Quad Opamp biasing

Discussion in 'General Electronics Chat' started by Peter Wan, Jul 16, 2009.

  1. Peter Wan

    Thread Starter New Member

    Jul 16, 2009
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    I have a single supply quad Opamp and the four Opamps are doing ampliflication and filtering in the circuit. All of them are biased to +VCC/2. Can I use just ONE voltage divider (2 resistors) to bias all four opamps at their +ve terminals? Or should I have separate voltage divider for each of them? See, I can save both resistors and PCB space if I can do that. The four Opamps are AC coupled in my circuit. Does coupling make a difference? I wish someone can answer my question.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you post your schematic? Then we will be better able to give informed recommendations.

    .png image attachments are preferred, as they are not lossy like .jpg images are.

    Click the "Go Advanced" button below the text box, and then "Manage Attachments" - navigate to where your image is on your local drive, select it, and then upload it.
     
  3. Peter Wan

    Thread Starter New Member

    Jul 16, 2009
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    Here is my schematic. Thanks!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK. You can go with just R10 and R11, however...
    Reduce them both to 1k to 1.5k. Also, use a 10nF cap from the junction to ground.

    If both resistors are 1.5k, that will result in just 1mA current flow through the voltage divider. 3v/3k=... The 10nF cap will help to keep things nice and quiet. Metal film resistors are less noisy than carbon or carbon film.
    [eta]
    You DO realize that LMV324 opamps are very slow, right? You shouldn't plan on getting much bandwidth out of them.
     
  5. Peter Wan

    Thread Starter New Member

    Jul 16, 2009
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    Thank you VERY MUCH. I appreciate.
     
  6. millwood

    Guest

    no. that's not going to work. you will need to have a resistor on the non-inverting end of each opamp to help isolate the input. Otherwise, the ac signal will all be "mixed" through the divider network.

    you should also chose the resistors carefully so that both inverting and non-inverting ends see the same DC resistance to the "virtual" ground. in this case, the inverting ends' DC resistance to ground is the feedback resistors. so the added resistors should have the same value as the feedback resistors, respectively.

    this allows for nulling of input bias current.
     
  7. millwood

    Guest

    that approach will not work: the capacitor will just short the ac signal for all opamps.

    you need the capacitor for supply decoupling and better ripple rejection. and you need additional resistors I suggested to isolate the a/c signal from the ground zero established by the capacitor.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    No.
    The opamps are inverting.
    The (+) input of each opamp has no signal and is for the DC reference voltage of half the supply voltage.
     
  9. steveb

    Senior Member

    Jul 3, 2008
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    I just wanted to point out one thing here. This is OK to save board space, but if you have a bad opamp shorting out the node, then all OPAMPs stages stop functioning and it's much harder to troubleshoot. With separate resistors, the bad OPAMP is very apparrent. There is also less risk of one bad OPAMP damaging another.

    This issue is more important if you have many OPAMP stages in a large board design.

    This is not necessarily a major concern, but I just thought I'd point it out, since I got a bite mark in the ass from this once. ;)

    EDIT: Also, certain OPAMP types (bipolar input with protection diodes for example), can behave oddly when they hit the rails and the feedback can't close the loop. There can be an input current that disturbes the node voltage. I just remembered the other bite mark.
     
    Last edited: Jul 17, 2009
  10. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Use an op amp to create the reference voltage (see attached).

    The output VBREF will be a very stable Vcc/2 (+9V as shown with a +18V supply).
     
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