Hi,
I have studied quite many schematics recently, but none of them could answer my question so far: How do multimeters do auto-ranging without relais and without blowing up their analog switch?
I have attached an example voltage divider as example. So if I apply 1kV at MEAS_IN, the divider would divide down to 1:20, which results in an input voltage of 50V at the 4051, which is certainly too much (and multimeters won't supply +.50V to Vcc/Vee).
I then thought: "Hey, why not add clamping diodes to each divider?". In theory, this was a good idea, however, I didn't realize that they clamp all the time, so if the clamping diodes at the 1:20 node clamps, I get the clamped output voltage divided down, not my input voltage.
I also have looked at several schematics for open source multimeters, including the OSHW Multimeter Project by stoney49 (which didn't show the input voltage divider at all) and the heise/c't Voltmeter for their lab (who use relais to switch inputs), so I couldn't answer this for now.
Any pointers in the right direction are appreciated.
Felicitus