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INA240: Using INA240 (any instrumentation amplifier in general) In a Feedback Loop

Rohat KILI�
Prodigy 130 points
Part Number: INA240

Hi all,

I've learned that using an amplifier (Av > 1) in a feedback loop may cause some instability issues. This is well analyzed in https://www.ti.com/lit/an/slyt103/slyt103.pdf. This can be experienced especially in a switching regulator if there's an amplifier (e.g. for current sense) before the comparator.

Considering a linear regulator's response is way faster than a switching regulator, would it still cause any instability issues if we use an instrumentation amplifier (e.g. INA240) in the feedback loop of a linear regulator?

  • 0
    TI__Mastermind 22650 points

    Rohat,

    Thanks for choosing E2E!

    I see a few things you need to think about here. As the attached app note explains, a challenge of stability here when using two amplifiers in a feedback loop is the further reduction of phase margin in the system. To determine this, you would need to know the primary and secondary amplifiers of your feedback loop to determine how phase margin would be affected here.  

    That said, the INA240 is not an instrumentation amplifier, but rather an analog output current shunt monitor. The loop is closed internally for this device, and the gain is predefined, based on device variant. As such, it in itself is a closed loop system, and his its own stability criterion. You cannot place the open loop response of the INA240 in the feedback loop as is shown in that app note as a result. Instead, the stability of the INA240 is dictated by the amount of capacitance the device can drive, as shown in the datasheet:

     

    Regarding how this would work with a true instrumentation amplifier, I am not sure how well this would work, given the buffer stages normally present in these devices. If you want to investigate this with an instrumentation amplifier, you can check in with our experts here.