# 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.

coolecho

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:

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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