+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Full Member level 2
    Points: 2,117, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jul 2007
    Location
    India
    Posts
    141
    Helped
    14 / 14
    Points
    2,117
    Level
    10

    Opamp characterization at system level

    hi friends,

    i got one problem.
    i have already one opamp, for which i checked gain, ugb and phase margin. everything is fine because i gave 1V ac signal to one input (DC bias voltage at two inputs) and did ac analysis.
    now my opamp is used in a big circuit having different blocks like bias, loads, and extra circuitry.
    can i do ac analysis of my opamp at this system level? if so, where should i give my ac 1V input ( because the two input nodes are already connected to somewhere in the big circuit)

    Please help me in this regard.

    Added after 3 hours 40 minutes:

    Hi, i got the solution like this.

    i calculated the overall capacitance the output node experienced.
    i put the same as load capacitance and test it as individual opamp.

    i hope the procedure is correct

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 1
    Points: 4,135, Level: 15

    Join Date
    Apr 2004
    Location
    shanghai,china
    Posts
    453
    Helped
    28 / 28
    Points
    4,135
    Level
    15

    Re: Opamp characterization at system level

    Quote Originally Posted by analog_prodigy
    hi friends,

    i got one problem.
    i have already one opamp, for which i checked gain, ugb and phase margin. everything is fine because i gave 1V ac signal to one input (DC bias voltage at two inputs) and did ac analysis.
    now my opamp is used in a big circuit having different blocks like bias, loads, and extra circuitry.
    can i do ac analysis of my opamp at this system level? if so, where should i give my ac 1V input ( because the two input nodes are already connected to somewhere in the big circuit)

    Please help me in this regard.

    Added after 3 hours 40 minutes:

    Hi, i got the solution like this.

    i calculated the overall capacitance the output node experienced.
    i put the same as load capacitance and test it as individual opamp.

    i hope the procedure is correct

    after the ac analysis, you can use the transient analysis to double check it.

    the overall capacitance the output node experienced can be used as the load.

    but the transient analysis is still neccessary.



    •   AltAdvertisement

        
       

  3. #3
    Member level 5
    Points: 2,044, Level: 10

    Join Date
    Aug 2005
    Posts
    81
    Helped
    5 / 5
    Points
    2,044
    Level
    10

    Opamp characterization at system level

    you can cut the loop at your op input then add a great r(about 1GOhm) connect them, then add a great c(about 1F) to the op input, give your ac1v to the other side of the cap, then you can run your ac simulation



    •   AltAdvertisement

        
       

  4. #4
    Member level 1
    Points: 1,714, Level: 9

    Join Date
    Jun 2005
    Posts
    36
    Helped
    7 / 7
    Points
    1,714
    Level
    9

    Opamp characterization at system level

    I think put all capacitors at the output is sufficient but not necessary in many cases.



--[[ ]]--