Help, please!

 

Help, please!

You need to look at the actual calculation being done. Then there are formal methods given the operation. However, it helps to formulate an example first.

What if you want a gain of -1, and you use two 10 K resistors for Rf and Ri. What if Ri was 2% low at 9800 and Rf was 2% high at 10200? The gain would be -Rf/Ri=-10200/9800=1.0408, which is about 4% too high. So the tolerances nearly add in this case.

So, you need to look at the operation you are doing. If you add two resistors, then the tolerances are the same, but if you multiply or divide, then the tolerances approximately add (at least for small tolerances).