Opamp gain error - depends on what? only open loop gain!!!

Qns: Assume Vdd : 0 to 3.3V
opamp DC gain : 60db
opamp is connected in unity feedback.
positive terminal is connected to 2.15 V.
Upto which precision my opamp will settle? (take settling time as infinity)


Ans:
since my supply is 0 to 3.3 V. we can assume 1.65 as reference voltage.
now at worst case;

compute 2.15 - 1.65 ==> 0.5 V
worst opamp error is : 0.5 / (DC gain) ==> 500uV

What is important in this to note??

in all text books they will assume this reference as 0 V. so they will simply tell that opamp gain error is given by;
% error = Adc /(1 + Adc*B) ; B - feedback factor.
using the above formula; % error = 999.0 uV ==> not exactly correct.
We can't use this formula blindly. We need to know initial operating point of opamp. worst case we will assume mid of supply voltage as reference voltage.


If there is any misunderstanding correct me.

I don't know how you get 990 uV. And a % error doesn't have a volt unit, I think.

The consideration about gain error reference point is basically correct. In practice, everybody ignores it as long as the accidental offset voltage will be larger than the contribution of different reference points. But an exact measurement setup definition should include a reference point. Classically it's 0 V for dual supply OPs.

jebaspaul said:

opamp gain error is given by;
% error = Adc /(1 + Adc*B) ; B - feedback factor.

Click to expand...

Please, can you verify/explain this gain error formula?

Additional question: How do you define "opamp gain error"?