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AD8227 stability

Hi there,

reading through the AD8227 datasheet, in the chapter "Driving an ADC" it is stated that loading the AD8227 output with a RC tau lower than 5us affects the device stability.
A 5us RC gives a pole at 32kHz. 

Can I please have more info about this? (i.e. phase margin of the output opamp)

Thanks

Flavio

  • Hi Flavio,

    Capacitive loads introduce additional pole to the amplifier which can cause instability. To compensate this, easy fix is to put series resistor between the amplifier's out and cap load to isolate the opamps output from the cap load and introduces zero on the transfer function. You can learn more of it on this link: https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

    It is recommended to set the zero frequency around one decade below the amplifier's bandwidth to ensure stability. Just use this general rule as a starting point. When we verify it on the  AD8227,  any RC combination as long as the time constant is above 5us, this part is stable.

    If you want to set a higher cut off frequency than what the datasheet is recommended, make use that you test it in the lab to see if the parts is stable or if there is a need to use ADC driver with better output drive capability like the AD8616.

    I hope this helps.

    Best regards,

    Emman

  • Hi Emman,

    thank you very much for your reply.

    Yes sure; however I was wondering if it's possible to have the forward gain (A(s)) of the output opamp. That way I can add poles/zero to ensure a decent phase margin.

    Unfortunately I couldn't find that plot on the AD8227 datasheet (I can only find the device closed gain plot over frequency).

    Thanks for your help

    Flavio