PCB design tips for Differential pair routing?

In this tutorial we are going to learn about PCB design tips for Differential pair routing?

Because some device technologies utilizes differential techniques, its worth explaining some of the advantages & key layout aspects of differential circuitry & comparing them to similar single ended op amp circuitry. Differential circuitry is superior to single ended circuitry for a number of reasons. The CMR of differential inputs lets balanced circuitry reject common mode interference, including GND noise, that would be amplified by single ended circuits. Also, a differential circuit’s balanced properties usually reduce non-linearities & improve distortion. ICs with differential outputs have inherent common-mode output noise that is cancelled if the DAC is followed by a differential-input amplifier or filter. Because a differential signal’s 2 conductors carry a balanced signal, reduced EMI generation & reduced
susceptibility to magnetic pickup are additional benefits of differential circuitry. Even “quasi-differential” circuitry, with its GND taken adjacent to a single ended source but shipped to a differential load as if it were the II half of a differential signal, is superior to single ended circuitry. This is because the small common mode interfering currents between the source & load are still reduced by the differential input. for examples of these concepts. Generally the single-ended connection in standard op – amp, although it could also apply to single ended connections into Devicces gain blocks if the GND referenced signals were instead referred to 2.5 volts. In general, the rules fall into one or more of these five categories:
Planes: There should be a continuous power system plane underneath the differential pair.
Length: Care must be taken to ensure that differential traces are of equal length.
Spacing 1: Care must be taken to place the traces as close together as possible.
Spacing 2: Care must be taken to ensure that the spacing between traces is constant everywhere along the length of the traces.
Impedance: Differential impedance rules must be applied.

Make D > 2S to minimize crosstalk.
Where
S: Space between the two traces of a Differential Pair
D: Space between two adjacent differential pair
2) Route the 2 traces of a differential pair as close to each other as possible after they leave the device
to ensure minimal reflection.
3) Maintain a constant distance between the 2 traces of a differential pair over their entire length.
4) Keep the electrical length between the 2 traces of a differential pair the same. This minimizes the
skew and phase difference.
5) To minimize impedance mismatch and inductance, avoid using vias.
To minimize the crosstalk, keep the traces for the differential pairs as short as possible. If the pair has to
be routed for an extended length, observe the following guidelines.
· Keep the differential pairs as far away as possible to eliminate cross talk between the pair.
· In PCB design ground strip should be on the top layer to separate the differential pairs.
· Use vias to connect the top layer GND strip to the bottom plane extensively(every several
hundred mils)
· Keep the distance of the differential pairs constant when routing the signal.
· Keep the distance of the differential pairs to the ground strip as a constant.
· Try to match the length of the differential pairs.
· the reciever pair closer to the Reciever pins of IC chips rather than to the transformer. This is because the crosstalk current depends on the physical layout of the signals & the characteristics of the board. The voltage strength of the crosstalk signal is equal to the impedance of the load times to the crosstalk current. The higher the input load, the higher the crosstalk created on the same board. Because it is impossible on the board to match the input impedance. Its better to keep the trace from the termination to the input as short as possible.

Why Use Buried Resistors PCB ?

In this tutorial we are going to learn about Why Use Buried Resistors PCB ?

Buried resistor technology replaces discrete resistors with thin film planar resistors laminated within multi layer PCB. Space previously occupied by discrete resistors may now be used for additional components, trace routing/eliminated to create smaller, denser boards. Using buried resistors can significantly shorten signal paths. This results in reduced lead inductance, shorter signal paths, & improved impedance matching. Assembly costs can be reduced when buried resistors replace enough discrete resistors on a board.

ANALOG EFFECT OF DIGITAL INTERCONNECTS IN PCB DESIGN

In this tutorial we are going to learn about ANALOG EFFECT OF DIGITAL INTERCONNECTS IN PCB DESIGN

In a truly digital world there would only be 1s & 0s. In electronics PCB design we represent them as “high” & “low” signals. When the input to the component is high voltage the component switches (this is known as the high threshold). When the input signal falls back to the low voltage, the component switches back. (this is known as the low threshold). Unfortunately, when you send a high-speed digital signal down a wire it gets distorted through internal reflections & coupling to other traces. This may cause the signal to momentarily jump above the high threshold or fall below the low threshold, causing components to switch when they shouldn’t. These effects are analog.

What is formulas to calculate the hole span for through hole axial components?

In this tutorial we are going to learn about What is  formulas to calculate the hole span for through hole axial components?

Standard Tooling

Metric Formula: Minimum Hole Span = [(Comp Body Length* x 1.112) + 2.36mm] – Lead Dia

Inch Formula: Minimum Hole Span = [(Comp Body Length* x 1.112) + 0.093″] – Lead Dia

Large Lead Tooling

Metric Formula: Minimum Hole Span = [(Comp Body Length* x 1.085) + 4.11mm] – Lead Dia

Inch Formula: Minimum Hole Span = [(Comp Body Length* x 1.085) + 0.162″] – Lead Dia

5mm Tooling

Metric Formula: Minimum Hole Span = [(Comp Body Length* x 1.109) + 1.40mm] – Lead Dia

Inch Formula: Minimum Hole Span = [(Comp Body Length* x 1.109) + 0.055″] – Lead Dia

5.5mm Tooling

Metric Formula: Minimum Hole Span = [(Comp Body Length* x 1.067) + 2.30mm] – Lead Dia

Inch Formula: Minimum Hole Span = [(Comp Body Length* x 1.067) + 0.090″] – Lead Dia

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WHAT IS VIAS IN PCB DESIGN ?

In this tutorial we are going to learn about WHAT IS VIAS  IN PCB DESIGN ?

Via is part from which the connection can be taken from one layer to the other for the continuity of the

track or the connection. Vias are identical to the pins used by elements except that they can be added or

removed individually. The purpose of vias is to provide connections between different layers. Don’t use

vias for adding more  elements to the layout, even if that seems easier than creating a new element. You can assign a name to a via even though you probably won’t ever want to.

1) Through hole via – A type of via that starts on an external layer & ends on an external layer –connects all the layer

2) Blind via – A type of via that starts from an external layer & ends at mid layer Example: layer 1 & 2 or last and last but one layer (called as blind bottom via).

3) Buried/embedded – A type of via that starts on a mid layer and ends on a mid layer. Example layer 4 & 5

4) Blind bottom – connects the middle layer and the bottom layer. for example in a 6 layer PCB, layer 5 & 6.

5) tented via —a via with dry film SM completely covering both its pad & its PTH. This completely insulates the via from foreign objects, thus protecting against accidental shorts, but it also renders the via unusable as a test point.

6) Dog-bone via: Its nothing but 2 Vias connected in Bone shape

7) Microvia: Blind & through vias are more frequently used with FBGA packages than buried vias. Blind vias can be more expensive as compare to  through vias, but overall costs can be reduced, because signal traces can be routed under a blind via, requiring fewer PCB layers. Through vias on the other hand, don’t permit signals to be routed underneath layers, which can increase the required number of PCB layers and overall costs.

1. A trace can move to opposite sides of an individual reference plane without significant effect.

2. The effect of the first via is the greatest, but the effects of additional  vias Diminish as more vias are added to the trace.

BEST QUESTION OF PCB DESIGN ?

In this tutorial we are going to learn about BEST QUESTION OF PCB DESIGN ?

  • Width Length Height Unit

Single Euro Board Size 100 160 25 mm

Double Euro Board Size 160 233.3 mm

Double MultiLayer Board 220 233 mm

  •  What is a clock skew?

Skew is the variation between the rising edge of one signal versus the rising edge of another signal. It can also be measured between the falling edge of one signal versus the falling edge of another signal.

  • What is a lossless trace?

All traces have some resistance (D.C. component), but its very small & using the transmission line theory, we neglect it & call the traces “lossless”.

  • How analog effect of digital interconnects ?

In a truly digital world there would only be 1s & 0s. In electronics we represent them as “high” & “low” signals. When the input to the component is high voltage the component switches (this is known as the high threshold). When the input signal falls back to the low voltage, the component switches back. (this is known as the low threshold). Unfortunately, when you send a high-speed digital signal down a wire it gets distorted through internal reflections & coupling to other traces. This may cause the signal to momentarily jump above the high threshold or fall below the low threshold, causing components to switch when they shouldn’t. These effects are analog.

  •  What is critical length?

The critical length, as used in Signal Integrity & in this document, is taken to be a quarter of the wavelength of the signal being transmitted, & physical discontinuities of this order have little or no effect on the signal.

What is Power Planes and Power Distribution In PCB Design?

In this tutorial we are going to learn about What is Power Planes and Power Distribution In PCB Design?

Which Supply to Use and Supply Decoupling

PWR leads coming onto the board should be decoupled to GND at the point where they enter the board.

That way, all return currents from the decoupling components return directly to the PWR supply without passing through the GND planes of the board. While inductive decoupling components   or resistive decoupling components can be effective, the best designs route ±18V to ±24V to each board & use small regulators, along with decoupling, to derive ±12V to ±15V for the board/individual devices. the single supply AC devices: route +8V – +15V to the individual boards, with separate +5V regulators & decoupling on each board. One of the advantages of using devices in designs is the fact that they only require a single +5V supply, without the need to decouple positive &d negative supplies. A/D converters & D/A converters, in particular, need regulators & decoupling close to them to reduce the possibility of noise coupling from the rest of the circuitry. Most A/D converters also have a low-current digital section that needs to be connected to the quiet analog supply & GND, usually through some decoupling. Having pins marked Digital  Ground  on an A/D converter usually means that they are the IC’s digital GND, not that they should be connected to the system’s DGND. Most A/Ds also specify that even their high-current digital sections should be connected to an analog supply & GND, using higher-power decoupling, to obtain best performance. In general, noisy digital supplies & GND need to be kept away from high-performance A/D converters, except in the area where the digital outputs are developed. In order to have analog PWR & GNDs on their supplies but DGND plane underneath their digital outputs, most A/D converters should straddle the split between the GNDs in some fashion.

IC Decoupling

All analog circuitry needs decoupling on its PWR leads to shunt both HF & LF noise to GND. Its generally

recommended that designers use either a 0.1 μF or 0.01 μF capacitor to decouple the PWR pins of each

analog IC to ground. At various distances, place larger value capacitors, usually 10 μF – 47 μF, in parallel with 0.1 μF/0.01 μF units. In particular, high performance ICs must have each supply decoupled to Ground. The smaller value capacitor should be placed as close to the IC as possible, the leads on these caps must be kept very short. The best technique is to have the PWR feed from the PWR plane through a via to the capacitor & IC pins, with the capacitor between the via & the IC. The GND connection is particularly important, & should be made with 3 – 4 vias connecting to the capacitor & thus to the IC pins. The inductances of  vias are then effectively in parallel. For examples of this technique. SM capacitors are best b’cos their connection pads have almost no lead inductance. In addition, SM electrolytic capacitors can be used for the 10 μF – 47 μF units. Both aluminum & tantalum capacitors are available in these values, with tantalum having the lower ESR but also being more prone to power supply transients or reversal.

WHAT IS BENEFIT OF GROUND PLANE IN PCB DESIGN ?

In this tutorial we are going to learn about WHAT IS BENEFIT OF GROUND PLANE IN PCB DESIGN ?

· Low inductance

· High capacitance

· Low EMI susceptibility

· Low EMI radiation

· Large PCB land area

· Always try to make design double and multi layer

· Signal track routing can be difficult

A ground plane provides several benefits:

• GND is frequently the most common connection in the circuit. Having it continuous on the bottom layer usually makes the most sense for circuit routing.

• It increases the mechanical strength of the board.

• It lowers the impedance of all GND connections in the circuit, which reduces undesirable conducted noise.

• It adds a distributed capacitance to every net in the circuit, helping to suppress radiated noise.

• It acts as a shield to radiated noise coming from underneath the board.

WHAT IS GROUND PLANE IN PCB DESIGN ?

In this tutorial we are going to learn about WHAT IS GROUND PLANE IN PCB DESIGN ?

Good grounding is a system level design consideration. Proper grounding should be planned into the

product from the first conceptual design reviews.

Separate grounding for analog & digital portions of circuitry is one of the simplest & most effective

methods for noise suppression. One or more layers on multi-layer PCB’s are usually devoted to GND

plane. If PCB designer will not take carefull then  the analog circuitry will be connected directly to these “GND” planes. Auto-routers respond accordingly–& connect all of the GNDs together, creating a disaster. GND & PWR planes are at the same AC potential, due to decoupling capacitors & distributed capacitance. Therefore, its important to isolate the PWR planes as well. Don’t overlap digital & analog planes. Place analog PWR coincident with analog GND, & digital PWR coincident with DGND. If any portion of analog & digital planes overlap, then generated capacitance between the overlapping portions will couple high- speed digital noise into the analog circuitry. This defeats the purpose of isolated planes.

“Separate GNDs” doesn’t mean that the GNDs are electrically separate in the system. They have to be common at some point, preferably a single, low impedance point. System-wise, there is only one GND – the electrical safety GND in an AC powered system or battery GND in a DC powered system. Everything else “returns” to that GND. All “returns” should be connected together at a main point, which is system “GND”. At some point, this will be the chassis. It is important to avoid GND loops by multiple connections to the chassis. Insuring only one chassis GND point is one of the most difficult aspects of system design. If at all possible, dedicate separate connector pins to separate returns, & combine the returns only at system GND. Aging & repeated mating causes connector pins to increase in contact resistance, so several pins are needed. Many digital boards consist of many layers & 100s or 1000s of nets. The addition of one more net is seldom an issue, but the addition of a several connector pins almost always is. If this can’t be done, then it will be necessary to make the 2 returns a single net on the PCB – with very special routing precautions. In PCB design should be keep analog traces short, Isolate Plane  & place passive components carefully if there are high-speed digital traces running right next to the sensitive analog traces. Digital signals must be routed around analog circuitry, & not overlap AGND & power planes. Most digital clocks are high enough in frequency that even small capacitances between traces & planes can couple significant noise. Remember that it is not only the fundamental frequency of the clock that can cause a potential problem, but also the HF harmonics. Genrally locate analog circuitry as close as possible to the I/O connections of the PCB. Digital designers, used to high current IC’s, will be tempted to make a 50 mil trace run several inches to the analog circuitry,  Skinny capacitor that couples noise from digital ground & power planes into the opamp, making the problem worse! one entire side of a PCB (or one entire layer, in the case of a multi-layer PCB) consists of continuous copper which is used as GND this is known as a “GND plane.” It will have the least possible resistance & inductance of and GND configuration. If a system uses a GND plane, it is less likely to suffer GND noise problems