Find min/max limits on an opamp circuit

Discussion in 'General Electronics Chat' started by coolecho, Oct 13, 2016.

  1. coolecho

    Thread Starter New Member

    Oct 12, 2016
    2
    0
    Hello,

    I need to determine the min and max output limits on the output of this opamp circuit. The resistors are 1%, and caps are 10%. VREF_A2D is accurate at 2.5 volts.

    Thanks in advance,
    coolecho
    upload_2016-10-13_8-23-7.png
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Welcome to AAC!

    You have a voltage follower with a divider on the input. Is this a trick question?

    Does the reference voltage vary?

    What is the purpose of R130?
     
  3. coolecho

    Thread Starter New Member

    Oct 12, 2016
    2
    0
    Hi Dennis,

    I'm not a designer so I don't know what R130 is for. The reference voltage VREF_A2D is very accurate 2.5 volts:

    upload_2016-10-13_8-55-45.png

    Thanks!
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    0.623V +/- 1% output at pin 1.
     
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  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    The intent of R130 is to improve the high frequency decoupling performance of C99. The actual effect is dependent on the load current characteristics of whatever the opamp is driving.

    If the reference voltage is assumed to be theoretically perfect and the opamp input impedance is assumed to be theoretically infinite, then you can calculate the voltage at pin 3 for all 4 cases of the +/-1% resistors. To each of those four voltages, add and subtract the opamp's max input offset voltage error. Now you have 8 voltages. Somewhere in there are the max and min output voltages, and that probably is what is required.

    If you want to go extra nuts, use the opamp's max input bias current to recalculate the four pin 3 node voltages...

    ak
     
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