AW: [sdiy] precision/matched capacitors??

[Czech Martin]

I have never heard or seen matched cap arrays, outside from
ICs, where they are very common. These integrated caps use
SiO2 or other "glas" isolation dielectrica, therefore they have
tight matching and also very little "hysteresis".
If you have a very good cap , like polystyrol, and watch the voltage
with a very high impedance amplifier, you will see this hysteretic
behaviour. From this point of view it is not sensible to sell
anything better then 1% tolerance.

Resistors OTOH can be very precise, 0.1% are of the shelf items,
and there are much better wire wounds. The tempco of these
can be very low, also. I think the best tempco you can get
for polymere caps is +-100ppm, note that the sign does not
seem to be determined at all. I think this is because of
the molecular structure of the dielectricum.
So, also the tempco point of view makes promises of better then 1%
accuracy
for usuall rolled caps  pointless.

If you need a very precise cap, use a air gap package rotary capacitor.
Or build your own air gap sandwhich.
Perhaps you can find such a thing @ HAM flee markets, or in very
old radios. Since these deviced use air as insulator, there is no
hysteresis effect, also the epsilon of air is very constant (given
that you work in dry climate). The insulation is near to perfect,
no leakage. Only the very best amplifiers can match this.
The capacitance is of course quite low.
(btw. such a device allows for differentiating a simple
tri wave into a precision square wave current, sub nA.)

So, comming back to your initial question, it seems that you look
for something impossible in terms of macroscopic rolled caps.

m.c.

> -----Ursprüngliche Nachricht-----
> Von: media at mail1.nai.net [mailto:media at mail1.nai.net]
> Gesendet: Mittwoch, 31. Oktober 2001 17:55
> An: synth-diy at dropmix.xs4all.nl
> Betreff: [sdiy] precision/matched capacitors??
> 
> 
> 
> You can buy precision resistors, you can buy matched 
> semicinductors, but
> can you buy precision caps, or number of matched caps built 
> into the same
> package??
> 
> THANX!!
> 
> 
>