[SI-LIST] Re: Current sense resistors and power integrity
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: "Ray Anderson" <ray.anderson@xxxxxxxxxx>
- Date: Fri, 22 Aug 2008 13:59:45 -0600
Ray,
the tempco of the PCB trace is 3900 ppm/deg C. That's 0.4% per degree.
Here's an Ohmite 3 milliohm that I recently tested in a high current pulsed
Wheatstone Bridge. CS3FR003E. It has a tolerance of 1% and a low tempco. The
downside is the size and the inductance of 25 nH or so. It's a 4-wire Kelvin
resistor.
Calibrating, or determining the value of such a resistor to better than 1%
takes some doing. Common bench DMM's don't have the accuracy (not enough
stimulus current). But these resistors can be calibrated by doing a
ratiometric measurement working down from a 10 ohm that the DMM can measure
accurately. For the 0.003 ohm one might go 10 ohms, 0.1 ohm, 0.003 ohm.
Here's a Stackpole 2512 SMD resistor that looks suitable. CSNL 2 0.003 1% R.
It too is 1% and has a tempco of just 50 ppm/deg C. With a careful layout
(don't run any current thru any portion of the 'Kelvin' traces) it will work
well and have a series L of 5 nH.
Dave Cuthbert
On Fri, Aug 22, 2008 at 1:20 PM, Ray Anderson <ray.anderson@xxxxxxxxxx>wrote:
> Dave-
>
> Where are you getting your 3mOhm 1206 resistors ?? :)
>
> If he is measuring current, he'd probably want pretty tight R tolerance
> (say < 2%). Perhaps a carefully design trace section whose R is
> nominally 3 mOhms would be the way to go.....
>
> -Ray
> Xilinx Inc.
>
>
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of DAVID CUTHBERT
> > Sent: Friday, August 22, 2008 12:11 PM
> > To: Mikhail Matusov
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Current sense resistors and power integrity
> >
> > If the resistor feeds a power plane (even a relatively small one) that
> is
> > decoupled with the correct capacitors this can work well. Without the
> > power
> > plane the resistor (say it's 1206 SMD) will add 2 nH in series with
> the
> > decoupling capacitor(s). All of this can be easily modeled in SPICE,
> > including the power plane.
> > Dave Cuthbert
> >
> > On Fri, Aug 22, 2008 at 12:51 PM, Mikhail Matusov
> > <matusov@xxxxxxxxxxxx>wrote:
> >
> > > Hi all,
> > >
> > > Is that a really bad idea to insert a small current sensing
> resistor,
> > e.g.
> > > 3
> > > mOhm, followed by a bulk capacitor in each power rail of a let's say
> a
> > DSP
> > > or a FPGA chip to be able to measure its power consumption?
> > >
> > >
> > > Thanks,
> > > =======================
> > > Mikhail Matusov
> > > Senior Hardware Design Engineer
> > > Square Peg Communications
> > > Tel.: +1 (613) 271-0044 ext.231
> > > Fax: +1 (613) 271-3007
> > > http://www.squarepeg.ca
> > >
> > >
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