Comparator applications: Difference between revisions

Revision as of 00:11, 8 August 2022

Revision as of 00:12, 8 August 2022

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== Threshold detector ==

[[file:Op-Amp Schmitt Trigger.svg|thumb|Circuit diagram for threshold detector with hysteresis]]

The threshold detector with [[hysteresis]] consists of an [[operational amplifier]] and a series of [[resistors]] that provide hysteresis.<ref>{{cite web|title=AN-49 Application Note, Rev. A|url=http://www.analog.com/media/en/technical-documentation/application-notes/AN-849.pdf|publisher=[[Analog Devices]]|accessdate=26 August 2017|format=PDF}}</ref>{{rp|5}}<ref>{{cite web|author1=Chaniotakis|author2=Cory|title=MIT OCW: Operational Amplifier Circuits: Comparators and Positive Feedback|url=https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-071j-introduction-to-electronics-signals-and-measurement-spring-2006/lecture-notes/24_op_amps3.pdf|publisher=[[Massachusetts Institute of Technology]]|accessdate=26 August 2017|format=PDF|date=2006}}</ref>{{rp|7}}<ref>{{cite web|first1=Art|last1=Kay|first2=Timothy|last2=Claycomb|title=Comparator with Hysteresis Reference Design|url=http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf|publisher=[[Texas Instruments]]|accessdate=26 August 2017|format=PDF|date=2014}}</ref> Like other detectors, this device functions as a [[voltage]] switch, but with an important difference. The state of the detector output is not directly affected by input voltage, but instead by the voltage drop across its input terminals (here, referred to as Va). From [[Kirchhoff's circuit laws|Kirchhoff's Current Law]], this value depends on Vin and the output voltage of the threshold detector itself, both multiplied by a resistor ratio.

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== Zero crossing detector ==

A ''zero crossing detector'' is a comparator with the reference level set at zero.

It is used for detecting the zero crossings of AC signals. It can be made from an [[operational amplifier]] with an input voltage at its positive input {{clarify|date=August 2017|reason=Which diagram?| text=(see circuit diagram)}}.

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<math>f_i=\frac{\Delta t}{2}</math>

where <math>\Delta t</math> is the time difference between adjacent zero crossings of FM wave

<ref>[http://sound.whsites.net/appnotes/an005.htm Elliott Sound Products] Application Notes : Zero Crossing Detectors and Comparators</ref>

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[[Image:Op-Amp Schmitt Trigger.svg|300px]]

In this configuration, the input voltage is applied through the [[Voltage divider#Resistive_dividerResistive divider|voltage divider]] formed by <math>{R_1}</math> and <math>{R_2}</math> (which may be the source internal resistance) to the non-inverting input and the inverting input is grounded or referenced. The hysteresis curve is non-inverting and the switching thresholds are <math>\pm \frac{R_1}{R_2}V_{\text{sat}}</math> where <math>V_{\text{sat}}</math> is the greatest output magnitude of the operational amplifier.

{{clear}}

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[[File:Op-Amp Inverting Schmitt Trigger.svg|300px]]

Alternatively, the input source and the [[electrical ground|ground]] may be swapped. Now the input voltage is applied directly to the inverting input, and the non-inverting input is grounded or referenced. The hysteresis curve is inverting and the switching thresholds are <math>\pm \frac{R_1}{R_1 + R_2} V_{\text{sat}}</math>. This configuration is used in the [[#Relaxation_oscillatorRelaxation oscillator|relaxation oscillator]] shown below.

== Relaxation oscillator ==

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[[File:Op-Amp Relaxation Oscillator.svg|300px]]

By using an [[RC network]] to add slow [[negative feedback]] to the inverting [[#Schmitt_triggerSchmitt trigger|Schmitt trigger]], a [[relaxation oscillator]] is formed. The feedback through the RC network causes the Schmitt trigger output to [[oscillate]] in an endless symmetric [[square wave]] (i.e., the Schmitt trigger in this configuration is an [[astable multivibrator]]).

== References ==