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Layout,
Bypassing, DECOUPLING,
Shielding and Ground Plane
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| Layout |
= Stability |
= E M C |
= Performance |
| Bypassing |
= Shunting |
= Diversion |
= Stabilization |
| Decoupling |
= Isolation |
= Separation |
= Noise Reduction |
| Shielding |
= Blocking |
= Impeding |
= Protection |
| ground
plane |
= Return |
= Sinking |
= Referencing |
Introduction
As digital gets faster and faster, it starts to look more like analog
than digital. It is necessary to have a good understanding of the Analog/RF
like properties of fast digital. Attention must be paid to such things
as Transmission Line effects like: impedance matching, series and parallel
termination, microstrip layout; propagation of very high frequencies to
and from the circuit; crosstalk, proper bypass, decoupling, shielding techniques,
as well as ground plane design.
Because of the speed of this demand (nsec) the frequencies are
in the many hundreds of MHz. And, any conductor/wire between the gate power
pin and the power source can look like an unacceptable high resistance
(.....).
In the world of fast edge sensitive logic, it is a simple matter to
conceive of "inappropriate communications" between logic elements through
the power rail.
Going back to basics: the definition of a voltage source is that it
has Zero Impedance out! If you could enforce that definition at the power
pin of every logic element, then life as we know it, would be GOOD!
Something as simple as placing a capacitor between Vcc and Grd. of each
logic element--chip, will effectively create this near Ideal power supply--Zero
ohms output impedance!
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Simple "L" Network |
Decoupling Device |
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AC Power Line Balanced
Decoupling
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Bypass, ,
Shielding and ground plane
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| Bypass, decoupling,
shielding and ground plane are the properties that allow circuits -- digital
and analog -- to function or work properly. The reason is simple: let's
say you have a cascade of amplifier stages that
are boosting or amplifying an otherwise weak signal. The input is very
sensitive to small signals, and successive stages are drawing progressively
more current in order to produce the larger replica of this weak input
signal. In doing so, the output stage draws large amounts of current at
varying rates. This large varying current is seen by the more sensitive
input stages through the common power supply rail, which serves all stages.
This can happen if the power rail, be it wire or PCB traces, is of sufficiently
high impedance. Even if the power supply were "Ideal," (zero ohms) this
can still happen: as the frequencies go higher, the inductive reactance,
of the leads or PCB traces, increases. For example, if some fast transitions
of the input signal caused a resulting perturbation on the power supply
rail to propagate down that rail to all of the other circuits, the resulting
effect can be oscillations or some sort of instability which could cause
distortion or even render the circuit inoperative. One can think of it
as inappropriate feedback between stages, facilitated by the power rail
not appearing as a virtual AC ground. |
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Decoupling
There are instances where the power distribution between stages cannot
be sufficiency bypassed. In this case, the designer might be tempted to
use several different power supplies. However, by supplying the DC power
to each stage through a separate inductor or "choke," while also bypassing
to ground that stage, the effect is the same. That is to say, the choke
offers a high impedance path to any errant signals or noise between stages,
while offering a very low resistance path to the DC power: this is known
as decoupling. Active devices such as voltage regulators can also be used
for decoupling stages.
In fact, considering the size of inductors as compared to surface-mount
voltage regulators: regulators might be the better choice. One might better
understand this by recognizing the fact that a choke or inductor is one
of the two needed components for a resonate circuit. Therefore, the combination
of decoupling inductor and bypass capacitor could just happen to resonate
at the wrong frequency. Having said that, it might be obvious that the
inductor needs to be as small a value as is reasonable, and the bypass
capacitor as large as practical. This is essentially correct, however,
there is still the possibility of the resonant frequency of this combination
to cause mischief. And, if that weren't enough, the inductor itself can
be self resonant. This is caused by the distributed capacitance between
windings, i.e., one turn of wire to the adjacent turn of wire, etc... |
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| One more thing to consider about chokes: the "Q"
or quality of the inductor has an effect on its efficiency. As previously
stated, the inductor should appear as a short circuit to the DC power it
is carrying, and a high impedance to any AC, i.e., no series "R." In the
practical world this isn't feasible. However, if heavy current carrying
chokes are required, then the choke must have higher "Q," i.e., less wire
which means lower "R." This can be achieved by using chokes with ferrite
cores, which need considerably less wire for the same value of inductance:
it is truly a multiplier of "Q." Also ferrite beads, i.e., very small donut
or tubular shaped ferrite, are regularly used for circuit isolation, effectively
preventing parasitic oscillations, etc. The down-side of ferrite, is that
it will change inductance as the current or flux changes. In the case of
large currents, it can saturate. However, by correct component choice --
frequency, AC and DC current, etc. -- ferrite is great tool for the designer. |
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1...Decoupling is used where the supply voltage cannot
be lowered, i.e., if one needed a noise-free +12 volts on a PC bus, say.
One could get a "clean" +12 volts with a voltage regulator... if only there
was +15 volts or higher to start with. But such is not the case. So you
use a high "Q" inductor (RFC choke) along with the proper bypass capacitor
to effectively lowpass filter the +12 volt supply rail. For a real noisy
supply you can use more than one inductor: multiple "pi" networks for example.
2...One of the most efficient inductors is the ferrite toroid. It has
high "Q" -- low "R" -- and because of its toroidal shape its fields are
confined, and therefore has little stray fields. The super star of high
"Q" inductors or transformers is the pot core. And of course, don't forget
the ferrite bead. Thread the wire through the bead once or several passes
and it may be just what the doctor ordered.
3...Decoupling is only as good as the components that you use. The capacitor
part of the network should be high "Q" and minimum inductance: the noise
is dropped across the inductor, and the capacitor must exclude the remaining
noise. Another way of saying it: in a perfect world the inductor is an
open circuit to noise (AC) and the capacitor is a dead short -- Zero, Nada,
Caput, Zilch; "This here parrot is dead." The slightest inductance in series
with that capacitor, and some very high frequency noise will come through
like Gang Busters!.... Anyway nuff said.
4...SMT or chip capacitors made of ceramic are best. Also, sometimes
in critical circuits, several size caps in parallel are appropriate, e.g.,
1ufd || .1ufd || .001ufd, etc. The reason for this is as the capacitors
become smaller in value, they also get physically smaller, hence less inductance.
However this is less the case with SMT caps: consult your capacitor data
sheets for the impedance verses frequency plots. Didn't he just say that? |
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Using Linear Voltage Regulators for Decoupling
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Linear Regulators
1..Read the data sheet. The needes and capabilities of the regulator
are in there somewhere; they might not jump out and bite you right away,
but they are there.
2..The use of three terminal linear voltage regulators, like the 78xx
and 79xx devices, is fairly straightforward. However, there are a few things
to remember: Always bypass -- there's that word again! -- the input pin
and the common pin with a ceramic capacitor no smaller than 0.3 ufd, and
use absolutely the shortest leads possible (there are some transistors
with pretty high f t in that regulator, and if you furnish enough reactance
of the right kind, Mr. Oscillation will visit you again).
3..If your regulator is furnishing power to a capacitive load, and the
primary power is removed -- like unplugging a PC card, or disconnecting
an experimental setup -- the charge in that capacitive load will cause
the secondary or output of the regulator to be more positive than the primary
or input. If this reverse voltage exceeds the regulator's ratings it will
blow up. To prevent this sort of failure, a diode is placed between the
input and output, such that, when reverse voltages are present, the diode
conducts preventing damage. (see Figure)
Protected Voltage Regulator
4..There will come a day (or night) when you may need an eight volt regulator,
and all you have is a 7805, five volt regulator. By inserting a voltage
equal to the difference in the common lead, "Voila," you have 8 volts.
You can do this by inserting a zener diode or a low resistance voltage
divider (or a pot for variability). If all else fails, insert a series
of silicon diodes (cathodes toward Grd.) @ .6 volts per, until you have
the desired output.
5..These regulators don't need an output capacitor per se, but a minimum
of 1 ufd is recommended to prevent fast load pulses from causing needless
error correction by the regulator. As for the primary or input capacitance,
it depends on the ripple content from the primary voltage: If the voltage
is straight from the rectifier, then obviously large capacitors are required
-- assuming a large load on the regulator's output. The greater the difference
between the input voltage and the output voltage, the less stringent the
capacitor requirements.
6...In the data sheet -- you know, that funny looking piece of paper
that causes you to squint, and makes your head feel funny -- In the data
sheet, there is information on forward drop, Vfwd, of the regulator at
some current. This means that if the primary voltage is near the desired
secondary voltage at some current, you may be in "Deep Dudu." The greater
the difference between the input voltage and the output voltage, the easier
life is: if the rating of the regulator is a 1.1 volt drop at 500 ma, and
you have a 5 volt margin -- say -- you are in fairly good shape; if you
have, on the other hand, a 10 volt margin, you're in great shape! |
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