How do true RMS digital multi meters work?

[alanb]

My guess would be that for each cycle there would be a series of measurements that would be squared, averaged and then square rooted. Am I correct?

If so on a decent meter how many samples per cycle would be taken on say a 50Hz input?

[ciccio]

Most meters use a dedicated IC such as Analog Devices' AD536A or AD736.
Other use analog multipliers.
If the instrument is DSP based, then the DSP can do the mathematics, and compute the RSM value directly starting from the A-D sampled values.

[Wytnucls]

Here is the PDF datasheet for the AD636, with a fair amount of information about the principle of operation:

[alanb]

Thank you ciccio and Wytnucls for the info.

[blackdog]

Hi,

Linear Technology app. note...
www.linear.com/docs/24931
www.linear.com/docs/4117

Kind regarts,
Blackdog

[alanb]

Linear Technology app. note...
www.linear.com/docs/24931

The last page of this application note, the good the bad and the ugly..... is fascinating.

Thanks blackdog.