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OPA836: OPA836 using issue

Herbert Wang
Intellectual 460 points
Part Number: OPA836

Tool/software:

1.First, the operation amplifier circuit test conditions: VDD voltage DC3.6V, S IN input signal suspended in the case.

 

2. The test waveform at each point is as follows: The waveform at ① is as follows:

②The waveform  is as follows:

③The waveform  is as follows:

④The waveform  is as follows:

3. Questions and how to improve (1) Why are the waveforms at (1) and (2) different, why are the waveforms at (2) cluttered? (2) In the case of input suspension, there is a signal at ④, how to solve it? (3) How can this circuit be improved to normally amplify the input 300KHZ, the amplitude of 10mV signal, and the output amplitude of 1V after amplification?

  • 0
    TI__Genius 13180 points

    Hi Herbert,

    Could you please explain what is meant by the input being suspended? Does this mean the input signal is altered or floated? The device itself seems like it is in an unstable state. A couple suggestions for this circuit to narrow down possible error sources is to try grounding the input signal. If you see there is still signals at the output, it is likely a stability problem. I would also reduce the values of the resistor divider used to bias the input. We try to stick with values in the thousand-ohm range as this limits noise in the circuit. As for the gain requirement, the device should have enough bandwidth, but we do have faster devices that can meet this goal. Another potential option would be a second stage; however, it would be good to first understand the reason this device is behaving this way. 

    Best regards,

    Ignacio

  • 0
    TI__Intellectual 1485 points

    Hi Herbert,

    I completely agree with Ignacio and his questions.

    1) Assuming suspended as floating, I created a TINA sim of your circuit. As he mentioned, there is stability issue in the circuit as the phase margin is only 33 degrees.

    2) If you just ground the input, the circuit is stable (PM = 85 degrees) as per the TINA sim that I have attached below.

    OPA836 Stability.TSC

    Thanks

    Aditya Gosavi

  • 0 in reply to Aditya Gosavi
    Intellectual 460 points

    Hello, after the input signal is grounded, the waveform is as follows, the problem still exists, please take a look, how to improve?

    1, the operation amplifier improvement circuit:

    2. The test waveform at each point is as follows: When the input is grounded, the VDD voltage is DC3.6V, and the test waveform at each point is as follows:

    ①The waveform  is as follows:

    ②The waveform  is as follows:

    ③The waveform  is as follows:

    ④The waveform  is as follows:

  • 0 in reply to Herbert Wang
    TI__Genius 13180 points

    Hi Herbert,

    Thank you for the update. Although this is not the goal with the design could you try to reduce the 4700-ohm feedback resistor and put a 1k resistor. Often a large feedback resistor results in stability problems, therefore reducing this value should ideally put the circuit in a more stable state.

    Best regards,

    Ignacio