Re: decoupling caps placement


"IanM" <Invalid@xxxxxxxxxxxxxxx> wrote in message
news:gioelo$qjh$1@xxxxxxxxxxxxxxxxxxxx
Tim Wescott wrote:
On Sun, 21 Dec 2008 16:33:47 -0500, tempus fugit wrote:

Hey all;

I've got a circuit that uses 3 4049 inverters. On this IC, the V+ is on
pin 8 and the ground is on pin 1. I know that the decoupling caps need
to be as close to the IC as possible, but how can I connect 1 end of
the
cap to V+ and the other to ground when the pins are so far away? Is it
sufficient to connect 1 end of the cap to V+ and the other to a nearby
ground node, or should the cap be connected close to the actual ground
pin of the IC? Also, do I use 1 cap for each IC? If so, (the ICs are
fairly close together) wouldn't the IC "see" the caps as being the
paralleled value of the 3 caps, thus reducing the available
capacitance?
I was going to use 0.1uF for the value of each decoupling cap. Would it
also be wise to use a larger (1uF or higher) cap in parallel?

Thanks

For that chip a 100nF (0.1u) ceramic up by pin 1, with a lead straight
to
pin 1 and another one straight to pin 8 will be more than sufficient.

You can think about reducing them later -- but why? Unless you're
trying
to shave deci-pennies off of the board cost, it's better to have too
many
bypass caps than just enough.

Some general comments on effective grounding and decoupling pitched at
hobbyists (pros will know how much they can save by cutting back on
both, but for the rest of us, the small additional cost is easily repaid
by time saved debugging a glitchy circuit):

By ground, I and most other dabblers in low voltage electronics mean
chassis, common rail, 0V, battery negative, Logic negative supply etc,
NOT a hard wired connection to a copper stake in the earth.
Electricians are different - when they say ground, they mean ground!

If building on veroboard or solderless breadboard, make sure you have
reliable power and ground rails. Except in exceptional circumstances,
DONT wire them point to point. (low level audio is an exceptional
circumstance with special layout and decoupling requirements as are high
gain RF amplifiers and high power circuits in general.)


Thanks for your reply Ian.

I am building on a non-etched board - one of those perfboards with little
copper pads you solder to (I assume this is the same as veroboard). What are
some of the special layout and decoupling requirements you mentioned in low
level audio circuits? I don't have any actual audio on this particular
board - it contains 3 voltage regulators (for 5, 9, and 12v) along with the
4049's in question, 2 8 bit latches, and a ULN2003 relay driver. The relays
are located on a separate board and switch audio signals from guitar fx
pedals in and out of the audio signal path.
Thanks


.

Relevant Pages

  • Re: decoupling caps placement
    ... I've got a circuit that uses 3 4049 inverters. ... pin 8 and the ground is on pin 1. ... sufficient to connect 1 end of the cap to V+ and the other to a nearby ... If building on veroboard or solderless breadboard, make sure you have reliable power and ground rails. ...
    (sci.electronics.basics)
  • Re: 1-in-4 sequencer freezes up--a noise problem?
    ... A diode between your + supply and 555 pin 8. ... A 470 uF cap from pin 8 to ground. ... trigger the circuit, but then continue on back to the top amber light. ... So the green light would consistently trigger the computer clock to ...
    (sci.electronics.basics)
  • Re: decoupling caps placement
    ... I've got a circuit that uses 3 4049 inverters. ... and the ground is on pin 1. ... close to the IC as possible, but how can I connect 1 end of the cap to V+ ... case you would put the cap next to the positive power pin and then to ...
    (sci.electronics.basics)
  • Re: Problem with 555 Monostable Circuit
    ... The cap on pin 5 is optional, and you could just as easily let pin 5 ... With Rt equal to 1.6 megohms, the width of the output pulse will be ... Professional Circuit Designer ...
    (sci.electronics.basics)
  • Re: WTB: Power supply for Heathkit "home" version of Fireball (Bally?)
    ... pin work in this Fireball home version pin. ... I also had to build a pass through circuit ... that carried fuses and returned back to the board for the 22v (coils ... board and then go directly off board again through fuses. ...
    (rec.games.pinball)