basic opamp issue

Discussion in 'General Electronics Chat' started by jut, Mar 20, 2013.

  1. jut

    Thread Starter AAC Fanatic!

    Aug 25, 2007
    I'm trying to buffer a 0.450V (from a voltage divider) with the LT1674 opamp. However, I'm seeing 0.470V at the output of the opamp. As far as I can tell, I'm within the operating specs of the opamp:
    vin range: 0.2V to 36V
    vsupply range: 2.2 to 36V
    I tried putting a 1.6k resistor from output to Vin- so input resistances are the same, but didn't help.
    What's gone wrong? This is quite perplexing.

    Here's the circuit I built on the bread board, with actual measured voltages:

  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    It is almost certainly oscillating. The datasheet says it is stable for gains greater than 5. "Stable" means "not oscillating".
    Why did you choose this part?
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Is that your entire circuit? What load is connected to the output?
  4. jut

    Thread Starter AAC Fanatic!

    Aug 25, 2007
    There's no load on the output. Well, I did have a DMM on there, so about 10M resistance.

    I chose this opamp b/c it was on hand, and seem to fit the bill; buffer a low voltage. But I should scope the output to see if it's oscillating.
  5. WBahn


    Mar 31, 2012
    I agree with Ron H, you are using an opamp as a unity gain buffer that is decompensated and not rated for use below a gain of 5. It is probably flailing.

    An easy mistake for someone to make. Live and learn.