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INA181: VREF impedance

Peter Schmuckal
Prodigy 230 points
Part Number: INA181
Other Parts Discussed in Thread: TLV9001, LMV321A, TLV431A

I'm looking at INA181 for a low cost device.  There is no specification of what the VREF pin impedance is or max expected current in/out of this pin.  All design examples use an opamp or shunt reg to drive this pin, but I would like to avoid the cost of opamp, and shunt reg uses too much current.  Can a simple R divider be used here?  If so, I need to understand the characteristics of the VREG pin and error consequences of using a divider.

Regards,

  - Peter Schmuckal

  • 0
    TI__Mastermind 27715 points

    Hello Peter,

    The REF pin is connected to one of the internal feedback resistors which set the overall INA181 gain, and is dependent on gain version. Please see the functional block diagram image below.

    The absolute values of these resistors varies significantly, as only the ratios are trimmed accurately in order to ensure low gain error. However, you can consider the resistance to be in the neighborhood of 500k.

    To preserve performance, the REF pin should be driven with as low of an impedance as possible. Any unbuffered resistor divider network which is connected to REF will create a mismatch with the internal resistors, decreasing performance of both gain error and common-mode rejection.

    Your best balance of cost, performance, and power consumption is likely to buffer with one of our cost-optimized op amps such as TLV9001 or LMV321A.

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • 0 in reply to Ian Williams
    Prodigy 230 points

    Thanks Ian,

    Understood.  Am using the A2 version for 5V high side sensing.  Also found the 500K Ref impedance buried in the text description.  Would be useful if that was listed in the specifications.

    I settled on a TLV431A low power shunt reg with minimized cathode current.  Assuming the above block diagram is accurate, then current will only be sourced, not sinked from the REF pin assuming high side measurement.

    Thanks

       - Peter Schmuckal

  • 0 in reply to Peter Schmuckal
    TI__Mastermind 27715 points

    Hi Peter,

    That sounds like a good option. Good luck with your design!

    May I ask that you please mark one of the responses as "Answered" with the green button?

    Best regards,

    Ian Williams