Hi All,
I'm working on the practice problems for TI precision labs : Opamp 2.1.
I'm a bit confused at the way the Vib is calculated between Problem 2 and Problem 3.
- In problem 2 solution, the Vib = Ib * Rf due to nodal analysis which makes sense to me.
- In Problem 3 solution, the Vib for U1(first stage) has an additional gain of Rf/R1 + 1. Shouldn't this calculation be the same as that of Problem 2?
Thank you for your help
Hello Huy,
Yes, you may, but the dominant error factors are from the voltage offset errors.
In problem 2: Vos=72.67uV and Vib=7.46pA, Vout=72.67uV*3 + 7.46pA*200k=219.5uV, which is agreed with simulation. In addition, the Ib bias current is only contributed 0.07% of total voltage errors of the op amp.
In problem 3 per the TINA simulation. U1 is OPA188 and U2 is OPA827, In the solution section in pdf, the U1 is OPA827 and U2 is OPA188 (the op amps are swapped in the solution section). The same conclusion" the dominant errors are contributed from Vos*Gain, not from Ib bias errors (~0.008%) in the simulation and calculation.
Regards,
Raymond
