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Voltage offset test

Discussion in 'Electronic Basics' started by Kingcosmos, Nov 20, 2005.

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  1. Kingcosmos

    Kingcosmos Guest

    I am trying to perform a simple voltage offset test in Spice for an
    OPA227. I have both inputs shorted to ground, +/-15V supply, and I am
    getting 48.9mV on the output. The datasheet shows +/-100 uV for the
    OPA227P and +/-200 uV for the OPA227PU. Either way, my test simulation
    is off by a few magnitudes. Is this a limitation of the simulation or
    is there a better testing circuit configuration for op-amps?
     
  2. So they are at the same voltage.
    So the input offset is being amplified by the open loop gain of the opamp.
    Since Spice can measure very small voltages, why not connect the opamp
    as a unity gain follower, with zero volts on the + input. The output
    will be at the offset voltage. Or connect the amplifier up as a
    non-inverting gain of 100 (1k between inverting input and ground, 99k
    resistor between non-inverting input and output, ground non-inverting
    input), and the output will be at 100 times the offset voltage.
     
  3. Kingcosmos

    Kingcosmos Guest

    So they are at the same voltage.

    Yes, they would be at ground potential.

    This bothers me a little. The typical open loop gain of the OPA227 is
    160dB. If the output I am getting is the product of the input offset
    and open loop again, then that would make the input offset incredibly
    small. Not that it is a horrible thing in fact it would be great. I
    wouldn't have thought that the input offset would be THAT small and I
    would expect the OPA227 to saturate to one of the rails. But I noticed
    another result. If I make the supplies smaller, +/-5V, then the output
    is 10mV. Why would changing the supplies make this drastic change?
    as a unity gain follower, with zero volts on the + input. The output
    will be at the offset voltage. Or connect the amplifier up as a
    non-inverting gain of 100 (1k between inverting input and ground, 99k
    resistor between non-inverting input and output, ground non-inverting
    input), and the output will be at 100 times the offset voltage.

    Thank you, I will try this and see what I get.
     
  4. The offset spec is either the typical value or the worst case. But
    actual offset can be anything smaller than the worst case for the
    given conditions, an I have no idea what offset is intentionally built
    into your model.
    For a real opamp, many things affect the offset voltage, including
    temperature and supply voltages (which change the internal temperature
    gradients). I doubt that all the things that alter the actual offset
    voltage are included in the model you are using.
     
  5. Kingcosmos

    Kingcosmos Guest

    The offset spec is either the typical value or the worst case. But
    actual offset can be anything smaller than the worst case for the
    given conditions, an I have no idea what offset is intentionally built
    into your model.
    temperature and supply voltages (which change the internal temperature
    gradients). I doubt that all the things that alter the actual offset
    voltage are included in the model you are using.

    Good show. At least this helps confirm my fears that it is the acutal
    Spice model. I will have to test the silicon itself to get some 'real
    world' values. BTW, I have cracked open the model and have no idea how
    to read them. Is there a tutorial that explains the syntax or teaches
    one to write a macro model? I am under the assumption that the
    'language' used in Spice has a structure like C (things like functions,
    variables etc) because in all honesty, it looks like meaningless
    sentences.
     
  6. Kingcosmos wrote:
    (snip)
    The choices available vary, somewhat, depending on what Spice you are
    using. There are models that are component level (sub circuits that
    use the standard component models and the schematic of the internal
    circuit of the chip, but these are slow, and to be accurate must give
    away all the design secrets of the chip. Some models are based on
    polynomials that curve match various internal nonlinearities. Some
    models are based on program secrets that are more accurate than the
    polynomial versions, but still hide the design secrets of the chip
    (LTspice does a lot of this for their chips). Some are based a
    combination of basic components in combination with all kinds of
    mathematical functions to simplify a full schematic model.

    Reverse engineering a difficult model can be a real challenge.

    Are you a member of the LTspice discussion on Yahoo groups? They have
    some very knowledgeable members.
     
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