opamp choice help - Page 1
Forum section: electronics / projects

author: kars
author_rank: Contributor
timestamp: January 16, 2018, 08:48:58 pm
content: hi everyone!for a schoolproject we need to use an opamp, the problem is there are so many availble.we narrowed the search down to the opamps availble at school.my question is, for what specification do i need to look in the datasheet? we already know the powersupply so thats sorted and so is the frequency bit.the opamp does need to be able to reach the Vcc, is there a spec for that?thanks in advance!

author: CopperCone
author_rank: Super Contributor
timestamp: January 16, 2018, 08:54:34 pm
content: Opamps capable of reaching the rail are called rail to rail opamps (marketing speak).There are graphs in parameters showing how close you can get to the rail, if you scroll down a bit

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2018, 10:57:38 pm
content: You haven't provided enough information to decide which op-amp to use.What's the supply voltage?You've mentioned the frequency but are you aware of the gain bandwidth product and the slew rate specifications?What are the input and output impedances?

author: bitbanger
author_rank: Regular Contributor
timestamp: January 17, 2018, 03:02:47 am
content: In addition to what others have asked, it would be best to explain the application (is it being used as a literal amplifier, comparator, etc). Help us help you!

author: kars
author_rank: Contributor
timestamp: January 17, 2018, 12:14:17 pm
content: 1)our supply voltage is 9v only, not -9v. the -bit isnt used.2)we use it at the end of an 8 bit r2r dac connected to an arduino as an output amp i presume.with it we have to 'draw' a house every 2ms with the shortest delay between a voltage change beeing 50 microseconds.and we need to generate a 480hz sine wave with an amplitude of 4.5v, this is why we needed the rail to rail opamp.to the question of hero999, no, this has not been tought in class (yet i presume)

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2018, 07:03:59 pm
content: Quote from: kars on January 17, 2018, 12:14:17 pmIs 4.5V the peak, RMS o peak-to-peak voltage?QuoteGain bandwidth product, is the frequency when the gain of the op-amp falls to 1.Slew rate is maximum rate of change the op-amp's output is capable of achieving. For example a slew rate if an op-amp has a slew rate of 1V/ms, then if it outputted a 1V square, the rise and fall times would be 1ms.Given your requirements, you don't need anything special as far as speed is concerned.One more question: does you want a surface mount or through hole package? My go to rail-to-rail input & output op-amp, the MC33202 is only available in SMT.http://docs-europe.electrocomponents.com/webdocs/14f3/0900766b814f3532.pdf

author: rstofer
author_rank: Super Contributor
timestamp: January 17, 2018, 07:54:27 pm
content: Quote from: kars on January 16, 2018, 08:48:58 pmOK, what are the op amps available at school?

author: kars
author_rank: Contributor
timestamp: January 17, 2018, 08:19:47 pm
content: Quote from: rstofer on January 17, 2018, 07:54:27 pmas i looked at the datasheets for the availible opamps, i noticed none of em are rail to rail so we got the green light to get one from somewhere else (local electronics store preferably)Quote from: Hero999 on January 17, 2018, 07:03:59 pmpeak to peak with the zero point at 4.5v. the peaks would be at 0v and 9vQuotethanks! that makes searching a lot easyer.Quotepreferably in dip package as it is less of a hassle to solder. there is a dip version of the MC33202, the MC33202p it is just impossible to get without a long waiting period. bit of a shame really.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2018, 09:17:38 pm
content: With 4.5 V peak to peak, there is a little headroom. So the OP does not absolutely need to be Rail to rail. Depending on the exact requirements (e.g. maybe us 4 V reference level instead of 4.5 V) one might just get away with TLC27x.A typical 12 V RR op would be the TS912.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 18, 2018, 06:25:34 am
content: To select an Op-Amp it is needed first to define the circuit specifications.Quote from: kars on January 17, 2018, 08:19:47 pmAre you sure that you need a R-R op-amp?QuoteThis does not make a ... lot of sense. The range of a 4.5Vpp signal, centered on 4.5V is between 2.25V and 6.75V. What peaks are on 0V and 9V?QuoteAdditionally:Quote from: kars on January 17, 2018, 12:14:17 pmWhat is the meaning of the sentence in bold?If all codes of 8-bit are used, the step change, for 480Hz signal, is every about 8 micro-seconds. Is the signal filtered/integrated before the amplifier?What is the role of this stage? Is it a buffer, a step voltage amplifier, an integrator, an amplifier of sinus signal or something else?Also the characteristics of the input source and the output load have to be defined.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2018, 09:54:03 am
content: Quote from: Damianos on January 18, 2018, 06:25:34 amI think he meant 4.5V peak, 9V peak-to-peak, with 4.5V of DC bias.Quote from: kars on January 17, 2018, 08:19:47 pmI believe they've discontinued the DIP variant. Many through hole parts are obsolete. In real life you'll find SMT parts used more than through hole.How about the LMC6482?http://www.ti.com/lit/ds/symlink/lmc6482.pdfIt seems to be widely available, but isn't the cheapest.https://www.mouser.co.uk/productdetail/?qs=QbsRYf82W3EByDDrj8Vijg==https://uk.rs-online.com/web/p/operational-amplifiers/1218816/One thing to bear in mind is, no op-amp has a truly rail-to-rail output, unless you can get one with a built-in voltage converter, which would be expensive. The output can never completely reach the rail, as it will have a series resistance, which will drop some voltage. Suppose the op-amp can only get to within 50mV of each power rail, with a 9V power supply and 9V peak to peak signal, on the output, there will be some distortion due to clipping which will occur, when the output tries to go below 50mV or above (9 - 50mV) - 8.95V.

author: kars
author_rank: Contributor
timestamp: January 18, 2018, 10:29:34 am
content: Quote from: Hero999 on January 18, 2018, 09:54:03 am2 euros does not seem that expensive, shipping will probably be more.Is there any difference between the LMC6482IN and LMC6482AIN?

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2018, 11:00:57 am
content: Quote from: kars on January 18, 2018, 10:29:34 amCheck the data sheet.