FreeLists / si-list / [SI-LIST] Re: Model of Capacitors

[Geoff Stokes <gstokes@xxxxxxxxx>]

Hi Bart

My experience has been that X7R is lossy enough to be a problem in the
regions above say 50 MHz, but a 10nF or 100nF X7R 0805 is sometimes really
quite effective up to around 300MHz.  Would be interested to know if anyone
has investigated decoupling the higher frequencies with these, comparing
against NPO types.  If the ESR gives significant loss in a medium impedance
filter application, it's also going to limit admittance in the decoupling
application - the decoupler is really a low impedance filter, for part of
its frequency range at least.

And NPO varies - the Murata GRM series (green dielectric) seemed better up
to 500 MHz than some others, when I looked in detail on an impedance
analyzer about 5 years ago.  And Rohm were also good according to published
graphs.

Some manufacturers give quite good detail.  As an example, see

http://www.johansontechnology.com/products/csc/drf.php

Cheers
Geoff 

> -----Original Message-----
> From: Bart Bouma [mailto:bart.bouma@xxxxxxxxx]
> Sent: 01 September 2003 10:37
> To: fred@xxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Model of Capacitors
> 
> 
> Hi Fred,
> I already replied to Zhangkun (directly I believe) on his 
> original post 
> and explained the temp. and voltage dependency of high K-materials.
> Modeling should be done at the applied voltage.
> 
> You're refering to X7R or worse class materials when saying +80/-20% 
> tolerance. These type of capacitors are indeed used in 
> decoupling apps etc 
> and typically have large capacitance values (typ. 100nF and 
> above), but 
> are strongly dependent on voltage and temperature.
> But they offer high capacitance-values, low inductance and 
> (very-)low ESR, 
> so quite suitable for decoupling/filtering. Change in 
> capacitance is not 
> critical here.
> For resonant circuits however one should never use such X7R ceramic 
> capacitors (also don't forget the piezo effect and the dielectric 
> absorption/memory effect these exhibit).
> 
> Instead there are NP0/C0G type/class ceramic multilayer 
> capacitors: low 
> values (upto several nF) but very stable against temperature 
> and voltage 
> and showing very low dielectric absorption values.
> These are used for microwave circuits, such as oscillators, resonance 
> circuits, hi-speed S&H etc. and have tolerances down to 1%.
> 
> best regards, Bart
> 
> 
> 
> 
> 
> 
> "Fred Townsend" <fred@xxxxxxxxxxxxx>
> 
> 31-08-03 19:45
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 
> Please respond to fred
> 
>  
>         To:     <zhang_kun@xxxxxxxxxx>
>         cc:     <si-list@xxxxxxxxxxxxx>
>         Subject:        [SI-LIST] Re: Model of Capacitors
>     Category: 
> 
> 
> 
> > Dear all:
> > I have one question about decoupling effect of caps. When 
> the voltage on
> > the caps is changing, does the decouling effect of capacitor change?
> 
> Simple answer, Yes!  Lots!
> 
> This why anyone using a capacitor value as anything other than a first
> order approximation is fooling themselves.  There are many 
> effects based
> upon temperature, voltage, history, age, diaelectric, etc.
> 
> This is also why a bypass type capacitor should never, ever, 
> be used in a
> critical value circuit (such as resonate circuits).
> 
> If you have any doubt about the classification of a capacitor 
> take a look
> at its tolerance.  Many bypass caps have a tolerance of +80/-20%!
> 
> Fred Townsend
> >
> > For example, there is one capacitor, 10uF/100V. I measure 
> its model by
> > means of VNA at low voltage. When the capacitor is used to decouple
> > -48V, could the model of caps be used?
> >
> > Best Regards
> >
> > Zhangkun
> > 2003.08.29
> >
> >
> > 
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