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OPA859: FB pin of OPA859

Part Number: OPA859


I have two questions FB pin of OPA859.

1) At "6 Pin Configuration and Functions" of datasheet,

    FB pin is I (input).

   However, in Figure 50, the FB pin is connected to the output of the amplifier, so it looks like an output.

   Is FB pin output?

2) When using OPA859 by voltage follower, is it correct to connect FB pin and VIN-?

Best Regards,


  • Hello,

    The questions you asked are good ones to discuss, I will do my best to help below.  I have a shorter section at the bottom with less explanation.  Also, your images you attached did not post/are not visible.  If you would like, you could repost/edit/reply with the images properly inserted; I would recommend using the 'paperclip' button on the command ribbon to insert any files/images.  Copy/paste usually does come through very well.  

    Regarding your first question about the FB pin on the OPA859, the reason this pin exists is to help with minimizing parasitic inductance and capacitance in the overall circuit.  Normally, routing a feedback network (or trace, for unity-gain buffer) around the op-amp would be required.  With the OPA859, the FB pin allows for short feedback networks via a trace or resistor between pin 1 (FB) and pin 3 (IN-). 

    I think you have a good understanding of this from how you phrased your question, so I will directly address your first point: I like to think about the FB pin as an 'input' in an internal sense, being an input within the op-amp feedback network, rather than an input along the greater signal path.  You will be able to use the FB as an 'internal output' to your IN- pin, but it is not quite the same as an output to drive your load (and progress along the signal path).  It could also be thought of as an 'input' for any external feedback resistor network. 

    Since the FB pin is really just a convenient pin with internal wiring to the OUT pin (pin 6), I agree it is confusing when input/output are used to define the FB pin functionality as well as the functionality of the typical input/output pins, such as IN+, IN-, and OUT.  One way to think about it is this: you would want to use OUT (pin 6) and not FB (pin 1) to drive your op-amp output, so perhaps that is a good enough reason for marking it as an 'input' within the overall op-amp implementation.  If the pin were labeled I/O or just O, the confusion might be slightly better or slightly worse.  I can check with my team on the exact nature of how to explain and reference these pins and their behaviors, so that you may be comfortable with your reasoning.  If they advise differently, I will update my response shortly.

    Addressing your second question, you are correct in connecting the FB pin (pin 1) to your VIN- pin (pin 3).  The datasheet mentions how the NC pin (pin 2) helps with the capacitive coupling issue between the two pins around it, so you are following correct implementation by wiring together pins 1 & 3.  If you had a different gain configuration, you would be able to place an SMT resistor between the pins as well.

    See this image from page 16 of the datasheet:  

    I am a bit overly-wordy in my answers, so I want to summarize what I think you need:

    1) The exact best name/definition for the FB pin is confusing; I would recommend you think of it as an internal feedback network connection point, where it specifically being input or output is less important.  It is truly just wired back to the OUT pin internally, and should not need to be used to drive output load.  I would say it is neither an input or output on the main signal path, but a part of the op-amp feedback loop.  Perhaps I/O or another designator would bring more clarity to future users.

    2)  You are absolutely correct here, wiring up your feedback network (and for your unity-gain buffer case, just a wire/trace) between the FB (pin 1) and IN- (pin 3) pins is the recommended implementation for this (buffer) configuration.  This will help reduce the number of long traces and layout headaches during your design.  I do not think it would be wrong to wire up the buffer in an more typical op-amp fashion (OUT pin to IN- pin), but there is no need to do so here.  Not using the FB pin would introduce additional board parasitics into your design, which would likely be unwelcome.

    Please ask any more questions you have or ask for clarity if you would like to do so, I can always explain differently or with images.

    Best Regards,

    Alec Saebeler

  • Hello Alec-san,

    Thank you for your reply.

    I failed to attach the figure, however I am very satisfied with your answer.

    The detailed explanation of 1) was very helpful.
    Also, thank you for agreeing to 2).

    Best Regards,


  • Hello Ikuo-san,

    I am glad my explanation was able to assist you.  If you ever have any additional questions regarding high-speed parts, please know you can always ask here.

    Best Regards,