Slew Rate?

Discussion in 'General Electronics Chat' started by ELECTRONERD, Jun 25, 2009.

  1. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Can someone clarify "Slew Rate" for me? I think that it defines what the maximum rate of change on the output of on op amp is, but do you want high slew rate or low slew rate? If you had "High slew rate" would your signal change a lot?

    Thanks in advance!
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A lousy old LM324 quad and a LM358 dual opamp have about the slowest slew rate of any opamp because they are "low power". Because the slew rate is so low their max full output frequency is only about 1kHz and at half full output the limit is 5kHz. Above these frequencies the output is a triangle wave with its amplitude dropping at higher frequencies.

    Most good opamps have a high enough slew rate to allow full output up to 100kHz.
     
  3. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    I see, so high slew rate is sought after. What applications need high slew rate?

    This looks lilke a pretty good little IC: http://focus.ti.com/lit/ds/symlink/tl3472.pdf
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Slew rate is defined by V/S, if memory serves 0.7V/µs for the 741. Try to go faster than that, as with a square wave, it becomes a linear line. Not the same as an integrator, but the end effect is the same (for the square wave). An integrator is also a low pass filter, which is what AG is talking about, but with one significant difference. Low level signals (with their lower voltage swing) don't run into this problem as much as high level signals.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Many audio opamps have a slew rate of 13V/us. Then their full output is to 100kHz.
    The 741 opamp runs out of slew rate when its full output frequency exceeds 9kHz.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    The thing of it is, while it affects frequency responce, it is not frequency dependent. It is a rate of change measurement.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    When the output of an opamp reaches its slew rate limit frequency then output signals become a triangle wave full of distortion and the output level drops at higher frequencies.
     
  8. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Hmmm...I see. So I want the highest slew rate I can get so that I can use it at higher frequencies.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Take a look at the attached PDF.
    On page 33 the slew rate is descibed.
    On page 41 there is a drawing about the slew rate with respect to the waveform.

    Greetings,
    Bertus
     
  10. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
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    Thanks Bertus.
     
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