[SI-LIST] Re: Kelvin current sense resistor
- From: Heyfitch <heyfitch@xxxxxxxx>
- To: Joel Brown <joel@xxxxxxxxxx>
- Date: Sun, 28 Feb 2010 23:47:43 -0600
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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