Measuring principle of AC & DC voltages with digital multimeters

Hi guys,
I have been scavenging the internet for articles on measurement principle of true rms value of AC by a digital multimeter & so for DC voltage.

Can any help me understand the principle of measurement or direct me to a good article..


Pls help!!

All I can say is DMM averages what is being measured to give you a reading. Seen a lot of 24VDC encoders changed because "it's only putting out 12 volts".