No need to digitize. There are analog multiplier and squaring circuits.<p><a href="http:&#x2F;&#x2F;www.analog.com&#x2F;en&#x2F;products&#x2F;linear-products&#x2F;rms-to-dc-converters&#x2F;ad637.html" rel="nofollow">http:&#x2F;&#x2F;www.analog.com&#x2F;en&#x2F;products&#x2F;linear-products&#x2F;rms-to-dc-...</a>