It is more along the lines of building a voltage divider with stable resistors. A !G ohm voltage divider would be more expensive to build with the same stability that you can get from 10M ohms. There are trade offs in all designs. The cost benefit ratio just is not there. If you really need high input impedance in a DC meter go differential. If you are looking for AC circuits then stray capacitance will really mess you up with a high impedance divider.
On Thu, Apr 10, 2014 at 12:27 PM, Andreas Jahn <andreas_-_j...@t-online.de>wrote: > Hello, > > perhaps its just to save the lifetime of the input range selection relays > to at least the warranty time. > Just a guess. > > With best regards > > Andreas > > Am 10.04.2014 19:58, schrieb Joel Setton: > > I think the 10 Meg default value became a de facto standard at the time >> of VTVMs (vacuum-tube volt meters), as a convenient value which reduced >> input circuit loading while remaining compatible with the grid current of >> the input triode. Designers of early solid-state voltmeters merely decided >> not to change a good thing. >> Just my $0.02 worth! >> >> Joel Setton >> >> >> On 10/04/2014 18:55, Steven J Banaska wrote: >> >>> As Tom said the 10M input impedance is used for the high voltage ranges >>> because it is a resistive divider (9.9M/100k) that can handle high >>> voltages >>> without much drift. Caddock THV or HVD are fairly common in precision >>> dmms. >>> >>> Typically you will find a high impedance (10G) path that can be used for >>> the ranges 10V and lower, but the 10M divider can be left connected and >>> will work for any voltage range by changing which side you measure. As >>> you >>> mentioned there can be an accuracy sacrifice when you have a high output >>> impedance from your source. I'm not sure why 10M is the default other >>> than >>> it may extend the life of the relay that switches the 10M divider in or >>> out. >>> >>> Steve >>> >>> >>> >> _______________________________________________ >> volt-nuts mailing list -- volt-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/volt-nuts >> and follow the instructions there. >> > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/volt-nuts > and follow the instructions there. > -- John Phillips _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
