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Op-Amp Accuracy

Discussion in 'Electronic Design' started by panfilero, Jan 28, 2013.

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  1. panfilero

    panfilero Guest

    I'm looking for an Op Amp to use for making bi-directional current measurements off of a low side shunt. I need to be able to read +/- 30A, I'm thinking I'll use a 1 mOhm shunt and read +/- 30mV across it, but I'm having a hard time understanding how to figure out the accuracy or precision I'd get from an Op Amp.

    I'm looking at using an INA210

    http://www.ti.com/lit/ds/symlink/ina210.pdf

    it has an offset voltage of +/- 35uV and the datasheet says Enables shunt drops of 10mV full-scale"

    I've found an example online that says if you have an offset of 1mV and need 1% accuracy you will need 100mV full scale across the shunt.

    I'm having trouble putting these pieces together offset voltage, precision,and full scale...

    much thanks!
     
  2. First, keep in mind "specsmanship". When an instrument guy says "1%
    accuracy", what they generally mean is that +/-1% of the rated full
    scale input is the maximum error (under some perhaps optimistic
    conditions such as 25+/-0.1°C and within 45 seconds of last
    calibration). This number is the smallest that can honestly be quoted,
    so it's a popular one to quote.

    So, +/-1% of a 30A full scale reading is +/-300mA. That means that you
    could have a +/-10% of reading error in a 3A measurement, and a
    300mA-ish measurement would be meaningless.

    A typically more useful spec is a percentage of reading (gain error)
    plus an offset (zero error). Your gain error will be a function of the
    resistor accuracy (trimmable) and the amplfier gain (not well
    controlled and temperature-senstive) relative to the gain required in
    your circuit. For example, if you're turning 30mV into 3V, you have a
    gain of 100, so your amplifier needs to have a gain of around 100,000
    to keep gain errors due to the amplifier around 0.1%. Not much of a
    problem at DC, but this can quickly become a problem if you need high
    frequency response- for example to accurately measure a 50kHz
    switchmode signal with a gain of 100, you'd need a gain-bandwidth
    product in the GHz range. It's also possible the shunt doesn't look
    like a pure resistor even at audio frequencies. To get 0.1m ohm of
    impedance at 50kHz takes only 0.3nH of inductance- which is not very
    much- about the inductance of 1mm of straight wire.

    Also, keep in mind that a stable offset is not inherently a source of
    error- it can be calibrated out (just measure the voltage on the
    amplifier output with no current and subtract that number from future
    measurements). If the offset changes with temperature- which it will-
    (and is uncompensated) or time, then it becomes an uncorrectable
    error. Same with gain errors. Temperature changes are both externally
    and internally provoked.

    There are several sources of offset error in making small DC
    measurements such as you are looking at doing- offset (Vos) and offset
    drift of the amplifer (TCVos), and thermal EMFs in the shunt
    connections. I assume you're using a 4-wire shunt resistor so the
    connections are properly designed. Asymmetry in the heat sinking can
    lead to temperature differences and thermal "tails" on the response
    etc.

    Generally, at least for relatively high frequency test purposes, it
    may be easier for you to use a feedback-type sensor with a good (and
    specified) bandwidth such as LEM products rather than trying to roll
    your own.
     
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