[volt-nuts] Best way to measure micro Ohms

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Wed Sep 27 18:51:25 EDT 2017

On 27 September 2017 at 22:22, ed breya <eb at telight.com> wrote:

> I just noticed this discussion recently, so I'm late to the party, but
> that never stops me from adding my one-cent's worth.
> David, regardless of the aluminum and other material issues, I think your
> initial idea of using a lock-in analyzer is definitely the way to go. I'm
> very fond of LIAs, although I seldom need or use them, so my opinion is
> somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204, an
> SR830 and SR850.

Yes, me too. I have used a number over the years, the nicest of which was
the Stanford Research SR830. The EG&G 7260 I own has rather user-hostile
interface. There's not even a power switch on the front, and I'm not sure
if there's even on on the rear.

The following link might interest others who don't know about these


> If you use an audio power amplifier for driving the experiment, you can
> rig it up so that the LIA can be used to measure the drive current as well
> as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so
> you put a few ohms in series with the output, then from there into a
> precision one-ohm sampling resistor, then into the RUT, forming a voltage
> divider. The RUT is expected to be in the micro-ohm region, which is many
> thousands of times smaller than the sampling R, so its tiny voltage drop
> will be negligible, allowing the sample voltage to be a good representation
> of the test current. You could also just treat the whole thing as a voltage
> divider and calculate the "exact" results.

I did purchase a Radio Shack 100 W Public address  I will set this up

> This could be fun and interesting. There are plenty of pieces and
> variables involved to experiment with to optimize the measurement, and lots
> of other tricks available to enhance it if necessary.
> Ed

Yes, with lock-in amplifiers there are an almost infinite set of
possibilities of how to do the actual measurement. I don't know if the
reference output is a sine wave or square wave on this unit. I have the
option of using a Stanford Research DS345 30 MHz function generator to
generate a sine wave if needed. There are almost a million things that one
can change.


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