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| Precise Offset Compensation Ohm Measurements and Validation of DMMs |
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| Dr. Frank:
--- Quote from: HighVoltage on January 02, 2016, 11:28:24 am --- What instrument did you use to get the impedance graphs over frequency? --- End quote --- I used an HP4149A impedance/gain-phase analyzer. |
| Vgkid:
Thank you for making these post Dr. Frank, they have been very informative. Of the 2 bench meters I own only one of them is capable of ocOhms. My 3456 does this, but when coupled with 100nplc it is a very slow process, only working as a 4-wire keasurement until several 10's of Kohm. |
| Kleinstein:
@HighVoltage: The change in inductance is a different thing: its just the not fully closed flield that is still incluenced by metal in some distance. With a core that is not alsolutely free from magentistriction you also have to care a lot about mechnical force on the core - this can also change a lot - so may need to keep it leveled to keep gravitational effects at least constant. I was allways wondering how they can measure high impedances (not just 100 K) so fast - so we see that we have to be carefull and it does not really work well so fast. Interesting that with insulators it's not just leakage currents and the capacitance but also dielectric absorbtion. So we can't just rely on the RC time constant beeing short - capacitance can still get us. |
| robrenz:
Excellent post. I wonder what aspects of this relate to offset compensated low ohms measurements on the Lom510A which uses 1A drive current for 166ms out of each 1 sec measurement interval. Obviously a problem on resistors with any significant inductance. |
| Kleinstein:
The inductance with low ohms resistors might in principle have similar problems, but usually you avoid a ferromagentic core in a parasitice inductor, not normally no relaxation to cause problems. Only the resitor material might be just on the border of getting ferromagnetic - so some magneto resitance might happen. The supprising thing with the parasitic capacitance is that the time constant is notjust given by RC, as one normally learns in school. The tricky part here was that there is also a much slower dielectric relaxation that can cause trouble. The simple RC current decays exponetially, so one can wait for something like 12 time constants and we are OK to the PPM level. But with an internal relaxation, a small fraction of the capacative current come much later, with time constant set by the material, not the RC combinaton. Even worse, the worst case relaxation current goes down quite slow. So just waiting may not be a solution to the ppm level. |
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