Questions on drift and tolerance

[SmallWalrus]

Hi guys,

I've been parts shopping for a little project that I'm looking to build, and I got stumped by this. The build requires a very exact matched pair of capacitors giving as close a total capacitance as possible to 1020pf (within 0.5% tolerance).

I noticed that RS has some silver micas that are rated to 1% tolerance for 1000pf which I can then combine it with another small silver mica rated to 0.5% tolerance, and get my 1020pf.

Alternatively, I can grab a LC meter and head down to the nearby electronics shop and cherry pick out some silver micas that are rated at 5% tolerance from their shelf. The store silver micas are much much cheaper.

My big question stems from the concept of tolerance and drift. Is tolerance only relevant for the actual measurement of a component with respect to it's rated value? Or will a component that was originally built to tighter tolerance be less susceptible to drifting as well?

I guess my queries can be summed up as, "Will I score a better deal (and not compromise anything) if I cherry pick out some nice 1000pf caps from the store, as opposed to getting the more pricey ones from RS?"

[PeteS in CA]

Re: Questions on drift and tolerance

The tolerance, as I understand it, is the initial tolerance at a given temperature (e.g. 25C). Things like temperature coefficient, voltage coefficient and aging are not included in the tolerance. Consequently, a 1uF, 50V, Z5U or Y5V ceramic capacitor at 70C and 40VDC applied could be as low as .2 or .3uF (working from memory). I think mica or COG ceramic could be good choices; how they age I'm not sure. I believe mica is used in military equipment (where stability and long life are critical); COG ceramic may be also.