what is the ad8676 best voltage follower setup?

I want to use AD8676 in voltage follower mode, whats the best setup, is it just just simple voltage follower model? sometimes they add stabilising resistors and caps in the feedback loop and 100ohm at the input ...

DS doesnt specify it, I will be using it for the analog mux buffering for measurements.

  • hi Hary,

    thanks for response, in my case impedance that pos input will see is 10kOhm, since neg input will be in the feedback loop, will I need to place 10kohm in feeback loop for bias current compensation, (how the compensation really work, does it create opposite voltage to the one on the pos input and therefore it compensates ?) do i need 100ohm res in series on the pos input ? sometimes they use capacitor in the feedback loop, would i also need it ?

  • big Mike,

    The feedback resistor is used to balance the impedances seen by the two inputs and compensate for

    the input bias current.  The AD8676 is a bipolar, so I would use a resistor.  A one kiloohm resistor is

    4 nV/rt-Hz, so your source impedance should be low, and thus the feedback resistor will be low.

    If it is too high, it will form a pole with the input capacitance, give you phase shift, and may lead

    to oscillations.

    Hary

  • Mike,

    A 10k is 12nV/rt-Hz, so don't use the AD8676.    Look at the ADA4622-2, ADA4610-2, or

    ADA4000-2.

    5.5nA times 10k is 55uV, so depending on your application, you may not care and can use a

    100 ohm feedback w/ no cap across it.

    What is your highest signal frequency?

    Do you need to go down to DC?

    Harry

  • hi Hary,

    in fact I will be mostly using it for DC measurements, ADA4622-2, ADA4610-2 seem to have large offset, ADA4000-2 even bigger.

    Regards,

    Mike

  • Hi Harry,

    Thanks for your help on this.

    ADA4522-1 looks very promising, ibias=150p, vos=5uV, p-p noise =117n, and dual supply from -/+22V, which i willbe using +/-15 as my max signal is 12V.

    I think I may go with this one.

    One final thing on the feedback resistor, could you elaborate more on it please? is there way of calculating fb resistor once source impedance is known ?

    Best Regards,

    Mike