OPA211: Question about Vib calculating in Precision Lab Video

Hello,

I think you'll find the two calculations yield the same result.  The first calculation is output referred, and second one is input referred.  If you multiply the input referred results by the noise gain: 1+RF/RI  (RF+RI) / RI) you'll find you get the same result referred to the output.

So, to directly calculate the error at the op amp output due to the input bias current you can use the first equation:  Vout_IB = Ib*RF

However, it's often desired to combine the errors from Ib, Vos, Vos_Drift, CMRR, PSRR, etc. together.  In this case since the other errors are already referred to the input it makes sense to calculate the error from input bias current referred to the input.  In this case you can use the 2nd equation:  Vout_Ib_RTI = RF*RI/(RF+RI) * Ib.  Notice if you multiply this input referred value by the noise gain (RF+RI / RI) then you'll get the same result as shown above: Vout_IB = Ib*RF.

Hi Collin,

Thanks for patient explain, very clear.

Another question came up when I review video 2.2. I think the error due to Vos of two cascaded OPA211 shall be (Vos1*G1±Vos2)*G2, rather than simply G1*G2*Vos as the lecture told.

How do you think? And there may be similar problem about the error due to Ib of two cascaded op amps.

Thanks.

Hello,

You're correct, that would be the more exact answer. The reason that term was left off was just for simplicity since it doesn't contribute very much to the final answer. Since the two op amps are the same, the offset voltages are roughly the same. Since the noise-gain of the first op amp is 101, the Vos1*G1 value will be roughly 101 times larger than the Vos2 value. Therefore the Vos2 term was omitted because it contributes to only (1/101^2)th of the final output voltage.