Electronic – Why do multimeters have an aluminium foil at the back


I was wondering what is the purpose of aluminium foil at the back of some multimeters? I did a continuity test and found that it is connected to the ground (COM) terminal.

My initial guess was that it was some kind of shield but since it wasn't fully enclosed and multimeters don't work in that high RF range it didn't seem plausible. Also, it wasn't some kind of ground plane as it is only connected to the board at one point via the spring.

What is it exactly?

Strange foil

Best Answer

It's a shield, but to keep RF out, not in. The multimeter does not use high frequencies internally, but it does have high-impedance internal nodes that could be affected by external sources of RF.