op amp RTI calculation

[houly]

Hello all,

I would want to know how I can calculate the RTI of an op amp ... I saw an AN that mention

RTI is equal to :

RTI=Vos + Ib+*R3 - Ib-*R1//R2

I'm not agree I have the opposite result for ib- and ib+ terms, for me RTI is

RTI=Vos - Ib+*R3 + Ib-*R1//R2 (I calculate the V+_ideal_input - V-_ideal_input).

https://www.analog.com/static/imported-files/tutorials/MT-039.pdf

another remark : in this application note, there is no Vos term ... !? is it normal ?

Are you agree with me ? or do I a mistake ?

regards

 

[erikl]

RTI=Vos + Ib+*R3 - Ib-*R1//R2
https://www.analog.com/static/imported-files/tutorials/MT-039.pdf

This depends on how the polarity of the offset voltage and the directions of the input currents are defined. I think the above equation is correct for the given schematic.

in this application note, there is no Vos term ... !? is it normal ?

Sure, the Vos term exists:
OFFSET (RTI ) = Vos + IB+•R3 – IB– • (R1•R2) / (R1 + R2)

d'accord?

 

[houly]

hello and thanks for your reply

I mean Voso not Vosi ! for the RTI we will have another term : Voso/G ...?

For your expression, I have the opposite because I use the RTI=V+ - V- of the ideal op amp not the oppposite.