Mike -
        It depends upon what your signal looks like. If it is purely
     sinusoidal, then you can just scale the amplitude. In fact, so long
     as you know the wave shape, you can do this. If you don't know the
     wave shape, there are a number of solutions. Jim Williams did a
     funky RMS to DC converter chip that is thermally based. Check out
     Linear Technology's Web site.
        My favorite tome on this subject (which I haven't seen since grad
     school days) was the manual for HP's 8-digit DMM. They had more ways
     of figuring out RMS than you can shake a stick at. I seem to remember
     a particularly novel random sampling method which worked in a
     statistical fashion. For reasonably stationary signals (in the
     statistical sense), I would imagine this to be the easiest to
     implement on a PIC...

     --- phd


______________________________ Reply Separator _________________________________
Subject: RMS - slightly off topic
Author:  pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU> at
WDI-INTERNET
Date:    8/2/97 11:12 AM


Hi all,

Sorry for slightly off topic - but it might be of use to others also.

I need to read 4 voltages at 60Hz and I need the true RMS value. I know
that I could use a true RMS detector/converter by Analog devices, but these
are just too expensive - any others around in small DIP packages ?

OR
Is there some 'appropriate arrangement' of LM324 or similar opamps that
might provide this conversion for frequencies around 50 to 60Hz ?

Any ideas ?

Rdgs

mike
Perth, Western Australia