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offset error adjustment circuit

Discussion in 'Electronic Basics' started by [email protected], Aug 9, 2007.

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  1. Guest

    The system I'm studying has
    integrator made of op amp(PA85),and they made additional
    circuit for offset error adjustment with OPA277.
    However,I've never seen the mechanism behind the circuit before.
    Reference I searched say that we can adjust offset error with
    potentiometer,but I still
    don't know how we can do such work with op-amp.(automatic adjustment)

    It will ve very helpful if I can attatch schematics,but I don't know
    how I can post bmp file.
     
  2. A group that allows graphic attachments is:
    alt.binaries.schematics.electronic
    Unfortunately, this (and all groups with graphic content) is
    not archived nor is it accessible through Google Groups, but
    only through direct access to a usenet server.

    If you contact your Internet service provider, they can
    probably tell you how to post directly to and read from
    their usenet server.

    Till you work that out, you may email me a schematic and I
    will post it to alt.binaries.schematics.electronic for you,
    under this thread title, with instructions to comment back
    here for you.

    Or you can locate one of the many free photo posting
    services on the web and put the schematic up, there, and
    bring us the URL for that location.
     
  3. John Larkin

    John Larkin Guest

    The PA85 is a power amplifier with mediocre input offset voltage and
    offset drift specs. A better, precision amplifier like the OPA can be
    used to measure and correct for the input offset of the PA85. The
    result is a "composite" amplifier, combining the best features of
    both.

    You can post a pic to one of the free image hosting sites, like
    Supload, and give us the link here.

    John
     
  4. Winfield

    Winfield Guest

    Why not try making an "ASCII schematic"? First, your
    browser or usenet program must be set to display in a
    fixed-spacing (non-proportional) font, such as Courier
    or Courier New.

    Then use - + | and similar symbols to draw your circuit.

    The advantages are substantial: Everyone can instantly
    see your circuit and think about it, and the circuit
    and discussion surrounding it are archived on Google.
     
  5. Kingcosmos

    Kingcosmos Guest

    I have seen circuits that use an inverting integrator, A2, from the
    output of A1 that feeds back to the non-inverting input of A1 for
    'auto-zeroing' DC offset.
     
  6. Guest

    Here is part of the schematic
    http://img.villagephotos.com/p/2007-8/1272856/schem.JPG
     
  7. I think this circuit is pretty simple to describe. Lets
    assume the PA85 amplifier has a 10 mV offset. If its +
    input were grounded, that offset would be forced to appear
    at the - input, and when the input voltage at the left were
    exactly zero, there would be 10 mV across the input voltage
    to current converting input resistance (made up of the total
    of R32, 35 and 37). That voltage would produce a current
    through those resistors that would ramp the integrating
    capacitor around the PA85 at a steady rate.

    The additional opamp (with an assumed offset that is a tiny
    fraction of that of the PA85) which according to this data
    sheet:
    http://focus.ti.com/lit/ds/symlink/opa277.pdf
    has a maximum of about 0.165 mV.

    Any offset voltage appearing at the - input of the PA85 is
    integrated (and inverted) by the OPA277 and that integrated
    output is connected as the reference for the PA85
    integrator, in stead of ground. This shifts the - input
    voltage toward zero and the OPA277 integration slows to a
    crawl as the voltage approaches the much lower offset of the
    OPA277. Equilibrium is reached when the voltage at the -
    input of the PA85 is the offset voltage of the OPA277, and
    the voltage of the + input of the PA85 is the negative of
    its offset plus the offset of the OPA277. The gain of the
    PA85 keeps its - input voltage very stable, in spite of any
    input voltages from the HV boost input, so the OPA277
    essentially ignores the actual input signal and responds
    only to offset.
     
  8. John Larkin

    John Larkin Guest

    The jfets-as-diodes trick looks a bit extreme, given that this is a
    pretty low-impedance circuit.

    I've seen a variant, where the precision opamp is in the forward gain
    loop, not just off to the side. There was an Apex appnote for
    improving both the DC and the noise behavior of one of their all-CMOS
    opamps that was very bad in both respects.

    John
     
  9. After looking at the data sheet for the PA85:
    http://www.apexmicrotech.com/mainsite/pdf/pa85u.pdf
    with something like 2 mV of offset, unless you pick a high
    priced version of the OPA277, the improvement with this
    circuit isn't that dramatic. If I was going to go to all
    this trouble, I think I might use an auto zero opamp.
    Perhaps something like an LTC1150 with worst case offset
    voltage of 10 uV:
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1027,P1310,D3092
     
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