AD8009 capacitive load drive capability

There Folks

I designed an input, output AC Coupled amplifier by AD8009. output ac couple circuit has a 47u capacitor.

are there any issues for ad8009 to working with this cap value as an output ac coupling?


    •  Analog Employees 
    on Dec 3, 2019 8:51 AM 11 months ago


    Can you show your schematic, 47uF seems too large. What will be the next stage after the AD8009? What is your load?


  • Dear Gauzz

    Thank you for the reply.

    the load that I said before is input of AD8302 and its input ac coupled capacitor.

    yes 47u seems too large. I plan to use AD8302 in a wide frequency range from 20Hz to 50Mhz.

    I plane to use AD8009 or AD8007 as last stage buffer of my analog front end circuit before AD8302.

    before AD8009 I have a resistive Attenuator, one AD4817 as a High Speed FET input buffer and a AD8337 as a VGA and then a AD8009/8007 as a buffer between the input circuits and AD8302.

    also my other discussion about AD8067+AD8009 vs AD4817+AD8009 composite amplifier is related to this circuits.

    after these AD8009 buffer, I have another signal path that need to amplify input by 80dB gain and I plan to do it by two composite amp that each has 40dB gain and multiplexed 0dB, 40dB and 80dB by a AD8174 with 0dB gain.

    these high gain stages must be ac coupled in range 20Hz to 50Mhz.

    so it is.

    I highly appreciated to have your kindly recommendation about the circuit.

    PS: the schematic is something like this

  • if im going to put a series resistor with ad8009 output, does it solve the problem due to large capacitive load or not? if it does, how can i calculate the proper value of series resistor based on value of ac decoupling capacitor?

  • according to datasheet, large cap load may be cause non-stability or oscillation in high frequencies, so what is this range? in 1 MHz or 2 MHz this non-stability could be accrued in 1 or 2 MHz till 30 MHz or amplifier may be oscillate in frequencies above this numbers?