How to use the Feedback (FB) and Power Down (PD) Pins of ADA4857 and ADA17

I am trying to Configure an ADA4857-2 as an Instrumentation amplifier and I have simulated the circuit in ADS using the model and it is working fine. Now that I am making the schematics,I have encountered two pins which I do not know how to use and they are FB and PD pins.

I was thinking I need to instead of connecting my feedback path to the Output pin, I have to connect it to FB pin and I can just connect PD to ground. Is that correct?

Lastly,I would also like to ask a question on how to connect the common mode voltage to ADA4857-2


  • Hi Sadeniji,

    You can connect your feedback to the FB pin or the OUT pin (leave the FB pin NC) either way. Since the FB and OUT pins are internally connected.

    The PD pin is used to power down the chip, which reduces the quiescent current and the overall power consumption. It is low enabled, which means that the chip is on with full power when the PD pin input voltage is low (see Table 8). Note that PD does not put the output in a high-Z state, which means that the ADA4857 should not be used as a multiplexer. (please see Applications Information section of the datasheet).

    May i ask what common voltage value will you use? Also may we ask what your application will be?



  • AS,

    The ADA4857 has the high speed, low distortion pin out.  One of the first op amps that had this pinout

    is the AD8000 and the data sheet has a good write up in the apps section as to why we did it.


  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:27 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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