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INA225-Q1: Current draw on REF source

Prodigy 20 points

Replies: 3

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Part Number: INA225-Q1

There is surprisingly little information on the REF pin on the datasheet.  If we are trying to use Vs/2 as our reference, is it sufficient to use a high precision voltage divider (say 10kohm, 0.1%) or do we need to feed such a source into an buffer opamp before going to the REF pin?

  • Mark,

    Thanks for using the forum to get your question answered.  The short answer is no you cannot use a divider without a buffer this is due to the fact that the Ref input is not a very high input impedance.  The lower resistance at your divider will help but when making exact measurements this may not be enough.

    Another alternative is to have that voltage as your reference when making the measurements so as that voltage changes your measurement will as well.  

    I am attempting to get of someone that knows the insides of the device a little better so that I can explain roughly what that impedance might be and if it changes as you adjust the gain as I assume it does.

    Best Regards,

    Javier Contreras, TI Sensing Products Applications Support

    Getting Started with Current Sensing Video Training Series

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and it is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

     

  • Hi Mark,

    the differential amplifier within the INA225 offers a brilliant common mode rejection of 105dB typical. But this requires that the feedback resistances of the internal differential amplifier are exactly balanced. So, the REF pin should always see a very low impedance as shown in figure 37 or 43. The resistance of a typical resistive voltage divider connected to the REF pin, on the other hand, would totally ruin this high common mode rejection.

    As a rule of thumb the imbalance of internal resistors should be less than 1/100,000 (=0.001%), if a common mode rejection of 100dB is planned. So, these TI's INAs are state-of-the-art.

    Kai

  • In reply to kai klaas69:

    Thank you kai and others!

    Mark

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