periodic voltage drop from a pulse generator when loaded

Discussion in 'General Electronics Chat' started by ariemeir, Jan 31, 2013.

  1. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Guys hi,

    Hope for some advice as I am not sure where to look.
    I have a a simple microcontroller-based pulse generator which shoots nice square pulses when no load is connected.

    The micro outputs a signal which is then fed to a AD8030-based voltage buffer which according to the specs is capable of sourcing/sinking ~ 160ma of current.


    Now when there is no load, there is no problem - i see beautiful square waves. When i connect a resistive load, i start seeing the waveform oscillating 15-20% around its nominal peak value (please see attached image).
    At first i thought, maybe the load draws so much current that the opamp cannot provide it, but when measured, the current is much less than the specified in the sheet (~20 ma). I put a 47u capacitor next to the opamp power supply but this shouldn't help since i see the voltage oscillating, not dropping. Indeed - it had no effect : i still observe this annoying oscillation.

    Would appreciate an advice from an expert.

    Kind regards,
    Lenny
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Can you post your schematic?

    What is the load? Purely resistive or not?
     
  3. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    thatoneguy, thanks for the fast response,
    attached is the schematics.
    The load is purely resistive.

    thanks again.
    L.
     
  4. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    one comment: the digital pot you see at the bottom divides the voltage and then the first of the ad8030 based amplifiers doubles it (ignore the resistor values - they are 100k and 100k in the real circuit). The last opamp is simply a buffer.
    The capacitor across the feedback resistor is not present at the real pcb.

    The lt1107 steps up 12v for the opamp from the input 3-5 for the micro.


    Thanks.
    L.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Try to replace the switchmode unit (LTC1107) with a battery just to see if the noise goes away.
     
  6. ariemeir

    Thread Starter Member

    Jun 24, 2011
    68
    1
    Thanks t07afre,
    Tried that, the noise is still there.

    In a way i kind of expected it to stay since the drawn current is small (20ma)
    and the capacitor at the powersupply of the opamp should take care of the ripples
    from the ltc1107. Is there something specific about this unit that made you suspicious ?

    Thanks.
    L.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Do you have a big electrolytic cap on the input of the LT1107? Your 3.3V supply may not be able to provide the peak current required unless you have it in place. The datasheet shows 47uF.
    BTW, your schematic shows LTC1107. I couldn't find a part with that number, so I'm assuming you meant LT1107.

    EDIT: If it doesn't work with a battery, that isn't the problem. I would still add the 47uF cap, though.
    Have you probed the 3.3V supply when this is happening?
     
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