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LMH6715QML-SP: LMH6715QML-SP

ishii-ko
Prodigy 50 points
Part Number: LMH6715QML-SP

Tool/software:

Hello.

I have a few questions about the LMH6715QML-SP. Please respond with answers.

1. Could you tell me the input impedance of the inverting input pins? (I think it's small because it's current feedback)

2. Does the direction of the bias current (IBN, IBI) (into or out of the op-amp) change depending on the input voltage?

     If so, at what volts does it change?

3. If possible, could you tell me the internal circuit (inverting/non-inverting) of the op-amp input stage?

     (I want to understand how it works, so it would be even better if you could explain the internal circuit in detail...)

Thanks and Regards,

koji

  • 0
    TI__Mastermind 26825 points

    Hello Koji-san,

    1. Yes that is correct. The input impedance at the inverting input pin is small because it is a current feedback, usually this is less than 1kOhms, around 500Ohms. 

    2. It depends on the input transistors (NPN vs PNP). Figure 15 in the datasheet shows negative bias currents, which suggests current direction is out of the amplifier input pins. 

    We can provide standard CFA input structure:

    From this app note

       


    From this app note:


        Also this PowerPoint is very useful.

        I hope above information is useful, if it didn't answer your design concerns please let us know.

    Thank you,
    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 50 points

    Hello. Sima-san,

    Thank you for reply.

    I have some additional questions for this device.

    - In the datasheet, page 12, equation(3), for IBN and IBI in Voffset, should the negative value be used when the value is negative? Or should the absolute value be used?

    - It is written in application notes that current feedback op-amps do not need resistors to cancel bias current like voltage feedback op-amp. Is this understanding correct?

    Thanks and Regards,

    koji

  • 0 in reply to ishii-ko
    Prodigy 50 points

    Hello.

    Could you give me an update on the situation?

    Thanks and Regards,

    koji

  • +1 in reply to ishii-ko
    TI__Mastermind 26825 points

    Hello Koji-san,

      I apologize for the delay. 

    1. For equation 3, in datasheet page 12, you should use the negative value when the value is negative. This will give you the offset voltage error when values are not equal. I agree that the +/- symbol before the bracket refers to value can be either positive or negative, which would assume then we would use absolute values within the equation. But, for CFB it is most likely not the same as it would be for VFB, which is why you should not use absolute values.

    2. It would be done a little bit differently, but you can add a resistor to cancel bias currents. 

    From this app note: https://www.ti.com/lit/an/snaa004/snaa004.pdf

    Thank you,
    Sima