It's completely true, but, as you say, it's
not the whole, very exhausting, story. I am commenting at the
very basic level of 'What about tolerance?' Your input is at a
second level of concern. I could add that it's not necessarily a
good idea to use a wide-tolerance component, because the wide
tolerance is needed because the dielectric has large temperature
and voltage coefficients of capacitance. But to keep things
simple, I won't add that. (;-) However, your assertion about 200
pF +/- 1% and18 pF +/- 10% is not quite true, I think: it gives
extreme values of 221.8 and 214.2, whereas 218 +/- 1% gives
220.18 and 215.82.
On 2024-10-28 09:16, Tony Casey wrote:
On 27/10/2024 11:20, John Woodgate
wrote:
It's
not clear what you are doing about component tolerances. You can
get 218 pF with a 150 pF and a 68 pF in parallel. That is just
within +1% of 216 pF. A single 220 pF is within +2%. If all your
four capacitors have +/-1% tolerance, your 216 pF will also have
+/-1% tolerance, but there are many additional stray
capacitances with this arrangement. Fewer capacitors, fewer
strays and simpler construction.
What you're saying about tolerance is only half-true. Paralleling
multiple capacitances doesn't change the worst cases. But it does
change the standard deviation. Assuming 4 equal valued capacitors
in parallel reduces σ by √4. If the nominal values of the
capacitors are not equal the situation is much more complicated.
In general, it is a much better idea, when trying to obtain a
non-standard value, to get as close as you can with one high
tolerance capacitor and add a much smaller one in parallel that
can be a much looser tolerance. You can get 218p with 200p and
18p. You only need to specify the 200p as a 1%. The 18p could be a
10% for the same effect on overall tolerance.
--
Regards,
Tony
--
OOO - Own Opinions Only
Best Wishes
John Woodgate
Keep trying
John Woodgate