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OPA2340: Step response abnormal oscillation

Part Number: OPA2340
Other Parts Discussed in Thread: OPA188, OPA2320, OPA388

Hi, Team

Customer O-Net just found an abnormal output oscillation from OPA2340.

It's just used as a buffer with, step signal to IN+, IN- connect with OUT, test OUT without any load.

I test it on the precision Labs test board, on circuit 5, the issue is reduplicated.

The oscillation will come up at the step response, 5us later after the output stable.

Can you help to check what's the reason caused this oscillation?

I also tried several other setting, but don't see it, for exmaple, with Gain =10, or with 100Ohm RL.

  • Harson,

    What you describe is not an oscillation but a known recovery time required after driving the output into a rail.  Even though the input common-mode voltage is literally rail-to-rail, output cannot follow all the way to neither rail.  And attempting to drive the output to either rail will triode one of the output transistors and result in collapse of one of the internal nodes inside the output stage.  As the output is then driven away from the rail, it takes the collapsed node around 5us to fully recovery from the non-linear condition culminating in the burst of noise you see on your scope picture as the node settles on its final value.

    The best way to prevent this condition is NOT to drive the output closer to the rail than specified in the data sheet. Another way is to add a pull down resistor from the output to an additional negative supply in such a way as to sink the quiescent as well as any possible load current. By doing so, the transistor in the output stage does not have to carry any current allowing the output to reach the rail while maintaining its linear operation.

  • Hi  Marek

    Thank you for your explain. I test OPA188, 170, 140, which does't show this recovery.

    I re-test and change input of OPA2340 with some DC offset to verify your explain, the output is good if not drives to 0V after start up.

    The situation we get is customer test AD8616 which is no such a recovery time from same test.

    Customer power supply is Single 3.3V.

    O-Net are trying to test with OPA2320. Is there any way to guide us which kind of amplifier doesn't have a significant recovery?

  • OPAx340 is a twenty year old rail-to-rail output op amp designed using a different output stage topology that can drive very large capacitive loads but it's proned to overload recovery problems discussed here. All other op amps mentioned by you are less than ten year old and use a different output stage - Monticelli Class AB output stage - that was under patent protection at the time of the introduction of OPAx340. Monticelli output stage cannot drive as much of capacitive load but it does not suffer from the kind of delayed overload recovery time discussed here.
  • Hi, Marek
    Thank you for your kindly help!
  • OPAx340 is still used by a lot of customers and the feature of <1mV rail to rail output is a significant advantage for customer with single power supply system.
    I've seen a lot of customers like this feature.
    For newer amplifiers I saw, mostly do not support this feature, OPA388 is the mostly close to.