Home-made Modular Synth teething trouble II

[Karl Helmer Torvmark]

On Thu, 3 Sep 1998, <<<marjan>>> wrote:

> For capacitors : large electrolytic caps have significant  impedance so
> you connect some ceramic 100nF in parallel which has minor inductivity
> (it's dielectric is feroelectric,meaning higher dielectric const. and
> high specific capacitance) for supply with some parasitic impedance
> Why : in oscillator circuits and esp. modern components there are very
> high current pulses inside it for making better speed performances 
> (eg. for fast charging of parasitic cap on cmos gate or for Miller
> capacitace iside compesated opamp ,fast comparators ,pulses with
> fast rising edges) so instant current spike can't be obtained from 
> big filter cap with high indictance (and inductance disables sudden
> current changes) without 2-3V drop ,so little ceramic cap enables 
> wanted electricity Q for that spike with just some 0.3-0.5 V drop as
> it has small inductance .Thats why you also need those caps across fast
> switching components onboard itself - closer to it - better as there's
> also unwanted impedance in copper traces too.It's good to solder them
> directly to IC on -/+ pins,on every ttl,cmos and comparator chip.
> Well it's maybe too much but it can't hurt.
> eg. using 10-100nF cap and 0.5V drop you have 5-50 nC charge.
> One logic gate has 50pF input,and for 0->5V rise it needs some 150pC .

Good advice. I use SMD ceramic 100nF caps, and solder them right under the
IC for maximum effect. I use separate voltage regulators for each module
(to reduce crosstalk to a minimum), and I usually find that the
electrolytics connected to the regulators provide enough low-frequency
decoupling (although in adverse conditions perhaps several electrolytics
are needed on each board).

-----------
Karl H.