OP AMP Stability Questions

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eladta

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Hi all,
I want to use the AD8638 OP Amp as a simple buffer.
I know that according to theory we need to check the phase of the open loop gain where the magnitude of it crosses 0 dB. According the datasheet page 8 it comes to 1 MHz, and the phase is aprox. 65 degrees (meaning stable). But what does it mean exactly ?
say i wanna use another freq. what than ? how do i check the stability ?
Are there any other parameters i need to check in terms of stability ?

Thanks, Elad.
View attachment AD8638_8639 op amp.pdf
 

With a simple resistive feedback that op amp will be stable for any frequency or gain with a phase-margin of 65 degrees. Typically you want at least 35-40 degrees. 1 MHz is just the gain-bandwidth-product, thus if the non-inverting gain is 2, the bandwidth would be about 500kHz.

You can check the stability by plotting the gain and phase of the circuit (Bode plot). Also look at the transient response of the circuit to a step or square-wave input. If the edges of the step output have significant overshoot or ringing then that indicates the circuit may be marginally stable.
 

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