signal clipping question

Discussion in 'General Electronics Chat' started by tpny, Dec 23, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    Why is the general recommendation for protecting IC input from transient spikes to use a diode pair like this article shows in the 'cpu dataline protection' figure. Diode has a forward voltage drop that prevents my input to see rail to rail levels at the IC. Is there another recommendation?

    http://www.electronics-tutorials.ws/diode/diode_4.html
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    diode pair is reverse biased and does NOT prevent your input from going rail to rail. it prevents it from going outside of the range (0-Vd, Vcc+Vd) where Vd=0.7V approx. therefore if Vcc is 5V, diodes would clamp the signal to range -0.7V to 5.7V.

    http://www.electronics-tutorials.ws/diode/diode9a.gif
     
  3. tpny

    Thread Starter Member

    May 6, 2012
    216
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    Oops. Yes, your right.

    I meant to ask what if I can't pass levels > 5.0 and < 5.7 to the input of my IC? What should I do?
     
  4. panic mode

    Senior Member

    Oct 10, 2011
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    not sure what you are asking.... are you asking how to limit signal to be within 0..5V and not -0.7..5.7V? what would be concern to ask something like this? do you only care about upper limit to be at 5V? you can get rid of the diodes and add zener 5.1V in parallel with input. or... you can replace both diodes by 5.1V zeners.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Most IC's can tolerate 0.7V outside of their range. If your's can't, then there are diodes with lower forward bias voltages around. Or, as pointed out already, you can use a 5.1V zener diode from the signal line to ground (reverse biased) to clamp it at only about 100mV above Vcc. But be sure that the current that is starting to flow at 5V, which can be quite noticeable that close to the knee, doesn't disturb the signal too much.

    Short of that, you could probably use an active "ideal diode" to protect the circuit.
     
  6. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    Should I put a resistor between zener and ground?

    Do you mean between 5~5.1V things get weird? Wouldn't anything below 5.1V just flow to IC input as normal? Thanks!
     
  7. panic mode

    Senior Member

    Oct 10, 2011
    1,320
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    before we toss dozens of ideas around and design something overly elaborate and complicated, why don't you just tell us what exactly you want to do?
     
    Ron H likes this.
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,854
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    Then the zener will lose its function.

    As the TTL IC can be provided the power 5V±5%=4.75~5.25V.
    If Vcc<4.75V then IC will not work properly, if Vcc>5.25V could damage the IC, so under 5.1V should be ok for the IC, and don't care about the 5~5.1V.
     
  9. dataman19

    Member

    Dec 26, 2009
    136
    29
    The thread is "signal clipping"..
    Yet Diode clamping for Logic Levels is being discussed....
    Am I confused, or what?
    I agree that using diode clamps for the input to audio ICs is a "bad idea" and will cause signal clipping (which is why it is not a generally accepted practice). But on the other hand, Logic levels are square waves (which is what a diode clipper will emulate when used with an audio signal with high peaks - in which the diodes will clip - giving the audio waveform flat tops and bottoms "ie: clipping").
    ...
    I agree with panic_mode the desires of the oP needs clarification.
    ..
    I again wish everyone a Merry Christmas
    ...
    or Happy Holidays if that floats your boat...
    ...
    Dave
    Phoenix, AZ
     
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