Re: DVM - VTVM - etc

[Stan Dodds <dodds@RICE.EDU>]

Michael Edmiston notes

> First, not all DVM are equal.  A reasonably high quality DVM will have
> an input resistance (for DC voltage ranges) of 10 megohms.  But less
> expensive meters only have an input resistance of 1 megohm. ......
> ..... Solid-state meters that are more
> equivalent to VTVM would be FET-input or MOSFET-input meters.

True, and some inexpensive meters do have MOSFET inputs. The HP model
E2373A multimeters we use are specified as 10 Megohms on their higher
ranges, but claim >1000 Meg on the 300 mV range. My own measurements
suggest nearly 10^12 ohm on that lowest range. For small voltages
these meters are usable as electrometers, and Francis Peterson (Iowa
State) developed a nice series of electrostatic experiments using DVMs
as electrometers. Given the variability of designs, checking the
actual specifications is the only reliable procedure.


> For high input impedance meters using active input components,
> regardless of whether they are vacuum or solid state. I am inclined to
> use the word... electrometer.

That would certainly be more accurate, especially as real vacuum tube
VTVMs did not necessarily have extremely high impedance. It was just
large compared to passive voltmeters. Finally, if one tolerates some
pedantry, it seems to me that a VTVM should have vacuum tubes, which
disqualifies all modern instruments.

Stan