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Driving an OP AMP input with no VCC

Discussion in 'Electronic Design' started by AJ, Nov 7, 2006.

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  1. AJ

    AJ Guest

    I have a similar question to one posted a few days ago. I am thinking of
    driving an OP AMP running at 5V configured as a follower with a 4.0V input
    via a 10K and there could be times where Vcc is removed while the input
    remains. Is it possible that this could damage the IC and should I use a
    74LCX541 buffer as previously discussed?

    Best regards,


    AJ
     
  2. default

    default Guest

    When you have the potential for input to exceed VCC or ground it is
    good practice to just clamp the signal at the input pin with diodes to
    carry the current up/down to VCC or ground. The diodes are reversed
    biased during normal ops.

    Good practice for long wire runs to inputs to keep induced voltages
    out and in industrial and automotive environments to clamp EMI.
     
  3. Jim Thompson

    Jim Thompson Guest

    Depends on the OpAmp. Read the maximum input voltage spec. If it
    defines maximum input as a relationship with VCC, then you may have
    problems.

    Be more precise in you question... OpAmp type, etc.

    ...Jim Thompson
     
  4. AJ

    AJ Guest



    I am using and MCP6044 running at 5V and I have the input clamped via BAT54S
    diodes to a 3.3V rail and ground. The problem is that my board may loose
    power while an input remains. I have a TVS on the 3.3V rail to prevent it
    going above 3.3V but this would mean that an input would be clamped at 3.6V
    while the OP AMP's Vcc would be 0V.

    Best regards


    AJ
     
  5. Jim Thompson

    Jim Thompson Guest

    I am unable to view a data sheet from Microchip's site. Firefox just
    hangs and sucks 100% CPU. Sorry.

    ...Jim Thompson
     
  6. Genome

    Genome Guest

  7. AJ

    AJ Guest

    Thanks anyway mate, I am not committed to the MCP6044, can you recommend one
    that would be suitable for my application or tell me what I should be
    looking for?

    Best regards


    AJ
     
  8. Kingcosmos

    Kingcosmos Guest

    What is your application? If you are looking to use an op-amp as a
    follower, you could look at the OPA333 from Texas Instruments. Not
    that fast but really good spec's in terms of low power, low voltage,
    and offset (rail to rail too!).

    With any op-amp (just about anyway) you need to pay attention to the
    data sheets absolute maximum ratings. Usually there will be a maximum
    rating on the input voltage;i.e. Vcc + 0.3V using the OPA333 as an
    example. If you go beyond this then you need to heed the maximum input
    current rating. Again using the OPA333 as an example, it is listed as
    +/-10mA and it is explicitly stated in the datasheet as well. A simple
    series resistor can be used to do this. You can also use clamps on the
    inputs too.
    As far as I am aware, this limited input current is continuous and
    would not damage the part and I would go as far as to say that it would
    not cause worry in terms of degradation or reliability either. The
    real fun is what happens when you are using a dual or a quad and one
    channel experiences an electrical overstress event while the others are
    driven normally. In your case if you are not going to have power
    connected, the point is moot; however, through my discussions with
    several colleagues and looking at test data it appears that you can
    treat each channel as a separate block within the IC. It is true that
    they will all ultimately share the same power rail and that there are
    many paths for current to flow during an EOS event. I will have to
    look at the test again to see how it was set up if you are interested.
     
  9. Jim Thompson

    Jim Thompson Guest

    All inputs and outputs................... VSS –0.3 V to VDD +0.3 V

    I think you will find most, if not all OpAmps have this restriction,
    so you need some way to protect it.

    Post your schematic on a.b.s.e and everyone here will take a look.

    ...Jim Thompson
     
  10. Fred Bartoli

    Fred Bartoli Guest

    Jim Thompson a écrit :
    So it's time for some update.
    Have a look at "over the top" opamps or comparators like LT1716 or LT1782...
    Bias current is going high over the supply rail, but it is OK for some
    applications.
     
  11. Jim Thompson

    Jim Thompson Guest

    It just occurred to me that the LM324 can take inputs > VCC.

    ...Jim Thompson
     
  12. AJ

    AJ Guest

    "Fred Bartoli"
    Here is a link to the schematic to make things a bit clearer....
    http://61.9.815.52/OP_IN.JPG . The output connects to a PIC analogue
    input and the idea is to be able to detect a high, low or float condition.
    The MCP6044 has VSS –0.3 V to VDD +0.3 V so maybe I will be ok with this
    configuration. I have done a bit of testing and I haven't blown an OP yet.

    I have seen these "over the top" OP's, didn't think of them, thanks for the
    suggestion.

    Best regards,


    AJ
     
  13. AJ

    AJ Guest

    Thanks for your reply mate, the MCP6044 does have vcc + 0.3V and I am
    connecting to the input via 10K so with any luck I will be ok, I haven't
    managed to blow one yet.
     
  14. Jim Thompson

    Jim Thompson Guest


    ...Jim Thompson
     
  15. AJ

    AJ Guest

  16. Fred Bartoli

    Fred Bartoli Guest

    AJ a écrit :
    Buffering with an opamp is unnecessary for a simple detection. And with
    your 30V input, you still have the same pb, whatever the 3.3V supply
    status. And thanks to the high leakage of schottkies, a floating input
    could well read a false high or low at high temperatures, depending luck.

    That one will clamp your input voltage and provides an output within the
    supply rails.
    If you're really cheap you can get rid of the 2 leftmost 100K resistors.
    Use any NPN-PNP signal transistors.

    1M
    VCC ___
    .----+----|___|-.
    | | |
    .-. | |
    100K | | \| | ___
    | | |-+------+-|___|-< 0-30V
    '-' <| | |
    | | | | 100K
    to PIC <---+----+ | |
    | | | |
    .-. >| | |
    | | |-' |
    100K | | /| |
    '-' | 1M |
    | | ___ |
    '----+----|___|-'
    |
    ===
    GND
     
  17. AJ

    AJ Guest

    "Fred Bartoli"
    Thanks a lot for your input Fred, very helpful.

    Best regards


    AJ
     
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