[sdiy] The TL072 , part 2

[Ian Fritz]

At 04:10 PM 2/26/2009, David G. Dixon wrote:
>Real op-amps require
>some small non-zero differential voltage to give zero output.  This small
>input voltage is Vio.  This is typically important for high-gain dc
>applications.


True as far as it goes.  The equation is
Vo_error = Voff ( R2 / R1 + 1 )

Look what happens when the gain is small.  The offset appears directly at 
the output!
Vo_error ~= Voff ( 0 + 1 )
So if you are attenuating to a small signal level, then the offset voltage 
can be relatively large wrt the signal.  This happens in the input summer 
of an expo converter if you use a 100k input resistor and a 2k tempco as 
the feedback resistor.  This is why I always use an OPA2227 or OP27 at this 
point.  An offset there leads directly to a temperature drift!

http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm
http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm

   Ian