Re: Measuring Large resistances

Fred Bartoli wrote:
Spehro Pefhany a écrit :
On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihatespam@xxxxxxxxxx>
wrote:

I am having problems trying to measure resistance values above
100MegaOhms. My Laboratory Ohmeter gives unstable values.
Is ther anyway apart from the usual dc bridges of getting accurate
resistance measurements.

Thanks guys.

Kreyen

100M isn't that high, well, depending on what kind of accuracy you're
looking for.
How much instability are you seeing? How much above 100M?
LMC6042 has 2fA (typical) input bias current and costs a couple
dollars one-off. A 100M 1% resistor with 50ppm/K tempco runs around five dollars. A 1G 1% resistor with similar tempco is maybe double that.
Most inexpensive DMMs on the lower ranges have very high input
impedance so the LMC buffer might not be necessary.



High value resistors are (depends on the model) somewhat unstable.
I currently have a batch of Dale 1G/1%.
They measure fine with 100V bias which is the datasheet measuring conditions. When measuring them at low voltage, they're all over the place, from +3ish% to +7ish%. Yep, not even grouped...
On the contrary I've some Caddok rated <0.02ppm/V!

A friend of mine worked at, IIRC, Vishay/sfernice on that specific high value resistors 'feature' and went auditing some of the 'production' lines (the quotes are his). He said the voltage dependency was mostly a prod issue. High value Rs is a very small niche market and as such, production inherited some highest tech tools, like hand lapping and the likes. Being labor intensive it was relocated in low labor cost countries, with poor buildings (he said, almost backyard :-) and less than ideal handling cleanliness.

That could explain a lot some of the strange behaviors...



One cute method is to build a current-controlled oscillator, with a CMOS op amp connected as a Schmitt trigger, a voltage reference, a small MOSFET for the reset, and a good integrating capacitor. When you stick a current into the capacitor, the output frequency gives you the current. (No, this isn't the most accurate possible VFC, but it works pretty well for ~1% accuracy or a bit better.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
.

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