comparator circuit. Once the output voltage switches, the threshold voltage seen at the non-inverting input also switches states. This should explain why the upper and lower threshold voltages depend on the output from the Schmitt trigger circuit.

To summarize, when the input voltage rises above V_hi, the output voltage switches from V₊ to V_{_}. Similarly, when the input voltage falls below V_lo, the output voltage switches from V_{_} to V₊. For a non-inverting Schmitt trigger, the high-to-low switching action occurs in the reverse direction. This property of hysteresis is set totally by choosing the resistors in the above circuit diagrams. For a non-inverting Schmitt trigger circuit, the input is simply connected to the non-inverting input net on the op-amp.

Output Waveform

The output waveform in the above circuit is shown below. In this waveform, we can see a comparison between cases with and without hysteresis. In the case of a waveform without hysteresis, noise on the input will cause the output to bounce between the rail voltage values. This is why we add some hysteresis to a comparator circuit to create a Schmitt trigger. By shifting the threshold for switching below the rising edge value (and vice versa for falling edge), a small amount of noise on the input will not affect the output voltage.

Schmitt trigger output without hysteresis (left) and with hysteresis (right).

The level of noise on the input signal that the Schmitt trigger can tolerate is approximately the same as the hysteresis window. RMS noise level is usually an appropriate metric for comparison when looking at noise in the time-domain. If you need to check your Schmitt trigger hysteresis window against the expected noise level in a circuit, you’ll need the right set of circuit design and simulation tools.

Simulating Schmitt Trigger Hysteresis

The primary tools for simulating Schmitt trigger behavior are DC analysis (for designing feedback loops) and transient analysis (for examining output waveforms). When building these circuits from op-amps or transistors, you’ll need to use standard models for these sub-circuits to produce accurate behavior from DC sweeps and time-domain simulations.

Designing Schmitt trigger hysteresis from an op-amp can be difficult without the best PCB design and analysis software. When you need to design and analyze your Schmitt trigger circuits, the front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for circuit design and evaluation. This application and the other design tools from Cadence give you access to a full suite of design and optimization features.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.