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INA240-Q1: Maximum capacitive load

Sherry Liang
Genius 10255 points
Part Number: INA240-Q1
Other Parts Discussed in Thread: INA240

Dear team,

I don't understand below spec. Does it mean the output capacitor should be less than 1nF?

Thanks & Best Regards,

Sherry

  • +1
    TI__Mastermind 18780 points

    Sherry,

    This is a typical spec for operational amplifiers, as output capacitance on the load will erode the phase margin and eventually cause the device to become unstable. You are correct that the spec is stating that the output capacitance needs to be less than 1nF. That being said, this spec refers to capacitance placed directly on the output pin. By isolating the output with a large resistance as you have (1kΩ), it is typically safe to use larger values, but you will need to test the resulting phase margin in your system to ensure that the stability it robust enough for your use case. 

  • 0 in reply to Carolus Andrews
    TI__Genius 10255 points

    Hi Andrews,

    Do we have any tool to simulate the phase margin?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 18780 points

    Sherry,

    you cannot really simulate phase and gain margins of our devices, as their loops are completed internally, and there is no way to break them to examine loop gain, Aol, etc.. This is why we specify a maximum capacitance that these devices can drive while maintaining sufficient margin to guarantee stability..

  • 0 in reply to Carolus Andrews
    TI__Genius 10255 points

    Hi Andrews,

    Thanks for your reply!

    Then is there any way for customer to evaluate the phase margin?

    Carolus Andrews said:
    it is typically safe to use larger values, but you will need to test the resulting phase margin in your system to ensure that the stability it robust enough for your use case. 

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Mastermind 18780 points

    Sherry, 

    By this statement, I simply meant that the customer should ensure that the output is stable in their system and that it converges to the expected value(s). The  As I mentioned in the above post, there is not a good way to actually quantify these values without significant effort. Closed-Loop Analysis of Load-Induced Amplifier Stability Issues Using ZOUT discusses the reasons behind why the capacitor would cause the device to become unstable, but the output impedance curve of the INA240 is needed to actually implement the strategies found inside. The INA240 output impedance curve is not available as of today for the INA240.