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Layout,  BYPASSING,  Decoupling, Shielding and Ground plane
 Layout = Stability = E M C = Performance
  Bypassing = Shunting = Diversion = Stabilization
  Decoupling = Isolation = Separation = Noise Reduction
  Shielding = Blocking = Impeding = Protection
  ground plane = Return  = Sinking  = Referencing
       
-//-Bypassing! -\\-Who Needs It?  Read: Ernie's Story
Read-a-Book: "High-Speed Digital Design: A Handbook of "Black Magic" (Prentice-Hall, 1993).
WWW: Signal Consulting The Art of High-Speed Digital Design http://www.sigcon.com
 
Introduction
Look!
Your life as a lover, as a voter and as an electrical engineering student, can be changed for the better: if you will read and try to understand--at least think about--the following. 
  
Warning!
If you ignore the above, you will more than likely end up behind the bus station, wearing a soiled World War I trench coat; an unlit stub of a cigar clinched between your rotted out teeth; a dirty paper bag partially covering your bottle of cheap wine in your left hand; trying to recall Maxwell's equations while writing them on the inside of the third stall from the left, in the Trailways rest room.
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, i.e., impedance matching; parallel and series termination; microstrip layout; propagation of very high frequencies to and from the circuit; crosstalk; proper power rail bypass & decoupling; shielding techniques; as well as ground plane design.
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One great source of noise on the power rail--Vcc, is from TTL (Transistor Transistor Logic) logic. In a typical TTL logic device (gate, flip flop, counter, register, etc.), it is fair to say that at any one time: half of the transistors are ON, and the other half are OFF. 

TTL is "Saturating Logic," i.e., a transistor is either fully off or fully on, and in Vsat [1] a saturated transistor requires more time to become unsaturated than it took to become saturated in the first place; this time difference is called storage time (Ts). 

Therefore, during any logic transition (one/zero or zero/one) All of the transistors are ON for this short time (Ts)--drawing twice its normal current. This latency can last from two or three nanoseconds to as long as fifteen nanoseconds--depending on the logic family. 

In systems using thousands, to hundreds of thousands of gates, there can be a tremendous demand on the power supply, i.e., supplying a constant, non-varying Vcc (5.0 Volts) to a load that is random and very high speed (fast di-dt). 

If one could supply power to all of the logic devices at a constant Vcc, everything would be "Peachy Keen." However, Life--and Murphy, do not allow anything dealing with Engineering Design, to be "Peachy!"   --Bummer! 

Because there is "Distance" between any power source and the logic it powers, a "conductor" or "wire" of finite length is required between this power source and the logic. 

Any "conductor" or "wire" have the properties of resistance and Inductance; both can cause problems where power is concerned. The resistance problem can be overcome by increasing the "cross-sectional area" of the conductor. The Inductive property is likewise amenable to increased cross sectional area, but in a very reduced sense.

Even what might appear to be a small inductance, at high frequencies, or very high frequencies, can translate to an unacceptable high impedance. 

At this point, it is worth noting that TTL logic devices (as with most--spell that ALL--active devices) have inputs and outputs--you already knew that didn't you! What you may not have considered is that the power rail--the Vcc pin--can be both, an input and output "port."  You only have to examine a schematic of such a device to see that the power rail is common to both input and output stages, i.e., the bias resistors of the input stage are connected to the same "Rail" as the load resistors of the output stages. 

This arrangement is not a Bad Thing, because it presupposes that this common power rail "point" will always be held at Zero Ohms! That is, it will be sourced by an "Ideal Voltage Source." 

Together now class: "a Voltage Source--by definition--has Zero Ohms Output Impedance! 

Right? 

Right! 

Did you get all that Thompkins? Somebody wake up Tompkins [1]--there in the back... 

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In the world of fast edge sensitive logic, it is a simple matter to conceive of "inappropriate communications" between logic devices (& elements within a device) through the unprotected, high impedance power rail. 

Because of the speed of this demand on the power supply--Rise and Fall times in the nanoseconds--the frequencies are in the many hundreds of MHz. And, any conductor, be it printed circuit trace, or wire, any conductor between the device power pin and the power source can look like an unacceptably high resistances of Ohms to hundreds of Ohms. 

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! 

--------------- TTL (Transistor Transistor Logic) is saturating logic, i.e., a transistor is either fully off or fully on, and when it is fully on it is said to be, in Vsat

It takes longer to turn a transistor off than it takes to turn it on: the time to turn on (risetime) = Tr the time to turn off = Tf (falltime) + Ts (storage time). Ts is the time required for the hole/electron pairs to move back across the base junction--taking the transistor out of saturation (Vsat). 
[1] Not his/her real name.
 

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TTL (Transistor Transistor Logic) is saturating logic, i.e., a transistor is either fully off or fully on, and when it is fully on it is said to be, in Vsat. 

It takes longer to turn a transistor off than it takes to turn it on: the time to turn on (risetime) = Tr the time to turn off = Tf (falltime) + Ts (storage time). Ts is the time required for the hole/electron pairs to move back across the base junction--taking the transistor out of saturation (Vsat).

..
 
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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Bypass, Decoupling, Shielding and Ground Plane 
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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B y p a s s i n g
The only way to maintain the integrity of the power distribution to the various circuits, i.e., to maintain the low impedance of a true voltage source (zero ohms), is to counteract the inductance of the finite length conductors that distribute the power to the circuits. This can be done by applying a sufficiency of capacitance between the supply pin of each device or stage and ground or common (using the shortest paths possible). 

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Click_It for Anim
Frequency Domain-->
Time Domain
Frequency Domain-->
Time Domain
Effects of Excessive Lead Length & Capacitor Type 
 
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-- Adding the Capacitor --

Wire Wrap, Ground Plane, Bypass Capacitor (SMT Chip Cap)
BYPASSING IS THE ONLY WAY to maintain the integrity of the power distribution to the various circuits, i.e., to maintain the low impedance of the true voltage source, to counteract the inductance of the finite length of the conductors that distribute the power to the circuits. This can be done by applying a sufficiency of capacitance between the supply pin of each device or stage and ground or common: using the Shortest Path Possible. 
 
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An illustration of the effects of Inductance in Excessive Lead Length.
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D e c o u p l i n g

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... 

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: a "pie" network for example. 

  2... One of the most efficient inductors is the ferrite torrid. 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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  S-h-i-e-l-ding   

Shielding can be anything from using a coaxial or shielded cable, to a sealed conductive chamber for circuit isolation. Shielding serves a reciprocal purpose: it protects the circuit it is shielding from outside noise or unwanted signals; and conversely, it contains its own signals and thus protects the outside world from interference of its own making. Shielding is mostly used to block electrostatic or "E" fields (Faraday shield). However, if ferrous metal (tempered Mu Metal works best for magnetic fields) is used then both electrostatic and some level of magnetic shielding is accomplished. This is especially useful where open frame transformers or unshielded coils are used and would otherwise exchange signals by mutual inductance. 
When is a Shield a Shield? 
One important requirement for a shield to be effective, is that there must be no currents flowing through the shield itself. This is best accomplished by connecting the reference or common, at only one point on the shield, thus preventing any flow of current. The reason for this, is that any current flowing in the shield material itself can produce secondary fields on the other side of the shielding material and thereby reducing the effectiveness of the shield. An extreme case of this might be a shielded cable, whose shield has a different potential at each end, and the resulting current flow in the shield, inducing unwanted noise into the center or shielded conductors. (In this situation one might find a remedy by disconnecting one or the other ends of the cable. However, this may not prove satisfactory in certain environments, and may require a "Guard" potential, which is some compromise potential.)
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A c t i v e   Shielding

There is an active form of shielding where fields of counter EMF (equal but opposite) are generated to cancel out the offending fields. A good and simple example of this is the AC power transformer, where a "shorted turn" is used to generate a nulling field. 

The shorted turn, is a seamless band of copper that wraps the transformer core in one direction. When cut by the rising and collapsing magnetic flux -- caused by the transformer action -- the shorted turn acts as a very low impedance, high current secondary winding, and generates a counter EMF, and because this winding is shorted, it generates a rising and collapsing magnetic field of opposite polarity thereby nulling the original stray magnetic flux. In some cases of severe common mode noise, the shield can be made to carry an equal but opposite noise current to counter the interfering noise. However, this is not for the faint-of-heart: any slight change of the mechanical or electrical parameters, and the canceling noise becomes the noise noise!
 

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G r o u n d  P l a n e 
A ground plane is a special and very important component of any circuit. In essence, it is the return path for all signals including the power distribution. The ground plane can be thought of as homogeneous for the DC power only. In all other situations it is strictly inhomogeneous. All this means that all grounds are not the same. As various circuits use the ground plane for their signal and power return paths, currents -- conducted and induced -- are caused to flow throughout the ground plane, and potentially can affect any or all other circuits, and can cause real problems. 

There are only two ways to model the ground plane in a complex signal environment: and nobody knows what they are!

One can start to understand the function and design (and FM) of ground planes if one does the following: 
1) Draw a map of all signals in a circuit, their inputs, outputs, paths, and their various connections to and from the ground plane; 

2) Then model the inductances, capacitances, parallel and series resistances while noting the power distribution paths and returns and their respective noise content; 

3) And don't forget all bypassing devices and their contributions to the model; 

4) Since the ground plane is mostly inductive, note must be taken of any other inductances in proximity to the ground plane, such as transformers, chokes, tuned circuits, etc., and their contributing fields at all relevant frequencies. 

The use of multi-layer printed circuit boards allow the use of multiple ground planes, as well as buried (under the signal layers) Vcc and Grd. layers. These layers are sandwiched together and act as a very efficient distributed bypass capacitor. A variation on this is to have the Vcc and Grd. layers as the outer or intermediate layers, thus shielding [see foot note 1] the buried signal layers; or some combination thereof. 
 

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Input / Output 
or
"Every Signal Should Have its Own Ground Return" 

When interconnecting signals--be they analog or digital, to or from any device, the ideal approach is to have each signal have its own return, or common path. In the best of all worlds, a balanced (equally referenced above ground) shielded twisted pair, driven by a high quality differential driver--having controlled rise and fall times and minimum skew--and received by a properly terminated differential receiver. Being that the world is not always obliging, a compromise is usually in order: as illustrated by the accompanying figs in Appendix A. 

Common Mode Noise 
Common mode noise is any noise that impinges equally on both the circuit input and its return path, with the same polarity, e.g., both sides of a differential circuit or twisted pair would have the same sign or polarity, and in a twisted pair situation with a differential input, there would be a rejection of all common mode noise. 
Differential Mode Noise 
Differential mode noise is any noise that is signed such that the input and the return path have opposite polarities. For example, a differential input, which would normally reject common mode noise, would be susceptible to this type of noise. 

Video
In the case of interconnecting base-band video signals: the accepted standard is, and has been, unbalanced 75 ohm coax (RG-59). The nature of standard RS-170 video is that it covers many many octaves, i.e., ~10Hz to 4.2MHz; and is a very fragile signal. If one wanted to pick the very worst transmission medium, it would be hard to pick one worst than 75 ohm unbalanced coax. The source of the most common and offending interference or artifact to base-band video, is the ubiquitous 60Hz AC power mains. This noise manifests itself an a "hum bar" that repeatedly and slowly creeps up the displayed video. A veritable fortune has been made by many companies over the years from specialized amplifiers and video processors (proc-amps) whose sole purpose was to overcome these faults. 

If the baseband video standard had been 150 ohm shielded twisted pair, instead of the 75 ohm coax, the television world would be a much safer place in which to raise our children. 
This problem can be ameliorated by the careful treatment of this coax in the following ways: first and foremost is to isolate the shield or braid of the coax. The source of the signal, e.g., the camera, should have its connector isolated from case ground, and the shield side of the video connector should be connected directly to the circuit board of the video source. From here to the destination, the signal common or shield, must never contact any external ground, common or any other coax's common or shield. Upon arrival -- whether to a proc-amp, distribution amplifier (DA), ADC, display, or whatever -- the coax shield must only connect to the signal return of the appropriate circuit board, and, of course, the coax must be terminated in its characteristic impedance of 75 ohms. 
 

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Analog / Digital 
One of the hardest things to design successfully, is one having both analog and digital circuits living on the same board or planer, and sharing the same power supply. Most designers who have yet to be burned, don't give it a high priority. In some cases analog and digital on the same real estate may not be practical, and in other cases it can be down-right hard. If one is to succeed, it will require that he/she master the above noise handling Analog/RF concepts. 

 

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Tips & Suggestions 

Ideally one should attempt the following:

Bypass 
1... .1 ufd or greater, ceramic capacitors (chip surface mount technology, SMT, preferably).

2... One capacitor per SSI, MSI TTL device.

3... The capacitors should connect Vcc and Grd. of the device, using the shortest path possible. (See figure)

4... Ceramic chip capacitors have the best high frequency characteristics, i.e., have least inductance and stay  capacitive reactive over a wider frequency range (offer the lowest impedance).

5... Tantalum will not substitute for ceramic. However, tantalum capacitors are a good adjunct to the ceramic.  That is, they offer low impedance to the lower frequencies.

6... In some high frequency analog (RF) or high speed digital circuits, it is appropriate to use several different  types of capacitors. Tantalum for low frequencies; large ceramic (e.g., 0.1 ufd) for the higher frequencies,  and  small ceramic, RF capacitors (~1nfd) for the much higher frequencies. The name of the game is maximum  capacitance with minimum inductance. Remember: when Xc = Xl you have resonance, and that's something  you  don't need in a bypass circuit!

 

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 Decoupling using "L"
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: a "pie" network 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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 Signal Distribution
1... "Every dog has its Day," and: Every signal needs its own Ground Return. Inputs and outputs should be as  separate as possible, and they must not share the same ground return. Think about it: very sensitive input,  very large signal output; a formula for Mister Oscillation.

2.. The layout should follow a flow, like the circuit diagram follows a flow: there is a "Goezinta" and a   "Goezouta."
 And, the power supply, and its ground return, should move in near the output part of the circuit -- where the  larger signal levels (and low sensitivity) live and away from the high sensitivity part of town.

3.. When two, or more, signals share the same ground return, there can be interference between the two. This is  really true for analog, but digital has some margin before it becomes troublesome. However, this is not a  license to steal: if this happens at more than a few places and a few times, the cumulative effect can come  home to roost in the form of intermittent trouble, and that my friend will drive you "Nuts," and you may never  track it down, NEVER!. 
 

 Parts Layout
1.. When laying out a board -- PCB or wire wrap -- try to keep all of the components for that particular part of  the circuit together. Don't think that the timing resistor or timing capacitor for a one-shot can live on some  other part of the board without causing timing jitter or even false triggering. And remember: Bypass! 

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 Crosstalk Problem:
1.. Crosstalk can happen when wire wrap wires are bundled. The tighter the bundle and the longer the wires run  together, the worse the crosstalk.

2.. The distributed capacitance between wires is the killer: with the rise and fall times in the single digit  nanosecond range, it doesn't take much to trigger an edge sensitive device.

3.. In PCB layout the name of the game is to crowd thin traces together and get data from point "A" to point "B,"  "C," etc... without using any board space. If the traces are too close and "stay together" for too great a  distance, it starts to look electrically like a wire bundle, a tight wire bundle: "A real Bummer Dude!"

 Solution:
1.. When wire wrapping, Don't bundle wires, EVER! Use the shortest wire run from point "A" to point "B." And:
 Don't worry what it looks like! In this life you have a choice: it either looks good and doesn't work worth a  Damn, or  it looks like Hell and works great -- it rarely does both. Remember the Hot Rodder's old saying: "If it  won't run, chrome it." Words to live by.

2.. On PCB layout: if a group of signals must run together more than a two or three inches (more or less depending  on the logic family) across the board: swap sides of the board and/or mix  traces so everybody has new neighbors.

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Power Distribution
1.. Distribution of power to and on a circuit board is fairly straightforward. In the case of several different power  supplies, e.g., +5, +15, -15, etc., the ground returns are the most important consideration. If there is a mix of  circuit types, Analog, TTL, CMOS, etc., then the analog grounds must be separate from the digital ground  returns. And, sometimes if there is a large concentration of fast TTL, and some CMOS logic: thought should be  given to separate supplies and, of course, separate ground returns. The reason for the latter is that TTL, by its  very nature, puts large amounts of noise into its supply rail, and though CMOS is robust, its merely a matter of  degree to what it takes to corrupt it. 

2.. There is a school (DUKE) of thought (no, wrong, can't be DUKE) that says that Vcc leads or runs should be as  short as possible. This is true as far as IR drop (resistance) is concerned, however, up to a point, the longer the  lead, the more inductance, hence the more decoupling. But, if you buy on to this idea, and its a really good  idea,  think about limiting the number of circuits feed by this long wire -- run more than one "long run." 

Summary
Implementing the suggestions in this paper will not be easy, and for-sure not always possible, but do the best you can and, who knows, your circuit might work... or NOT.

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   Voltage Regulators

Linear Regulators
1.. Read the data sheet. The needs 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)

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, "Viola," 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 Du du." 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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Constant Current Generator
1... A voltage regulator is, by definition,  a "Voltage Source," which ideally  supplies a constant voltage  regardless of the changing load  (within reason). A current source,  likewise, supplies a constant current  regardless of the changing load. A  good constant current source, or generator, can be constructed using the three terminal regulator tied as in the  diagram. The VR maintains a constant voltage (e.g., 7805: 5 volts) across the series sense resistor: a constant  voltage across a constant -- or fixed -- resistance, yields a constant current. "Gee, that sounds remarkably like  Ohm's law." 

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