Calculating RMS of a given output

Bobb Fwed

I want to calculate the RMS values of a sine wave output. For example, how would we calculate the RMS value of a perfect output from a Propeller IO (3.3Vpp, ~40-Ohm),

I found this formula for sine waves: Vpp / 2 / 1.414 = RMS (Vpp / 2.828 = RMS) (of course wikipedia has a completely different formula -- someone may be able to explain the difference).
But how do we account for the resistance/impedance of the output.

I'm trying to determine how much power I can output from an OpAmp with 13Vpp and 38-ohm. I want to compare that value to other OpAmps and ultimately match it up with a transformer I'm using.

jmg

Bobb Fwed wrote: »

I found this formula for sine waves: Vpp / 2 / 1.414 = RMS (Vpp / 2.828 = RMS) (of course wikipedia has a completely different formula -- someone may be able to explain the difference).

There is no difference. Wiki says a/sqrt(2), where a is the amplitude, or half the peak-peak value, so is the same as you mention.

Bobb Fwed wrote: »

But how do we account for the resistance/impedance of the output.

I'm trying to determine how much power I can output from an OpAmp with 13Vpp and 38-ohm. I want to compare that value to other OpAmps and ultimately match it up with a transformer I'm using.

Assuming those numbers as meaning opamp can drive 13vpp into a 38 ohm load, then your RMS voltage is 4.596V - is the same power, as 4.596 V DC
Power into a load is V * (V/R) so into 38 ohms, you have ~ 556mW

kwinn

You need to take the current the op amp can supply into consideration. If the op amp can only supply a maximum of 100mA the maximum voltage on the 38 ohm resistor will never be more than +/- 3.8V.