Uncertainty with a log function

[sprinks13]

Homework Statement

Im trying to find the uncertainty for a log function. I have that

f = 20log(V_in/V_out)

and i want to know (delta)f

Homework Equations

f = 20log(V_in/V_out)

The Attempt at a Solution

I was going to do it the calculus way:

f = 20log(V_in) -20log(V_out)

df = 20*f*[(dV_in/V_in) - (dV_out/V_out)]

but this doesn't seem right to me. I've never seen subtraction in an error calculation before. And I am not sure if this is a relative error or a percent error. I want a relative error (with units), the only problem is that my function has no units, soI'm not sure what I should do in that case (if that makes sense).

Is there another simple formula for log function errors?

 

[Mapes]

http://en.wikipedia.org/wiki/Propagation_of_uncertainty#Example_formulas" has a pretty good explanation.