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Single Supply Op-Amp

Discussion in 'Electronic Design' started by Love a Sheep, Oct 27, 2004.

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  1. Love a Sheep

    Love a Sheep Guest

    Hi, I need an op-amp configured so that it acts in differential mode
    (to subtract two signals) but with a single supply. It can be
    ac-coupled.
    I know how to do this with a dual power supply but I am unsure how to
    do this with teh singel supply.

    Thanks
     
  2. Andrew Holme

    Andrew Holme Guest

    View in a fixed width font
    || ___ ___
    -||--|___|-----------+-----|___|---+
    || | |
    | |
    | |
    VCC | VCC |
    | | | |
    .-. | | |
    R1 | | | | |
    | | | |\| |
    '-' +----|-\ |
    || ___ | | >-----+-----
    -||--|___|-+--------------|+/
    || | |/|
    .-. |
    | | |
    R2 | | |
    '-' |
    | |
    | |
    === ===
    GND GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    R1 and R2 are equivalent to (R1 || R2) returned to half rail.
     
  3. Country Loon

    Country Loon Guest

    Thanks but this is so frustrating - I cannot read your diagram. I have tried
    all manner of fixed-width fonts.Could you describe it maybe?I recognise the
    rail splitter on the + input consisting of R1 and R2 which has an R-C series
    netowrk as its input if I am right.What value is this resistor which is in
    series with the capacitor.
    At the - input we have the usual feedback arrangement with ac coupling via a
    capacitor.There are two grounds - one of them has the supply splitter but I
    cannot make out what the other grounds.

    Tom
     
  4. Country Loon

    Country Loon Guest

    I have it now! A weird font gives me the picture. (MS Mincho!) However, I do
    not know what the series resistor feeding the supply splitter is? I figured
    out most of the circuit myself but not this resistor.
    My guess is that the Thevin equivalent of R1 and R2 (ie R1 in parallel with
    R2) must be the same value as the feedback resistor to the - input and the
    series resistor is the onther one.

    Thanks

    Tom
     
  5. Andrew Holme

    Andrew Holme Guest

    Courier is fixed width. I usually paste them into Notepad or view them in
    Google groups.
    Yes, if R = (R1 || R2) then all the other resistors could be R.

    In practice, you might want a third resistor and a capacitor to decouple
    power supply noise at the top of the divider.
     
  6. Wim

    Wim Guest

    You might try the following link, it's an application note from Texas
    Instruments "A Single Supply Op-Amp Circuit collection"

    http://focus.ti.com/docs/analog/cat...stract.jhtml?familyId=78&abstractName=sloa058

    Wim
     
  7. Tam/WB2TT

    Tam/WB2TT Guest

    If you use an op amp where the common mode range extends to 0, and the input
    voltage is small enough, you don't need to do any of these things that
    people are talking about.

    Tam
     
  8. legg

    legg Guest

    ......unless the expected resulting output would have to go negative
    (ie any AC ground-referenced input signal)

    sheesh

    RL
     
  9. Tam/WB2TT

    Tam/WB2TT Guest

    OK, you got me. I don't remember the detail, but when I used an op amp like
    that, I think the input signal was DC coupled and never went below some
    VCE(sat). Now if we knew what the DC level of his two input signals is (or
    can be made), the whole discussion might be moot.

    Tam
     
  10. Now this is a point that always got me.

    Look at the specs of op amps. Assume a perfect one. Both inputs at *exactly*
    the same voltage. What's the output?

    According to the specs and text books it's zero. But is it? Is it not halfway
    between V+ and V- ? Which may be far removed from zero.

    This seems cause great fun in simulators.

    Gibbo
     
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