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question about calculating true rms on non-linear signals

**Psi**:

I have a microcontrollers adc channel connected to a current sensor, however this sensors output voltage is not linear to current. This isn't so much of a problem as i have the formula to correct the response of the sensor and produce accurate current values.

What i want to know is if i can sample and calculating the RMS value of the raw non-linear sensor output and then convert this rms value to rms current using the formula later on.

Does anyone know if that will work?

I know it will work if i use the formula first and then do the RMS calculations on the true current values but i want to know if it will work the other way around.

**ziq8tsi**:

--- Quote from: Psi on April 07, 2011, 11:32:18 am ---What i want to know is if i can sample and calculating the RMS value of the raw non-linear sensor output and then convert this rms value to rms current using the formula later on.

--- End quote ---

No, rms(f(x)) is not generally expressible as g(rms(x)) for any function g. If it was we could take f as sqrt, and then calculate rms in terms of the (absolute) mean.

You might be able to save work by moving some linear parts of the function out of the rms, or by simplifying the function at the expense of some accuracy. But probably your best chance, if the function is slow, is a lookup table on the adc output values.

**Psi**:

yeah, i suspected that might be the case

thanks for your help

**scrat**:

If you know the formula, you could integrate and calculate its sqrt analytically, then apply the obtained formula on the MCU.

**Mechatrommer**:

LMGTFY... RMS d' WIKI, check under the discrete section and formulation. its possible i think.

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