[SI-LIST] Re: capacitance and inductance values of SMT resistors?

There really isn't just one answer for any given resistor, because it
depends on how it is mounted.  The standard face-up configuration creates
an inverted-U shaped path, which is a tiny half turn inductor.  RF
engineers using standard resistors in the signal path sometimes resort to
upside down mounting to minimize that.  The shunt capacitance is
determined by the mounting pads, the surface areas of metallization on
the part and their separation, plus the Dk of the resistor substrate (~10
for alumina), the air around the part, the Dk of the PWB it is mounted
on, and a small contribution from stray capacitance to nearby components.

Because of mounting dependency, the best bet for an accurate number is to
make a small, representative PCB and mount the highest value resistor
available in your choice of style (1 to 10 Megohms typical) and measure
the capacitance.  Then do the same with the lowest available value and
measure the inductance.  You'll also want to measure the capacitance with
no part installed and inductance with the gap shorted by a flat strap so
that you can compensate for the board contribution.

There is also a way using the marker features of a TDR such as the
Agilent 54753/4, but that is probably more than you are asking for here.

Orin Laney

On Wed, 12 Nov 2008 11:21:52 -0800 "Bowden, Ivor"
<ibowden@xxxxxxxxxxxxxxxxx> writes:
> Thanks Ray, that's what I meant, shunt capacitance, in parallel with 
> the resistance, I apologize for the mis-nomenclature. I don't need 
> exact values, just want to compare smaller and larger SMT packages. 
> -Ivor
>  
> 
> -----Original Message-----
> From: Ray Anderson [mailto:ray.anderson@xxxxxxxxxx] 
> Sent: Wednesday, November 12, 2008 11:15 AM
> To: Bowden, Ivor; si-list@xxxxxxxxxxxxx
> Cc: Ray Anderson
> Subject: RE: [SI-LIST] capacitance and inductance values of SMT 
> resistors?
> 
>  
> 
> Series capacitance in a resistor ?? Not unless it is open. 
> 
>  
> 
> The resistor would have shunt capacitance across the resistor as 
> well as
> 
> series inductance.
> 
>  
> 
> Don't have any numbers handy at the moment...
> 
>  
> 
> -Ray Anderson
> 
> Xilinx Inc.
> 
>  
> 
> > -----Original Message-----
> 
> > From: si-list-bounce@xxxxxxxxxxxxx
> 
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> 
> > On Behalf Of Bowden, Ivor
> 
> > Sent: Wednesday, November 12, 2008 11:03 AM
> 
> > To: si-list@xxxxxxxxxxxxx
> 
> > Subject: [SI-LIST] capacitance and inductance values of SMT 
> resistors?
> 
> > 
> 
> > Hi SI Experts,
> 
> > 
> 
> > 
> 
> > May I please have quick comment on the comparative capacitance and
> 
> > inductance of SMT resistor parts based on package (0402, 0603, 
> 0805,
> 
> 1206)
> 
> > size? While I know parasitic capacitance and series inductance 
> should
> 
> be
> 
> > lower for the smaller sizes, I'm not sure about series 
> capacitance.
> 
> Any
> 
> > links to manufactures' datasheets or test cases with this 
> information
> 
> > appreciated.
> 
> > 
> 
> > 
> 
> > 
> 
> > Thanks,
> 
> > 
> 
> > 
> 
> > 
> 
> > Ivor Bowden
> 
> > 
> 
>  
> 
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