ADA4528 oscillation

Hello, 

         I'm using a ADA4528-2 as a non-inverting buffer (single sided, 5V). I'm seeing a 200mV Pk-Pk 120kHz oscillation when a constant DC voltage is applied to the +IN pin. My initial circuit contained a 47 ohm resistor to isolate the circuit from any capacitive loading. I was able to eliminate the oscillation by incrementally increasing the resistor value to 1k ohm. This circuit feeds a op amp based current source with a input capacitance of 1uF.

Can you shed some light on why this oscillation occurs? Additionally can you suggest a alternate low power/low noise op amp that my be less sensitive to capacitive loading? 

Scott

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    •  Analog Employees 
    on Aug 19, 2018 7:20 PM

    Hi Scott,

    Opamps are usually sensitive in driving  capacitive load. This capacitive loading in conjunction with the inherent output impedance of the opamp forms an additional pole that makes the opamp unstable and oscillates. You can see the detailed discussion of driving capacitive load on this link: http://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html

    You mentioned that you remove the oscillation by using 1kohm resistor, do you see any trade off on that compensation on your circuit that makes you look for alternative opamps? You can also  see on the article above that the easiest way is to put series resistor at the output of the amplifier (out of the loop compensation) that you may want to try.

    As mentioned on the article, driving heavy capacitive load needs some compensation to avoid instability. Though we have some opamps that can drive any cap load from linear technology: http://www.analog.com/media/en/reference-design-documentation/design-notes/dn107f.pdf. But you need to check if it covers all the spec requirements that you need. If you could just share your intended applications then we might be able to suggest more suitable parts for you.

    Best regards,

    Emman