| Specifications | Rarely Asked Questions \Why buy an expensive comparator when an op amp is just as good?\ RAQ, RAQs, Rarely Asked Questions, Bryant, op amps, comparators Analog Devices |
| Business section |
![[designnews.com]05.15.06 DESIGN NEWS27 Rarely Asked Questions Strange but true stories from the call logs of Analog Devices S](https://www.catagle.com/38-1/RAQ_comparatorsOpAmp_1.png "[designnews.com]05.15.06 DESIGN NEWS27 Rarely Asked Questions Strange but true stories from the call logs of Analog Devices S")
|
|
|
|
|
||||||||||||||||||||||||||||||||||
|
|
|
|
|
||||||||||||||||||||||||||||||||||
| Specifications | Rarely Asked Questions \Why buy an expensive comparator when an op amp is just as good?\ RAQ, RAQs, Rarely Asked Questions, Bryant, op amps, comparators Analog Devices |
| Business section |
| Specifications | Rarely Asked Questions \Why buy an expensive comparator when an op amp is just as good?\ RAQ, RAQs, Rarely Asked Questions, Bryant, op amps, comparators Analog Devices |
| Suggested Link Details/Purchase | |
| Content | [designnews.com]05.15.06 DESIGN NEWS27 Rarely Asked Questions Strange but true stories from the call logs of Analog Devices SPONSOREDBY RAQ’s SPECIALADVERTISINGSECTION Have a question involving a perplexing or unusual analog problem? Submit your question to: A.Because it isn’t. My advice to anyone tempt- ed to use an operational amplifier (op amp) as a com- parator is the same as Mr. Punch’s advice to those about to marry? “Don’t!” But since my good advice is as likely to be taken as Mr. Punch’s this piece contains some advice for those who are going ahead anyway. Comparators have differ- ential inputs and a rail-to-rail output. So do op amps. They have low offset, high gain and high common mode rejection ratio (CMRR). So do op amps. But comparators are designed to work open-loop, drive logic circuits, work at high speed even when over-driven, and accept large differential input voltages. Op amps are intended to work closed-loop, driving simple resistive or reactive loads— and are not designed to recover quickly from overdrive. But op amps are cheaper, often come in four- or even six-packs (so often one is spare and begging to be used), and compared to most comparators, have better offset and bias current specifi- cations. Using an op amp as a comparator may lead to difficulties for three major reasons: speed, logic drive capability, and various effects of input structures. Comparators are designed to work with large differential inputs, while op amps are intended for closed-loop operation with both inputs at very similar potentials. If an op amp sees a differential input voltage of even a few millivolts its internal circuitry may saturate. Recovery may be very slow and recovery time may vary considerably with the level of over-drive and from device to device. Such variability and loss of speed may be unwelcome in a comparator. If an op amp has rail-to-rail output and is operated on the same supplies as the satu- rating logic it is driving (CMOS or TTL) then interfacing is not hard. But if the op amp and logic supplies differ, then additional interface circuitry, which may be quite com- plex, is necessary to produce the correct levels. Finally, op amp inputs usually have high impedance and low bias current. But if a dif- ferential input voltage of more than a few hundred millivolts is applied to them this may no longer be the case, and all sorts of less than ideal behaviour may occur. Higher levels of over-drive may also do minor dam- age to an op amp’s input stages, resulting in slowly accumulating long-term changes in performance which may be overlooked dur- ing laboratory development. To learn more about op amps and comparators, Go to: http://rbi.ims.ca/5696-110 raq@reedbusiness.com Contributing Writer James Bryant has been a European Applications Manager with Analog Devices since 1982. He holds a degree in Physics and Philosophy from the University of Leeds. He is also C.Eng., Eur.Eng., MIEE, and an FBIS. In addition to his passion for engi- neering, James is a radio ham and holds the call sign G4CLF. Comparators & Op Amps—May They Never Meet (Or Good Advice From Mr. Punch) Q.Why buy an expensive comparator when an op amp is just as good? DNX060515RAQ2.qxd 4/26/2006 10:54 AM Page 1 |
| Suggested Link Details/Purchase | |
| Following Datasheets | RAQ_converterFront-end_1 (1 pages) RAQ_crosstalkin_converters (1 pages) RAQ_datasheets (1 pages) RAQ_DC_Parameters_1 (1 pages) RAQ_dieTemperature_1 (1 pages) RAQ_diffAmps-1 (1 pages) RAQ_diodes_1 (1 pages) RAQ_FeedbackPin_1 (1 pages) RAQ_filters_1 (1 pages) RAQ_Foolish_Question (1 pages) |
Check in e-portals |
World-H-News Products Extensions Partners Automation Jet Parts |
| Sitemap Folder | group1 group2 group3 group4 group5 group6 group7 group8 group9 group10 group11 group12 group13 group14 group15 group16 group17 group18 group19 group20 group21 group22 group23 group24 group25 group26 group27 group28 group29 group30 group31 group32 group33 group34 group35 group36 group37 group38 group39 group40 group41 group42 group43 group44 group45 group46 group47 group48 group49 group50 group51 group52 group53 group54 group55 group56 group57 group58 group59 group60 group61 group62 group63 group64 group65 group66 group67 group68 group69 group70 group71 group72 group73 group74 group75 group76 group77 group78 group79 group80 group81 group82 group83 group84 group85 group86 group87 group88 group89 group90 group91 group92 group93 group94 group95 group96 group97 group98 group99 group100 Prewious Folder Next Folder |
