Parasitic capacitance of SMD resistors

[Jeroen Belleman]

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

 

[Jeroen]

A few days ago there was some discussion about parasitic
capacitance of SMD resistors.
[...]
Measuring one using my HP8753D network analyzer, I got
50fF. Not too bad an agreement for a five-minute lash-up.

I calculated, roughly, 100 fF, based on the alumina between the end
caps. But the resistance element effectively 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?

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

 

[Jeroen]

On Fri, 03 Dec 2010 17:12:04 +0100, Jeroen Belleman

A few days ago there was some discussion about parasitic
capacitance of SMD resistors.
[...]
Measuring one using my HP8753D network analyzer, I got
50fF. Not too bad an agreement for a five-minute lash-up.

I calculated, roughly, 100 fF, based on the alumina between the end
caps. But the resistance element effectively 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?

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.

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

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

Jeroen Belleman