[sdiy] bypass caps for TTL

[Harry Bissell Jr]

--- Cynthia Webster <cynthia.webster at gte.net> wrote:

> .1 Mfd is typical up to 3 Megahertz,
> (twenty years ago .01 was more popular)
> 
> If your circuit is high-speed say around 20
> megahertz then you'd
> actually want to go smaller since the recharging
> time for the cap is
> quicker.  faster = smaller such as a value of
> 0.047uF.

either I don't understand what you are saying or its
bass ackwards.

In  almost no case do we want the cap to significantly
discharge.  The larger cap will actually have better
performance. It can supply more energy.

The problem is really inductance.  The inductance of
the power supply leads cuases a delay in the current
reaching the chip. As it cannot get the current from
the power supply fast enough, it will draw from the
low inductance local source (bypass cap)

Keeping the 5V and ground runs CLOSE together
(parallel) or using ground and power planes is the
best way to keep inductance down. DO NOT MAKE LOOPS
!!!

The reason that a .047uF cap might be better than a
.1uF cap, is self inductance.  The smaller cap might
have less internal inductance, so it will have less
delay in supplying current transients.

In ceramic caps, the difference between .1uF and
.047uF is probably marginal.

The best solution would be a ceramic cap across the
power supply pins. They used to make IC sockets with
the caps RIGHT across the pins. Expensive, but at that
time they were cheaper than using a multilayer board.
Today they are probably rare.

If you are building by hand, I'd run the cap
diagonally on the back right across the IC pins. Can't
get closer than that.

WRONG WRONG WRONG is to put the cap at one end of the
IC and make a big loopy trace around the outside. The
bigger the loop, the higher the inductance, the slower
the IC gets the current it craves !!!

Generous use of good low inductance caps of larger
size
(can you say "tantalum" ? :^) is a good idea every few
ICs along the bus.

Also, given a choice I'd consider the use of the 74HC
or 74HCT cmos series ICs.  The HCT have the same
threshold voltages at TTL... just watch out for the
fan out (number of loads you can drive from a single
IC).

Use TTL only for a couple of oddball chips and you
will cut your troubles down a lot...

H^) harry



> 
> I use .1 Mfd  ceramic bead capacitors all the time
> with success,
> although there are a zillion different types that
> would work,
> these are fairly compact.
> 
> Here is the part number for www.mouser.com
> 
> 581-SA115C104K
> 
> Best Wishes!
> 
> 
> Cynthia
> 
> http://www.cyndustries.com/
> 
> > 
> > Thanks,
> > 
> > Richard Moore
> > 
> 
>