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Offset Voltage in Isolation Amplifier???

Discussion in 'Electronic Basics' started by [email protected], Jan 20, 2005.

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  1. Guest

    I use isolation amplifier (e.g.ISO124).
    I get considerable offset voltage.
    Since its a isolation amplifier, how to nullify offset voltage???
  2. Chris

    Chris Guest

    The data sheet specifies V(os) of +/-20mV typ., +/-50mV max. It also
    shows a V(os) which is very dependent on temp (+/-0.2mV/degree C) and
    supply voltage (+/- 2mV/V). This is not good news if you're looking
    for precise control of offset voltages in a DC/low frequency signal

    If you're going to stick with the INA124 (it is lower cost than the
    alternatives), you might want to use a unity gain op amp with an offset
    adjust at either the input or the output side. One other way would be
    to use a multi-pole reed relay and a sample-hold setup to achieve
    chopper stabilization, but that is almost certainly more trouble than
    it's worth here.

    If you want to deal with the stability and drift problem, though, you
    might want to upgrade to the Analog Devices AD202. It's more
    expensive, and has lower bandwidth, but if you've got +15V on the
    receiving end, it will give you better offset, stability and drift
    figures, and also gives you a free +/-7.5V supply at the signal end.
    Also, since there's an op amp signal input, you can build in your
    offset adjust more easily. I've used it successfully many times.

    There is one other thing, which I'm not sure is really relevant in your
    case. The AD202 is transformer isolated, which might give you more
    reliable protection than the capacitive isolation of the ISO124.
    However, that doesn't make much difference in most cases, and the
    ISO124 is rated to 1500VAC. It may be at least partly superstition,
    but I just feel more comfortable with transformer isolation from a
    reliability standpoint as well as a safety one when dealing with
    dangerous differential voltages.

    Good luck
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