Hi Shane,
When you say "next to" and IC, do you mean from the +5V, to ground?
Yep. There are some IC sockets who have a 100nF capacitor connected from
the pin on the bottom left to the pin on the top right, i.e. the most
common IC power pins.
Resistors I got, Capacitors frustrate me...
It is not that hard to understand: Capacitor act just like resistors for
AC. The higher the frequency, the more current can flow through a
capacitor. The complex impedance is Z := 2*Pi*(-i)/f*C, where f is
frequency in Hertz, C is the capacity in Farad and i is the imaginary
basis, e.g. i^2 = -1. This just means that the impedance (the
resistance, basically) approaches zero if the frequency approaches infinity.
In our example: Really fast disturbances can be seen as some very high
frequency (look at the Fourier transform of the signal). These
disturbances will be shorted by the small capacitor of 100nF due to
their high frequency, so that this high frequency does not corrupt your
circuit.
Slow signals are not affected. In the other limes, the frequency is zero
(i.e. DC signals), so the impedance is infinite. We already know that:
Capacitors do not conduct DC current.
Hope this helps.
Best regards,
Jens
On Thu, Jul 21, 2011 at 4:16 AM, Nick <[email protected]
<mailto:[email protected]>> wrote:
Decoupling should be done EVERY time on EVERY chip - analogue or
digital - right adjacent to the supply pins.
It costs pretty much nothing, and saves a whole bunch of trouble. Just
do it.
Part of the need to do this is that chips today are much much faster
than they used to be, so where frequency response would roll off
before oscillation, nowadays even standard opamps can have GBWs in the
MHz to 10s of MHz range, and logic goes far far higher.
On the analogue side, I'm currently restoring some Quad amps - the
amount of pure twaddle on the www about using loony opamps like the
OPA627 and much faster (in "audiophoolery" faster = better) class A
drivers & output stages - recipe for high-frequency instability - the
circuit were designed to use the inherent limits of the original
devices.
Maybe I'll just spend 1000 bucks on some speaker cables and
unidirectional 99.99999% OFC internal cabling. Not.
Nick
On Jul 21, 9:50 am, jb-electronics <[email protected]
<mailto:[email protected]>>
wrote:
> Hi,
>
> > (2) Sprinkle capacitors across power and ground all over your
circuit.
> > Preferably as close to the power and ground pins of each chip as
> > possible. Usually they're 0.1uf (100nf) ceramic capacitors.
Some big
> > chips require you to use several near them, so read you
datasheets.
> > Chips are fast. Very fast. They can either generate very brief
short
> > circuits (in the ballpark of 10nS), and/or be susceptible to these
> > very short glitches on the power rails.
>
> I cannot stress enough how much pain this will spare you. I recently
> built a combined volt- and amperemeter with a 2x16 LCD readout on a
> rather small pcb, and I did point-to-point-wiring like I always
do, and
> it did not want to work. Some weird oscillations at the volts
ADC. The
> first thing I did was inserting a 100nF cap next to every (!)
IC, and
> bam, problem solved.
>
> Jens
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