Comparators

B. Furman

15APR2014

Learning Objectives

Explain the basic comparator action considering inputs to an op-amp

Construct an inverting or non-inverting comparator using a comparator with open collector output

Explain the concept of hysteresis to avoid ‘chatter’ around a reference level

Mechatronics Concept Map

Controller(Hardware & Software)

System toControl

Sensor

SignalConditioning

PowerInterface

Actuator

UserInterface

PowerSource

BJ Furman 22JAN11

ME 106ME 154ME 157ME 195

ME 120ME 297A

ME 106ME 120

ME 106ME 190ME 187

ME 110ME 136ME 154ME 157

ME 182ME 189ME 195

ME 106ME 120

ME 106

INTEGRATION

The Comparator A specially designed op-amp,

optimized to switch Vout fast If V+ > V-, then Vout Vs If V- > V+, then Vout 0 V

But usually output is ‘open collector’ Can pull low, but… Needs external resistor (pull-up) to go

high

Intro to Mechatronic Design, Fig. 11.17, p. 245

Intro to Mechatronic Design, Fig. 11.18, p. 245

Typical op-amp output stage and some comparators (Push-Pull (Totem Pole))

Open collector output stage (typical of most comparators, such as LM339)

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Inside the LM339 comparator

www.national.com/ds/LM/LM339.pdf

Simple Non-inverting Comparator

Intro to Mechatronic Design, Fig. 11.19, p. 246

Intro to Mechatronic Design, Fig. 11.20, p. 247

Inverting Comparator

Inverting Comparator with Hysteresis

Add hysteresis (i.e., a change in Vref that depends on Vout by feeding back to the non-inverting terminal When the input voltage rises to the

threshold, the threshold drops to a lower value

When the input voltage drops to the threshold, the threshold is raised to a higher value

See equations and method in the Intro to Mechatronics text, pp. 247-248

http://www.ermicro.com/blog/wp-content/uploads/2010/03/comp_10.jpg

Non-inverting comparator with hysteresis

Window comparator

http://www.sparkfun.com/datasheets/Widgets/Window%20Comparator-v15.pdf

Two comparators with two Vrefs: VrefHigh and VrefLow One non-inverting (IC1A) One inverting (IC1B)

The ATmega328 implements an analog comparator

ATmega328 datasheet, p. 239

A comparator is at the heart of a successive approximation analog-to-digital converter

From Necsulescu, D., (2002). Mechatronics, Prentice-Hall, New Jersey.