The Use of Comparator Hysteresis in Circuit Design

Key Takeaways

●     Learn about the application of comparator hysteresis.

●     Gain a greater understanding of the types of comparators.

●     Learn more about the various comparator applications.

Operational amplifiers provide comparator functionality

Within every company or business, whether they are a workforce of 100,000 or one, they rely on a Financial Officer or CFO. Their responsibilities include developing budgets, monitoring transactions, and preparing financial reports.

Overall, these responsibilities rely on the individual's ability to compare products, suppliers, and services to ensure a company's fiscal health. Now in the field of electronics, we have devices that perform similar comparison actions, and they are called comparators. Their functionality is equally important to the overall functionality of the applications that employ them. Let’s take a look at what they are and how they work. 

What Are Comparators and What Are Their Types?

A comparator is a device for comparing a measurable property or thing with a reference or standard. In other words, a comparator is an electronic circuit for comparing two electrical signals. We use comparators to examine or analyze two currents or voltages. The comparator compares these voltages or currents at its two inputs.

Functionally, this means that a comparator takes two input voltages,  compares them, and then provides a differential output voltage that is either a low or high-level signal. On a broader scope, we can use a comparator to sense when a discretionary varying input signal achieves a defined threshold level or reference level.

You can often find comparators in use in electronic applications that drive logic circuits. The classification or types of comparators include mechanical, electrical, pneumatic, optical, electronic, sigma, and digital. Generally, a comparator's design is without feedback to afford open-loop configurations.

What is Comparator Hysteresis?

Hysteresis is the phenomenon in which the value of a physical property lags behind changes in the effect causing it. For example, when magnetic induction lags behind the magnetizing force. You can also define it as a concept in physical science. Conceptionally, a system's output depends not only on its input but also on the history of its past inputs. This concept is logical since history affects the value of an internal state. 

Furthermore, hysteresis happens in ferroelectric materials and ferromagnetic materials. One example of this is when ferromagnetic materials become permanently magnetized. This means that the ferromagnetic material will not revert to zero magnetization even after removing the magnetizing field. You can only coerce the ferromagnetic material back to zero with sufficient heat or a field in the opposite direction.

Under these conditions, applying an alternating magnetic field will create what is called a hysteresis loop (magnetic hysteresis). Magnetic hysteresis is also the effect that provides the element of memory for HDD. More importantly, the predictability of hysteresis makes it an ideal stabilizing mechanism.

The Use of Hysteresis in Comparators

A comparator is a relatively simple device that differentiates between two different input signal levels. However, even a minute noise variation on its input signals will result in instability in the comparator. Its output level will momentarily bounce, and in mechanical switch applications, this causes unintended switching. Therefore, we use hysteresis as a stabilizer since it is the easiest way to resolve this problem.

Hysteresis affords stable switching behavior in a comparator circuit, but it can also define the duty cycles for output square waves. In switching applications, we apply hysteresis by including a positive feedback loop between the output and one of the inputs. By doing so, it defines the threshold for switching as the input signal falls and rises.

Noise on the input signal of a comparator circuit will create multiple transitions as the input signal rises. We classify hysteresis as a form of positive feedback that can create two distinct threshold levels. So, when designing a comparator circuit with an external hysteresis, the amount of hysteresis should be more significant than the maximum peak-to-peak noise you expect in your system.

The use of comparators ranges from applications in heartbeat sensor circuits, smoke alarm circuits, and even humidity monitoring systems for soil. In each of those mentioned applications, a comparator serves a role in supporting life or preventing death. Therefore, we cannot overstate their importance as electronic devices. Neither can we overstate the importance of the use of hysteresis, which provides stabilizing functionality to comparator circuits.

Vision measuring instrument utilizing an Optical comparator to  inspect a PCB circuit board

For all of your simple and complex circuit designs, Cadence Allegro contains all the tools and features you need to get it done right the first time without costly errors or setbacks. The front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for circuit design and simulation. If your company needs more information on comparator hysteresis or how its use can benefit your company’s designs, then view our Hysteresis in Analog Circuits page.

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