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OPA567: Output Pulse(noise)

cafain
Mastermind 6365 points
Part Number: OPA567

Hi, staff

Please ask questions using the waveforms on page 9 of the OPA567 data sheet (SBOS287A).

Q1.
The lower left waveform “ENABLE 10Ω Load” has a ± pulse output.
After ENABLE transitions from Low to High,
the output has a +1V pulse about 1us later and a -1.5V pulse about 7us later.

Why is such a pulse output?

Q2.
The output of the bottom left waveform "ENABLE 10Ω Load" is valid for 8us after the rising of ENABLE,
but the input vs. output delay is unknown because the input waveform diagram is not published.
I think there is no delay because it is analog.

Q3.
The lower right waveform “ENABLE 1Ω Load” has ± pulses output.
Why does the pulse amplitude after ENABLE decrease when the output addition is small?

Q4.
What countermeasures are required to prevent pulses on the output?

best regards

cafainOPA567 Out-pulse.pdf

  • 0
    TI__Guru* 93105 points

    Hello Cafain,

    I searched to see if I could find any of the original design documentation from when the OPA567 was being developed 20 years ago, but nothing was located so we don't have any detailed information about the enable/shutdown related output behaviors. 

    The Typical Characteristics graphs indeed show the behaviors that you ask about in Q1 through Q3. About all I can do is speculate what is occurring because I don't have any information about the internal transient behaviors of the OPA567. The behaviors has to do with the output stage response and reaction to the enable/shutdown circuit control. Do note that this circuit is fast with times in the microseconds so they are very transient behaviors. The initial smaller positive voltage pulse occurs right when the enable pin goes from shutdown state to enabled state. The much larger negative going pulse might be due to a shoot through current in the output stage transistors when the actual turn-on of the transistors occur. Lowering the load resistance to 1 Ohm such that the output is closer to a short to ground may be diverting the current directly through the output to ground instead of through both output transistors.

    Do note that the OPA567 datasheet states, "Examples of output behavior during disabled and enabled conditions with various load impedances are shown in the typical characteristics section. Please note that this behavior is a function of board layout, load impedances, and bypass strategies. For sensitive loads, the use of a low-pass filter or other protection strategy is recommended" so there may be some external factors at work as well. Whenever I see a voltage level momentarily drop below ground in a single-supply op amp circuit that is usually an indicator of a stored charge being discharged somewhere in the circuit.

    The above paragraph suggests that the addition of a low-pass filter might be applied for sensitive loads.Certainly a low-pass will affect a transient's characteristics and may attenuate it. However, an RC or LC low-pass at the output can introduce stability concerns and must be carefully selected.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Mastermind 6365 points

    Hello Thomas san

    Because it is an old device, you can understand the situation where there is no design data.

    Also, the cause of the noise appearing on Output immediately after ENABLE is unknown, I understand your idea.

    Think agan.

    Thank you very much.

    cafain