• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 49 subscribers
  • Views 611 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Why oscillate when I use OPA890 or OPA690 as a unity gain buffer

user104719
Prodigy 20 points
Other Parts Discussed in Thread: OPA690, OPA890

I use OPA890 or OPA690 as a unity gain buffer. But the output show a small  oscillation.

According the datasheet, I directly connect Output and inverting input  in using OPA890 ,and

connect Output and inverting input  with a 25Ω resistor in using OPA690.

Add a small resistor below 100 ohm at output for driving pure CAPACITIVE LOAD about 50p.

But After I doing all this,  a small  oscillation still exist for 200mV p-p.

Are these two amp not suitable for unity gain buffer?

  • 0
    TI__Genius 12631 points

    Hello,

      Both amplifiers are unity gain stable, however having said that they could be susceptible to external parasitics based on the layout of the board and passive components. Can you please tell me if you are using a custom PCB/ TI Evaluation module or are you breadboarding this? Furthermore, have you added sufficient bypass capacitance close to power supply pins? I assume you are using +/-5V Supples. Are you using surface mount components or through holes.? Have you followed the layout guideline mentioned on page 26 of the OPA690 datasheet?

    Looking at the datasheet I can see that the OPA690 could be susceptible to instability (Page 7, figure 1) shows some peaking in the unity gain mode, however the TINA simulations show sufficient phase margin for this to not be an issue. Does removing the capacitive loading improve stability? What is the frequency of oscillation?

    The OPA890 looks extremely stable in unity gain configuration and I am very surprised you are having oscillation issues. What is the frequency of the oscillation here? Again, could you please try removing the capacitive load and checking if the oscillations go away? I think it is highly unlikely as the 100Ohms should be sufficient isolation, however it is an easy thing to check.

    Samir