Playing with Op-Amp's

Discussion in 'General Electronics Chat' started by Weird Tolkienish Figure, Aug 10, 2015.

  1. Weird Tolkienish Figure

    Thread Starter New Member

    Sep 16, 2013
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    Looking to build an analog computer to do some rather complicated math, what op-amps would you recommend for hobbyists?

    I was thinking of this one:

    http://www.ti.com/product/lm324

    I would like to incrementally play with op-amps until I can do some of the projects mentioned at this website:

    http://www.allaboutcircuits.com/textbook/semiconductors/chpt-9/computational-circuits/

    I have built circuits with the 555 and such. As far as Newbs go, I'm pretty newbish, but I feel I have grasped most of the basic concepts.

    Will I need an oscilloscope? This will drive my budget contraints up, and space issues (and annoyed wife, lol).

    How bad are noise problems with these?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    LM324 is probably okay to start with but, without more info on the complicated math you want to perform, can't say if it's a good choice. There are other opamps with lower input currents, offset voltage, and can swing to the positive rail (LM324 should be able to get to the negative supply in split supply mode).
    I'd say mandatory for analog circuits. If you decide to get one, do your research. USB scopes are cheap, but there's a reason they say "you get what you pay for". Some of the inexpensive digital scopes seem to have poor resolution (compared to my 30-40 year old Tektronix which cost $8K when it was introduced). Prices for analog scopes (used, don't think you can buy new) wax and wane; they seem to be at a premium now.
    Depends on application.
     
  3. #12

    Expert

    Nov 30, 2010
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    Isn't that like looking for a good bicycle to get to St. Louis quickly? I thought about doing this experiment 40 years ago when every op-amp was a PIB for accuracy. Today, you have op-amps available that make your concerns about noise and my concerns about offset voltage a thing of the past...WAY past.

    You should go to a vendor site like www.mouser.com and learn how to use the parametric search engine. You will be amazed at what's available.
     
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  4. crutschow

    Expert

    Mar 14, 2008
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    The LM324 is a cheap, common op amp, but it has a significant input offset voltage and input bias current which can introduce errors in the output, especially if you have an integrator in the circuit.
    That may be okay for simple op amp functions but, depending upon your budget, you might want to look for one with a lower input voltage offset and input bias current.

    If you are computing any AC or time-varying functions, you will need an oscilloscope.
    For DC type functions, a multimeter will be fine.

    What type of "complicated math" do you want to do?
     
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  5. Weird Tolkienish Figure

    Thread Starter New Member

    Sep 16, 2013
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    I shouldn't have mentioned the "complicated math" bit. My idea was that I work at a statistical software company, I thought it might be neat to build analog equivalents of some of the simpler calculations, like calculating p values for normal distributions.

    But, as the fellow said about building a bike to go to St. Louis, I'd like to start real simple, with a simple model that is forgiving enough on input voltages (hopefully 9v or 12v maybe) so I don't fry the thing when I hook up some wrong wirers, or that doesn't leave me scratching my head because it's not really meant for hobbyists.

    Ideally I'd like to build very simple adders, subtractors, dividers, multipliers with op-amps. Just at a learning phase here.

    Hope that makes more sense.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Note that you can't make true multipliers and dividers with op amp alone.
    Those require a non-linear component, such as diodes or a Gilbert Cell.
     
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Not to contradict you on that last statement, #12, but I prefer www.digikey.com much better than mouser's for parametric search purposes... just a matter of personal taste...
     
    Last edited: Aug 10, 2015
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  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The LM324 is an old, trusted, dear friend, and for the basic functions in post #5 is good enough for learning-level precision. Plus, the little puppy is very robust. Compared to more modern designs it has all kinds of input errors, but with +/-15V supplies you should be able to see real results. Analog computers of the 60's used discrete component amplifiers with chopper stabilization. Not much bandwidth, but impressive precision given the tech at the time.

    ak
     
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  9. #12

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    I like the, "smart" filter at mouser. It eliminates all the options that result in, "zero parts meet your criteria, please guess again".
     
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  10. cmartinez

    AAC Fanatic!

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    Digikey has upgraded his parametric engine by showing you the number of available parts with the parameter selection that you've made before you hit the "Apply Filters" button. That more or less helps you avoid the annoying situation that you've just mentioned.
     
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  11. #12

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    I believe you are missing some information. Mouser greys out all the choices that would cause zero results so you don't even have to look and see if you have a zero before clicking on, "apply filters". You can not choose a zero result option at mouser if you turn on the, "smart" filtering option.
     
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  12. cmartinez

    AAC Fanatic!

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    Which reminds me why I prefer digikey... mouser's site is slower, at least for me, maybe it's because of where I'm located.

    EDIT: But what I actually do all the time is make a list of the items that I found at digikey and then search specifically for them at mouser... sometimes the price difference is important
     
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  13. #12

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    Lucky me. I have 15 Mega somethings per second.
    How good is it?
    When AAC takes a whole second to load a page, I know something is wrong at AAC. ;)
     
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  14. cmartinez

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    showoff.... :rolleyes:
     
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  15. Weird Tolkienish Figure

    Thread Starter New Member

    Sep 16, 2013
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    I ordered 10 LM324N's for $5 on amazon, free shipping.

    Looking at the mouser site, they make dedicated analog IC's for division and multiplication, i'm thinking I might just use those, rather than try to use the "non-linear" components described in this thread.
     
  16. dl324

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    Seems straightforward enough using LM324. Division and multiplication are just subtracting and adding logarithms.

    One thing to note about LM324 is that the output won't tolerate an indefinite short to the positive rail; shorting to ground is okay...
     
  17. dl324

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    I live in a rural area and we've only had broadband available in our area for a couple years. Before that, it was dial-up or wireless. Now that the FCC has changed the definition of broadband from 4Mbps down/1Mbps up to 25/3, broadband is no longer available because our provider throttles uploads for the majority of their customers to 1Mbps for uploads. I'm on the wrong side of the digital divide; again...
     
  18. DickCappels

    Moderator

    Aug 21, 2008
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    Please DO use the LM324. It has a much better chance of helping you experience some of the more interesting phenomena such as offset voltage, input noise, slew rate limiting, cross-over distortion, etc. No kidding, it would be valuable experience for you.

    The pin-out of the LM324 is a very common one and later if you desire you can unplug the LM324 and replace it with one that is closer to ideal.
     
  19. Weird Tolkienish Figure

    Thread Starter New Member

    Sep 16, 2013
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    Ordered an O-scale train for $200 and 10 LM324 PIN's for $5 and yet I'm equally excited about both, lol.

    Which one was the better deal? :D
     
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  20. dl324

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    Your Wife probably won't be too thrilled about your new toy if you try to get approval for a scope purchase... I would have gone for a piece of test equipment over the train 100% of the time; but that's just practical me;)
     
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