[SI-LIST] Re: Kelvin current sense resistor
- From: DAVID CUTHBERT <telegrapher9@xxxxxxxxx>
- To: Heyfitch <heyfitch@xxxxxxxx>
- Date: Mon, 1 Mar 2010 06:57:38 -0700
It takes current to accurately measure a 0.001 ohm resistor. Assuming 10
uV RMS noise a 1 amp source is needed to keep the noise induced error to
1%.
Better yet a 10 amp source and a 0.01 or 0.1 ohm known reference resistor is
the thing to use.
Take the specs of your measurement equipment and run an error analysis to
see if it's up to the task. A 4:1 TUR (Test Uncertainty Ratio) or better is
the target.
Dave Cuthbert
On Sun, Feb 28, 2010 at 10:47 PM, Heyfitch <heyfitch@xxxxxxxx> wrote:
> Joel,
> I once wanted to verify that the current-sense 2-terminal resistor on a
> DIMM
> riser card was indeed 5mOhm+-1% as advertised. To do that, I used Agilent's
> nanoVoltmeter/micro-Ohm-meter in the Kelvin configuration (and a colleague
> to do the soldering for me :-) It turn out that making such a measurement
> was no small feat. The first thing I learned that it was impossible to
> calibrate to zero in a repeatable manner while holding the probes with your
> hands. A contact resistance is very important. You need a firm fixture.
> Once
> you make the fixture, the finite size of the terminals becomes a problem:
> it
> is difficult to bring together all terminal into one point to set zero.
> The
> systematic error of such measurement was very sensitive to the size of a
> solder ball that connected the 2 pins into something like a 'H' to connect
> the 4 terminal. The solder ball was the horizontal bar of that 'H'.
> Empirically, I found that it had to be of some minimal size, which lowered
> its resistance, thus pushing the terminal farther apart. After much ado, I
> was able to get repeatable measurements that seemed to make sense. Anyhow,
> to make a long story short, this is better approached by trial and error
> than calculations, I think.
> Probably unnecessary, but there is a good app note from Agilent
> http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf. I once found it
> useful when doing wafer-level measurement of the nmos varactor C-V curve at
> baseband.
> - Vadim
>
> On Thu, Feb 25, 2010 at 4:35 PM, Joel Brown <joel@xxxxxxxxxx> wrote:
>
> > Sorry if this is not considered signal integrity related.
> > I am using a 0.001 (1 milli-ohm) resistor to sense up to 60A of current.
> >
> > They make 4 terminal Kelvin resistors so that the voltage drop created by
> > the current flowing through the PCB to resistor connection do not cause
> an
> > error in the measurement.
> >
> > I can also do a similar thing with a 2 terminal resistor by making two
> pads
> > on each side, one large one for the current and a small one for the
> sense.
> >
> > However when I calculate the resistance of the solder connection, using a
> > resistivity of 1.21 E-7 ohm.m, a pad size of 2.54 x 5.46 mm and a joint
> > thickness of 10 mils,
> >
> > I get a resistance of 2 micro-ohms, resulting in an error of 0.4% This
> > would
> > say I don't really need a 4 terminal connection and I could even
> calibrate
> > out the error. Is there something I am missing?
> >
> >
> >
> > Joel Brown
> >
> > Chief Electrical Design Engineer
> >
> > Z Microsystems, Inc.
> >
> > 9820 Summers Ridge Rd.
> >
> > San Diego, CA 92121
> >
> > Tel: 858-831-7011
> >
> > Fax: 858-831-7001
> >
> > <mailto:joel@xxxxxxxxxx> joel@xxxxxxxxxx
> >
> > http://www.zmicro.com
> >
> >
> >
> >
> >
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