Comparators B Furman 15 APR 2014 Learning Objectives

Comparators B. Furman 15 APR 2014

Learning Objectives n n n 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 Power Source User Interface ME 106 ME 120 Controller ME 106 (Hardware & Software) Power Interface INTEGRATION ME 106 ME 154 ME 157 ME 195 Signal Conditioning ME 106 ME 190 ME 187 ME 106 ME 120 Actuator Sensor System to Control ME 110 ME 182 ME 136 ME 189 ME 154 ME 195 ME 157 ME 120 ME 297 A BJ Furman 22 JAN 11

The Comparator n A specially designed op-amp, optimized to switch Vout fast If V+ > V-, then Vout Vs ¨ If V- > V+, then Vout 0 V ¨ n 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 Typical op-amp output stage and some comparators (Push-Pull (Totem Pole)) Open collector output stage (typical of most comparators, such as LM 339) Intro to Mechatronic Design, Fig. 11. 18, p. 245 http: //www. ermicro. com/blog/? p=1578 ¨

Inside the LM 339 comparator www. national. com/ds/LM/LM 339. 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 n 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 ¨ n 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 n Two comparators with two Vrefs: Vref. High and Vref. Low One non-inverting (IC 1 A) ¨ One inverting (IC 1 B) ¨ http: //www. sparkfun. com/datasheets/Widgets/Window%20 Comparator-v 15. pdf

The ATmega 328 implements an analog comparator ATmega 328 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.