Cap FAQ Additions (long post)

[Mark Amundson]

JBaker wrote:
> 
> Well, I asked for it and I got it in a big way.  -Jeff

I have an addition to the Capacitor FAQ I would like to share. This data
comes from Analog Devices' "Analoq Dialog" 30-2 from 1996. The data is
tabular and concentrates on dielectric absorption (DA)criteria. This is
a
photocopied page I keep within eyeshot of my desk/workbench and should
be
helpful to most designers of circuitry.

TYPE		Typ. DA		Advantages		Disadvantages
----------   ---------------    --------------------   
------------------

NPO Ceramic	<0.1%		Small case size		DA generally low
				Inexpensive		Limited to small values
				Good stability
				Wide range of values
				Many vendors
				Low inductance

Polystyrene	.001 to .02%	Inexpensive		Damaged by temps >+85C
				Low DA available	Large case size
				Wide range of values	High inductance
				Good stability

Polypropylene	.001 to .02%	Inexpensive		Damaged by temps >+105C
				Low DA available	Large case size
				Wide range of values	High inductance

Teflon		.003 to .02%	Low DA available	Relatively expensive
				Good stability		Large size
				Operational above +125C	High inductance
				Wide range of values

MOS (on chip)	.01%		Good DA			Limited availability
				Small			Only small values
				Operational above +125C
				Low inductance

Polycarbonate	.1%		Good stability		Large size
				Low cost		DA limits to 8-bit apps
				Wide temp range		High inductance

Polyester	.3 to .5%	Moderate stability	Large size
(Mylar)				Low cost		DA limits to 8-bit apps
				Wide temp range		High inductance
				Low inductance(stacked film)

Monolithic	>.2%		Low inductance		Poor stability
Ceramic				Wide range of values	Poor DA
(High K)						High volt coeff.

Mica		>.003%		Low loss at HF		Quite large
				Low inductance		Low values (<10nf)
				Very stable		Expensive
				Avail. in 1% values or better

Aluminum	High		Large values		High leakage
Electrolytic			High currents		Usually polarized
				High voltages		Poor stability
				Small size		Inductive

Tantalum	High		Small size		Quite high leakage
Electrolytic			Large values		Usually polarized
				Medium inductance	Expensive
							Poor stability
							Poor accuracy

----------------------------------------------------------------------------

FAQ notes:

When describing Bypass caps, AF coupling caps, VCF caps, VCO caps, etc.,
please caveot the mentioning of specific values with a mention of their
dependance on load impedance ("... depending on load or input
impedance.").

Regarding microphonic capacitor effects; ceramic caps and some plastic
caps
may have some vibration induced voltages due to "piezo-electric"
effects.
Some ceramic dielectrics are worse than others regarding this effect.

Bypass cap recommendations may vary from designer to designer. The
famous 0.1uf
ceramic is good choice audio supply bypassing, but 0.01uf may be a
little better
if you mix higher speed circuits like logic on the same supplies. Do not
forget
to retain 10uf or greater aluminum or tantalum electrolytics on circuits
as well.
They may not as location sensitive as the ceramics but act as local
energy 
storage reserviors when the DC supply is more than a few inches away.

Mark Amundson P.E.
Principal RF and Analog Development Engineer
Guidant Corporation