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OPA3692: How to change the gain of the fixed gain buffer to +1?

DeWei Lan
Prodigy 250 points
Part Number: OPA3692

I design fanout buffer with OPA3692.

Channel 1 and 3 are set to gain = +2 normally, so I just assign in1- in3- to GND.

But the ch 2, I'd like to set to gain = +1, 

Do I just disconnection(open) in-2 as this picture?

AFAIK, open the pin could introduce noise into IC via this opened pin, as here mentioned:

e2e.ti.com/.../the-unused-op-amp-what-to-do

 

Is there any other better solution?

  • 0
    Guru 273500 points

    When you leave pin 4 open, the input of the opamp is not floating because it is connected to its output through the 402 Ω resistor. This is safe.

  • 0
    Prodigy 250 points

    Thanks @Clemens Ladisch 's reply.

    So if I set pin 4 to float, I don't need to care about noise being introduced into the IC through pin 4?

    (I've read some references that say floating pins are similar to antennas and may introduce high frequency noise into the IC.)

    An idea come to me:

    As above picture, I connect 1M resistor or higher resistor than 1M.

    The gain will be 1+(402/402+1M) ~= 1

    Is this approach a good solution?

  • +1 in reply to DeWei Lan
    Guru 140200 points

    Hi DeWei,

    the section "Gain Setting" of datasheet clearly says what to do when a gain of +1 is wished: 

    "For a gain of +1, leave the –IN pin open and drive the +IN pin with the signal."

    DeWei Lan said:
    Is this approach a good solution?

    No, this approach is not good because with the 1M resistor you add an antenna now.

    Why are you worried? The internal -input of OPAmp is isolated by the left 402R resistor and the internal -input of OPAmp is not floating but via the other 402R resistor low ohmically connected to the output of OPAmp.

    Don't confuse the "-IN B" input pin with the "internal -input" of OPAmp! Bruce is not discussing the "-IN B" input pin but the "internal -input" of OPAmp. And the "internal -input" of OPAmp is not floating here.

    Every voltage follower (non-inverting amplifier with a gain of +1) is working like this. So everything is fine.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 250 points

    Many thank @kai klaas69:

    My bad, I have not notice datasheet has been said it.

    OK, I'm confused about -INB and internal -in(in the others word: -in of general OPA ).

    Now I know they are completely different, thank you so much!

  • 0 in reply to DeWei Lan
    Guru 140200 points

    Good luck Relaxed