# [volt-nuts] Best way to measure micro Ohms

John Phillips john.phillips0 at gmail.com
Fri Sep 29 02:37:01 EDT 2017

```Ed,
It is not sinusoidal AC. It is a DC voltage which is allowed to settle
before it is measured, It is then measured a few times to get an average.
The excitation current is then turned off to allow the resistors to cool
before the DC voltage is reversed and the process is continued. The key
factor is that all measurements are taken in a stable state.
There is a difference between reversing DC and AC.

Years ago I ask a guy how he got resistance values plotted to as many
digits as he did on a set of bridge resistors.
He told me he used an HP 3457A in statistics mode and measured the voltage
drop on the sent then revised the current exciting the bridge and took the
same measurements again.
He then averaged the values together to get the ratios of the 2 bridge
arms. He called it reversing DC. The first time I remember hearing the
term.

AC current sources are inherently less stable than DC current sources and
AC measurement system are inherently less accurate than DC system. If you
use AC rather than DC you are going to give up at least one and maybe 2
least significant digits. You would not be having this discussion if you
were willing to throw that much accuracy away. The standard resistor used
in the test typically has a DC documented value as well as a documented
temperature coefficient and an undocumented AC value with an undocumented
frequency coefficient and undocumented AC temperature coefficient.

True AC measurements are not nearly as accurate as DC measurements. HP
3458A has a mode for measuring AC where it takes a bunch of DC measurements
at high speed and calculates the root mean square. It works best on
repetitive low noise sinusoidal signals. For the correct signal, it is the
most accurate AC mode. For other signals, there is an AC to DC converter
that feeds a DC measurement system. In that case, the vast majority of
error comes from the converter. If the converter method gives you a tighter
range of readings than the DC method you know you have dirty
signal/distorted when compared to a sine wave or not very repetitive. It
works a lot like a sampling O-scope. Both of which depend on a solid
trigger signal.

On Thu, Sep 28, 2017 at 11:17 AM, ed breya <eb at telight.com> wrote:

> Hmm. Alternating the direction of the current repeatedly and processing
> the results - sure seems like that is fundamentally an AC measurement too,
> despite using DC measurement equipment.
>
> Ed
>
>
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--

*John Phillips*
```