how to caculate a standard deviation?

[tz0003]

hi all,
i am reading waveforms from an oscilloscope. once the wave form is flat, the computer will ask one pulse generator to send out a pulse. to determine whether the waveform is flat, I use the standard deviation as an indicator: if the standard deviation exceeds 20% of the average, the computer will treat it as not flat.
I saw there seems to be an already-written VI for this. but I cann't find it. and I also wonder how long it will take to caculate standard deviation for 500 data points

thanks,
tao

[Jonnie_5]

The standard SD vi is under Functions->Mathematics->Probability. 500 points are calculated in less than a microsecond (faster than the VI profiler's resolution).