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LPV812: Oscillating

Seishin
Genius 4135 points
Part Number: LPV812

Hi,

We have a question about LPV812 Op-amp oscillation from our customer.

■Question
We are trying to build the following circuit using an LPV812.
This application need to operate it with low power consumption for battery operation.

But the output of LPV812 oscillates on our prototype board.

Could you please let us know why would this circuit oscillate?
And what could be the solution to avoid the oscillation?

Actually this Oscillation can be suppressed by adding an additional circuit,
but that requires additional cost. So we want to make sure it works properly with this schematic.

Meanwhile we tried to replace this Op-Amp with another manufacturer's amplifier,
and it does not oscillate even without an additional circuit.

■Customer’s schematic diagram

Best regards,
Seishin

  • 0
    Guru 140200 points
    Hi Seishin,

    in order to keep the supply current of LPV812 extremely small, the internal circuitry of LPV812 is very high ohmic. Because of that the chip has also a very high open loop output impedance. This can conflict with the load capacitance coming from a cable or a 1:1 scope probe. A snubber of 10k/100n at the output, as shown in your schematic, can stabilize the OPAmp then.

    You should also connect a filter cap from the + input of OPAmp to signal ground. But, oops, that would even furtherly increase the costs... :-)

    Kai
  • 0
    TI__Mastermind 23425 points

    Seishin

    There’s a nice little write up in back of the datasheet for explaining why the amplifier can oscillate when driving capacitive loads. In order to improve your circuit performance remove the 100k resistor and replace with a  short. Next change your 100 ohm resistor to something greater than 10k ohms. Something between 10k and 50k should allow you to drive anybload you want. The optional components you have shaded should not be needed.

    In reference to other parts not oscillating, I am not surprised if these amps have higher quiescent current. Cap load drive capability is a trade off for lower power consumption. But adding one resistor should solve that problem for you.

    Chuck

  • 0 in reply to Chuck Sins
    Guru 140200 points
    Hi Chuck,

    the snubber is a really good option. I was surprised to see how good it works in the simulation.

    Kai
  • 0 in reply to Chuck Sins
    Genius 4135 points

    Dear Chuck and Kai,

    Thank you very much.

    I understand.

    Best regards,
    Seishin