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Parasitic capacitance of SMD resistors

A few days ago there was some discussion about parasitic capacitance of SMD resistors.

From the dimensions of a 1MOhm 1206 resistor, assuming all the field is in the alumin dielectric, and pretending it is otherwise a simple flat-plate capacitor, I found it should be in the 35fF ballpark.

Measuring one using my HP8753D network analyzer, I got

50fF. Not too bad an agreement for a five-minute lash-up.

That's a factor of 6 below John's results. You were right, Win, 300fF was way too high.

Jeroen Belleman

Reply to
Jeroen Belleman
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I calculated, roughly, 100 fF, based on the alumina between the end caps. But the resistance element effctively shunts sections of the alumina, and that varies with value and frequency, so maybe there's no single correct value for C.

My AADE meter reported a lot higher than my Boonton, numbers like 0.3 and 0.1 pF respectively. I think the AADE is also affected by the resistance itself.

How did you mount/fixture it?

John

Reply to
John Larkin

Hey, thanks for the measurement Jeron!

George H.

Reply to
George Herold

I mounted it in series between two SMA connectors. The NA S21 is basically a straight line with positive slope of 20dB/decade over several decades, characteristic of a pure fixed capacitance.

I'm at home now, and the NA is in the lab. I'll post a picture on Monday, if anyone wants to see the measurement.

Regards, Jeroen Belleman

Reply to
Jeroen

What was the frequency range? Any conjectured influence of the resistance element would happen at relatively low frequencies. Things should start to deviate from the ideal slope in the low MHz range, roughly where Xc approaches 1 meg.

John

Reply to
John Larkin

Agreed. I was looking well above that, in the 10 MHz to 1 GHz range. I'll have another look on Monday.

Jeroen Belleman

Reply to
Jeroen

o

You might try a few lower R values, (If it's not too much to ask.) It would be interesting, to see if you see anything like John.

George H.

Reply to
George Herold

I ran a SMD resistor on a network analyzer once for yucks. I don't recall the LCR model, but I do recall the L was virtually that of the electrical length of the resistor.

Reply to
miso

Here's another measurement.

Tek TDR/sampling head and fixture:

ftp://jjlarkin.lmi.net/0805_res_fix.JPG

This is the TDR (lower) and TDT (upper) with no resistor, just the gap in the CPW trace:

ftp://jjlarkin.lmi.net/0805_nores.JPG

And here's the TDT blip, the signal that passes through a 1M 0805 resistor:

ftp://jjlarkin.lmi.net/0805_res_TDT.JPG

If you do the math on that, it works out to 47 fF.

So the AADE meter, and maybe the Boonton, don't measure this capacitance very well. I still don't know if the resistance element changes C as a function of frequency, but maybe it doesn't.

John

Reply to
John Larkin

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