Trying to understand AD8561 circuit

Discussion in 'General Electronics Chat' started by CMJ, Aug 7, 2012.

  1. CMJ

    Thread Starter New Member

    Aug 6, 2012
    2
    0
    Hello,
    I am assisting an engineer at my job with a redesign of a product. The AD8651 shown in the attachment is in an differential system. If does not function the way we need it, but before we make any changes we would like to understand why the circuit was designed the way it is. Unfortunately, the person that originally design the circuit is no longer available. We believe that the circuit is setup with hysteresis, but are not sure why pin 8 was feedback to pin 3. The engineer I am working with says that normally when you feedback, you would feed pin 2. Any ideas on why the signal was feedback to pin 3 would be appreciated.

    Thanks in advance!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    3,354
    For hysteresis you need to apply positive feedback.
    This can be accomplished by feeding back pin 7 to pin 2 or pin 8 to pin 3.
    Hence the circuit appears to be correct.
     
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  3. CMJ

    Thread Starter New Member

    Aug 6, 2012
    2
    0
    Thanks for your input MrChips. After further conversations with the engineering I am assisting, I found that he has made changes to the circuit not shown on the attachment that feeds the AD8561 circuit. He says the circuit the way it is, it does not function like it should but when the 100K ohm resistor (R625) is removed the circuit works fine. I think that the changes he made to the circuit that feeds the AD8561 have improved the signal, which now does not require the use of hysteresis.
    Also, he is considering raising the resistor values of R190 and R187 to 10K ohm.
    Would you happen to know how high a resistance he could use on the input before it starts to affect the bias current on the AD8561 in a differential system?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,415
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    I agree that the first thing to do is to remove the 100kΩ feedback resistor and analyze what is the problem.

    Changing R187 and R190 from 249Ω to 10kΩ will make a huge difference in the time-constant with the C472 100pF in parallel. This, rather than the bias current, will have a greater effect on the performance of the circuit.

    Remember that this is a low-pass filter and the rise/fall times will be affected. The AD8561 has response time of 7ns and now you are limiting the input transitions to 1μs.
     
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  5. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Going to 10K input resistors creates many problems. If the resistors are not matched some portion of the common mode noise is changed to differential mode noise because the time constants are different. The input transitions will become slow so chatter might occur (preventing chatter is why hystersis was used). There could be an input offset bias problem caused by large value input resistors. Response time becomes so slow that the fast AD8561 may not be needede.
     
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