How to test the input offset voltage of a OP Amp?

[ZengLei]

Hi all
I try to figure out some ways to get the input offset voltage of a OP Amp,so i search it in the internet,finally i found one as the pic below.

here:in the pic

Vos=input offset voltage
Ios= input offset current
Ib= input bias current

Then there is a formula in the right side of the pic,
But i think this formula is totally wrong~!
becasue it didn't take the Ios into consideration!
i think the Vout is not just made by the Vos,the Ios has very great contribution to the Vout,but the formula ignore the effect of the Ios to the Vout.~~~~~


So pls give me some advices about how to get the Vos of a OP correctly,and how to build a correct circuit to test the Vos of a OP Amp.

Thanks~~~~~~~~~~

 

[monnoliv]

In my opinion, the Ios contribution is negligible since the source resistors are 10[Ohm]. Ex AD8615: Ib and Ios < 600[pA] then that leads to 6 [nV] max, that far less then the 800[uV] max Vos specified.
Regards,

 

[laktronics]

Hi,
The test circuit is correct. The values of Rf and Rs are kept low at 10 K/10 ohms to reduce the effect of offset current in the measurement. You can see practically, that the offset current will only produce an output voltage equal to Rf x Ios, whereas the offset voltage will be amplified by the voltage gain of 1+ Rf/Rs = 1001.
For measurement of offset current you will similarly use a high feedback resistor and an equal resistor from + input to ground ( no resistor from -ve input to ground). The offset current in this case will be Rf(large) x Ios.
Regards,
Laktronics

 

[A.Anand Srinivasan]

the equipment needed to measure these is expensive... these values are too low... following the datasheet is a better idea...