Opamp input noise issue.

Discussion in 'General Electronics Chat' started by EngineerErrnie, Oct 18, 2010.

  1. EngineerErrnie

    Thread Starter New Member

    Oct 18, 2010
    I built a classic opamp non inverting circuit and hooked it up to a function generator running at 1KHz. The opamp is a TI THS4601 FET Input Wideband OpAmp, which has the following specs:
    # Gain Bandwidth Product: 180 MHz
    # Slew Rate: 100 V/us
    # Maximum Input Bias Current: 100 pA
    # Maximum Input Offset Voltage: 4 mV
    # Input Impedance: 10E9 ohms || 10 pF.

    Rf is 7K and Ri 2.6K and function generator ouput is grounded through a 47 ohm resistor to terminate the 50 ohm line (didn't have a 50 ohm resistor). I can't figure out why I am getting all this additional ripple added to the opamp non inverting iput (function generator output).

    Any thoughts? This is driving me nuts! Pic one is the input and output version, Pic two shows the function generator signal I disconnect it from the inverting input of the the opamp. Pic three shows the ripple up close.

    Thanks, and this is my first post :)
  2. tom66

    Senior Member

    May 9, 2009
    You haven't specified your power supply; it could be noise on the rails.
  3. EngineerErrnie

    Thread Starter New Member

    Oct 18, 2010
    I am using a high end BK Precision 1761 DC power supply with plus 5 and minus 5 volt rails. I seem to have the same problem at other voltages too.
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    When riple is involved it's best to try and figure out where it's coming from. Take a ouple of caps and bypass the input rails at the amp - if it's a high frequency rpple try 0.1 uF, if it's a lower frequency use an electrolytic. See if the ripple decreases.
  5. Pich

    Senior Member

    Mar 11, 2008
  6. EngineerErrnie

    Thread Starter New Member

    Oct 18, 2010
    Thanks guys!!! the ,1uF caps did the trick.
  7. EngineerErrnie

    Thread Starter New Member

    Oct 18, 2010
    interesting article. I just assumed that I was using a fancy power supply and short leads so I would be ok, guess not. I have a perfect signal now. I still wonder why it would make the function generator output generator ripple . The Function generator was connected directly to the high impedance of input of the opamp.
    Last edited: Oct 18, 2010
  8. russ_hensel

    Distinguished Member

    Jan 11, 2009
    Ripple is usually 60 hz from power supply. What I think you saw was instability in the op amp because of inadvertent feedback thru the power supply lines.
    This is a very high speed op amp ( video amp perhaps ) 180 mhz is high rf. Normally not used at speed like 1 k hz or the like, do use when things must go real fast.