Use ALL the pins?

Discussion in 'General Electronics Chat' started by KLillie, Feb 23, 2015.

  1. KLillie

    Thread Starter Member

    May 31, 2014
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    Just curious. I came across this trying to answer a post. If you had a dual op-amp, what what would be the harm in trying to use both instead of dealing with unused pins?
     
  2. #12

    Expert

    Nov 30, 2010
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    If you had two shoes, what would be the harm in wearing both of them?

    Nobody said you have to unuse half of a dual op-amp and go buy another chip so you can use half of that one, too.
     
    Last edited: Feb 23, 2015
  3. KLillie

    Thread Starter Member

    May 31, 2014
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    But see, if I'm told I gotta go through all the trouble of putting on my fake leg...I'm wearing that shoe!
     
  4. #12

    Expert

    Nov 30, 2010
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    I think that means you understood me.

    The fact that there is a way to handle extra amplifiers if you don't need them doesn't mean you have to go buy too many amplifiers and not use some of them.
     
    atferrari likes this.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    It really depends on the application. Because amplifier noise is uncorrelated even when two amps are on the same chip, there can be a noise advantage to having two amps in series, each with half the required circuit gain, compared to a single amplifier providing all of the gain. Generally speaking the two-amps-in-series also gets you wider bandwidth, which you can use to improve phase margin and high-frequency distortion.

    ak
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Putting two op amps in parallel is a fine way to damage both devices.

    Besides, there is way less hassle in dealing with unused pins then in not dealing with them.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    For the same bandwidth, the noise of two op amps in series will always be larger than using a single amp, even if the two noises are uncorrelated (they add as their RSS sum).
    But if the first op amp is configured to have significant gain, then the contribution from the second op amp can be made negligible.
     
  8. KLillie

    Thread Starter Member

    May 31, 2014
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    This was kind of my thought also AnalogK. Why not just use half as much gain and use both? I don't know maybe it was a silly question, but it sounds like using both "just because" could introduce more noise into the circuit?
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    In general, there isn't any harm -- that's why they put multiple amps in the same package.

    There are some applications where two circuits can adversely interact when they contain opamps (or other devices) that share a piece of silicon, but by the time you are dealing with such applications you are usually dealing with a lot of other low-level and subtle issues, too.
     
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