20-08-2017, 06:51 AM
I'm wondering how DMMs achieve a very high input impedance on the mV range while being a standard 10M on all other voltage ranges. I suppose that the basic meter has a very high input impedance from a FET input opamp. Higher voltage ranges need multiplier resistors and so must have a defined impedance. I also wonder how well protected the mV range is against overvoltage. A thousand volts, or the maximum allowed for the meter, applied to any normal voltage or resistance range shouldn't bother the mater at all. Is the Hi Z mV input likely to suffer?
As Nick says, when you're dealing with picoamps, humidity and cleanliness are everything.
As Nick says, when you're dealing with picoamps, humidity and cleanliness are everything.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
