# Summing amplifier results dont agree with theory

1. Jan 15, 2009

### thedemon13666

i had to do a study using a summing amplifier to see if the results i got agreed with theory, the answers i got was something like 9.62, the answer i should of got was 10, now this doesnt seem that far away, BUT the errors didnt agree at all, they only agreed within 4 standard errors, so i was looking for a reason why they may not have agreed.

My orginial thought was that:

The summing amplifiers summing equation made an assumption that the current inside of the op am was 0 and as a result wud overestimate the summed voltage.

I spoke to my instructer and he said it would be negligable.

So i dont really have a reason why

The theory equation is

$$Vout = -(\frac{Rf}{R1}V1 + \frac{Rf}{R2}V2)$$

the measured value i get is -(9.620±0.005)V
the expected value i get is -(10.00±0.01)V

as you can see they only agree within 4 standard errors of each other.

Im a bit stumped at a reason why

2. Jan 15, 2009

### turin

We don't have enough information to help you. I would immediately suspect that the resistor values introduce more error than you have anticipated. Did you actually measure their values? If not, and you truly have such small error, then you are using quite expensive resistors.