Hysteresis Comparator Calculation

NOTE: Use the minus sign "-" for negative voltages.
Positive Supply Voltage: (V)
Negative Supply Voltage: (V)
Results:
Vth (High Threshold Voltage):
(V)
R2/R1(Ratio resistor): 
Vtl (Low Threshold Voltage):
(V)
Vr (Reference voltage): 
(V)

A Hysteresis Comparator is a type of comparator circuit that includes feedback to introduce hysteresis, which helps to stabilize the output and prevent erratic switching when the input signal is noisy or slowly changing.

What is a Hysteresis Comparator?

A hysteresis comparator is a device that compares two voltages or signals and outputs a high or low signal based on which input is greater. The key feature of this comparator is that it has a hysteresis effect, meaning that the output changes state at different threshold levels depending on whether the input voltage is increasing or decreasing. This hysteresis is achieved by feeding back some of the output to the input.

The main difference between a standard comparator and a hysteresis comparator is that in a hysteresis comparator, the threshold voltage for switching between states is not fixed but depends on the output state, introducing a window of uncertainty for switching. This prevents the comparator from switching too quickly due to noise or minor fluctuations around the threshold.

Why Use a Hysteresis Comparator?

  • Prevent noise-induced switching: In applications where the input signal is noisy or fluctuating around the threshold, a standard comparator might switch back and forth erratically. The hysteresis adds stability by requiring a larger change in the input before the output switches.
  • Improved noise immunity: Hysteresis ensures that small, rapid changes in input (which could be caused by noise) do not trigger unnecessary switching.
  • Sharpening the transition between high and low output states: The hysteresis increases the robustness of the system and sharpens the output response, making it more predictable in the presence of small signal fluctuations.
  • Used in applications like Schmitt triggers: The hysteresis comparator is often used as a Schmitt trigger in systems that require a clean transition between binary states (e.g., digital systems or pulse generation).

How Does a Hysteresis Comparator Work?

The core functionality of a hysteresis comparator is based on positive feedback. In the simplest case, when the comparator's output switches, it changes the threshold at which the comparator will switch again, depending on the output state.

  1. Comparator Circuit: A typical comparator compares the input voltage with a reference voltage. When the input voltage exceeds the reference voltage, the output switches from low to high (or vice versa, depending on the design).

  2. Hysteresis Implementation: In a hysteresis comparator, a feedback resistor is connected from the output to the positive input of the comparator. This feedback alters the reference voltage based on the output state, creating a different threshold voltage for rising and falling input signals.

  3. Thresholds:

    • When the input voltage rises, it must cross a higher threshold to switch the output state.
    • When the input voltage falls, it must cross a lower threshold to switch the output back.

This creates a dead zone or hysteresis window, preventing the output from switching if the input is within this window.

Hysteresis Comparator Calculation:

To calculate the hysteresis threshold voltages, you can use the following formula for a non-inverting comparator with hysteresis:

Where:

  • Vth​ = hysteresis threshold voltage (the voltage at which the output switches)
  • Vref​ = reference voltage
  • Rf​ = feedback resistor (from output to positive input)
  • R1​ = resistor from the positive input to ground
  • Vout​ = output voltage (high or low)

The feedback resistor Rf​ is what introduces hysteresis, and the thresholds will be different for when the input voltage is rising versus when it is falling.

Example:

If you have:

  • Vref=1.0V
  • Rf=10kΩ
  • R1=10kΩ
  • Vout​ could be either 3.3V or 0V depending on the state.

Then you can calculate the upper and lower threshold voltages at which the output will switch.

When Should You Use a Hysteresis Comparator?

  • When dealing with noisy or fluctuating signals: Hysteresis comparators are essential in systems where the input signal may be noisy or has small fluctuations that would otherwise cause a standard comparator to oscillate between high and low states. It smoothes out the transitions and reduces erratic behavior.
  • In Schmitt triggers: For example, digital circuits often use Schmitt triggers (which are hysteresis comparators) to clean up noisy or slow input signals to ensure clean digital transitions (high/low).
  • In pulse-width modulation (PWM) circuits: Hysteresis comparators can be used to stabilize PWM circuits, making sure the switching threshold is clearly defined.
  • In low-frequency switching applications: Any system where you want the output to respond only to significant changes in the input, like temperature control systems or light intensity sensing.

Example Use Case:

If you’re designing a system to monitor the temperature and trigger a fan when the temperature exceeds a certain value, you might use a hysteresis comparator to avoid the fan turning on and off rapidly due to small fluctuations in temperature around the set point.