Multimeter input protection redux

[Marco]

As I said in the #373 comment thread the original video left out a fair bit of the input protection circuitry, among which the use by Fluke of BJTs as zeners for final input protection of the ADCs which would certainly have been interesting enough to mention.

I'm also rather interested in how higher digit CAT rated multimeters (such as Metrahit 30m) handle things, doesn't the tempco of the PTC at what I would guess is around 10000 ppm negative temperature coefficient (initially, before it goes up) start to affect precision? A quick look at some older service manuals for the fluke 8505A 6.5 digit bench meter showed it had "CAT forget about it" protection.