Signal Integrity Issues And Pcb Design

The Importance of Impedance Control in PCB design (and how to avoid Signal Integrity problems)
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Signal Integrity Design Considerations for High Speed Design Exceeding these guidelines can lead to serious ground bounce problems. Every signal trace should maintain the same impedance no matter where it goes. Signal traces may be of any practical impedance value 40 ohms to ohms is common. The same design may have signal traces with a variety of different impedance values. However, a single trace should not change impedance values over its length.

For example, if a trace starts on one board layer and switches to another layer, the designer must ensure that the trace on the second layer has the same impedance as the first. If the layers are different distances from their respective reference planes, the widths of the signal traces should be adjusted accordingly.

In general, if the distance to the reference plane is increased, the trace width must also be increased in order to maintain the same impedance. For more information on transmission-line effects and simulation, see the text references at the end of this document. A simulated transmission line that exhibits ringing or overshoot indicates an unacceptable amount of signal reflection. Signal reflection occurrs when a signal wave encounters an impedance discontinuity. To repair the ringing or overshoot, you must eliminate the impedance discontinuity in one of three ways:. For further information on termination, see our resources page.

Clock signals require special attention for two reasons. First, it is critical that their timing not be marginalized by noise - this can lead to false clocking of data. Second, clock signals often run at a higher frequency than data; they can be more troublesome as noise sources. Pay attention to any traces running in parallel for long distances.

Signal Integrity for High Speed Design

Simulate any suspicious traces using a PCB crosstalk simulation tool to determine if they will cause problems. If you confirm crosstalk as a problem, manage it by separating the traces or decreasing their distance from the associated reference plane decrease dielectric thickness.

How to avoid crosstalk and signal integrity issues on your PCB

XPower is part of the design flow. These tools provide a guideline for power supply requirements and are essential for thermal planning. The power supplies should rise from less than 0. The rise should not be inhibited by a current trip, or current foldback. Current limit behavior is acceptable based on the "Power-On Ramp Up Current Requirement" specification from the data sheet. The voltage rise vs. Avoid dwelling at a voltage, or having a 'plateau', even if it is acceptable power supply behavior.

If the voltage increases beyond the minimum operating voltage and then drops below it, you will experience incorrect power behavior. If the power supply voltage falls below the absolute minimum operating voltage when the device is off, it should not rise immediately back to the nominal operating voltage when turned on without first discharging to below 0. You may need a resistor to bleed off the filter and bypass capacitor charges to meet this condition.

Aside from meeting the dynamic power requirements determined by the Power Estimator , the power supply must also be able to supply the minimum specified startup current specified in the datasheet. Determine the die temperature using the power figure derived from the Power Estimator, information about the device package, and the maximum ambient temperature in the operating environment. Further information on thermal planning and management is located on page 1 of XAPP This enables debugging in the final system.

This is critical with BG and FG packages which have limited access to device pins. You may also provide ground and VCC pins in the header for convenience six pins. Xilinx - Adaptable. Make the connection of low-impedance vias to device GND pins very convenient. Provides a path for return currents. Every Signal Trace is Within One Signal Layer of a Continuous Reference Plane Every trace in the stackup should either be adjacent to a reference power or ground plane, or only separated from the closest reference plane by one signal layer.

High-frequency Capacitor Within 1 to 2 cm of Each VCC pin High-frequency bypass capacitors are the smallest capacitors in the bypassing network. Bypass Capacitor on Each Vref Pin Because of their high input impedance, Vref pins are succeptible to noise coupled in from surrounding signals. Simultaneous Switching Outputs. Signal Paths PCB traces and terminations. Each Trace has Constant Impedance Every signal trace should maintain the same impedance no matter where it goes.

Products as old as the Western Electric crossbar telephone exchange circa , based on the wire-spring relay, suffered almost all the effects seen today - the ringing, crosstalk, ground bounce, and power supply noise that plague modern digital products. On printed circuit boards, signal integrity became a serious concern when the transition rise and fall times of signals started to become comparable to the propagation time across the board. Very roughly speaking, this typically happens when system speeds exceed a few tens of MHz. At first, only a few of the most important, or highest speed, signals needed detailed analysis or design.

Lower Frequencies

As speeds increased, a larger and larger fraction of signals needed SI analysis and design practices. For ICs, SI analysis became necessary as an effect of reduced design rules. In the early days of the modern VLSI era, digital chip circuit design and layout were manual processes. The use of abstraction and the application of automatic synthesis techniques have since allowed designers to express their designs using high-level languages and apply an automated design process to create very complex designs, ignoring the electrical characteristics of the underlying circuits to a large degree.

However, scaling trends see Moore's law brought electrical effects back to the forefront in recent technology nodes. With scaling of technology below 0. As a result, the wire delays needed to be considered to achieve timing closure.

Signals & Signal Integrity In a Nutshell

T is the trace height or copper thickness. With higher clock rates and signal speeds in every digital design, attention to SI and PI is crucial to designing a well performing product. This can cause erroneous data or clocking pulses to appear, and these can be very difficult to track down in some circumstances. There are two main multipachage topologies: Tree and fly-by. Picking the right location and selecting the right number and value of the decoupling capacitors play an important role here.

In nanometer technologies at 0. At these technology nodes, the performance and correctness of a design cannot be assured without considering noise effects. Most of this article is about SI in relation to modern electronic technology - notably the use integrated circuits and printed circuit board technology. Nevertheless, the principles of SI are not exclusive to the signalling technology used. SI existed long before the advent of either technology, and will do so as long as electronic communications persist.

Signal integrity problems in modern integrated circuits ICs can have many drastic consequences for digital designs:.

PCB Signal Integrity | Simulation | Electronics Notes

The cost of these failures is very high, and includes photomask costs, engineering costs and opportunity cost due to delayed product introduction. Therefore, electronic design automation EDA tools have been developed to analyze, prevent, and correct these problems. In CMOS technologies, this is primarily due to coupling capacitance , but in general it may be caused by mutual inductance , substrate coupling , non-ideal gate operation, and other sources. In analog circuits, designers are also concerned with noise that arise from physical sources, such as thermal noise , flicker noise , and shot noise.

These noise sources on the one hand present a lower limit to the smallest signal that can be amplified, and on the other, define an upper limit to the useful amplification. In digital ICs, noise in a signal of interest arises primarily from coupling effects from switching of other signals. Increasing interconnect density has led to each wire having neighbors that are physically closer together, leading to increased crosstalk between neighboring nets.

As circuits have continued to shrink in accordance with Moore's law , several effects have conspired to make noise problems worse:. These effects have increased the interactions between signals and decreased the noise immunity of digital CMOS circuits. This has led to noise being a significant problem for digital ICs that must be considered by every digital chip designer prior to tape-out.

There are several concerns that must be mitigated:. Modern signal integrity tools for IC design perform all these steps automatically, producing reports that give a design a clean bill of health, or a list of problems that must be fixed.

Douglas Brooks

However, such tools generally are not applied across an entire IC, but only selected signals of interest. Each of these fixes may possibly cause other problems. This type of issue must be addressed as part of design flows and design closure. Re-analysis after design changes is a prudent measure. On-die termination ODT or Digitally Controlled Impedance DCI [4] is the technology where the termination resistor for impedance matching in transmission lines is located within a semiconductor chip, instead of a separate, discrete device mounted on a circuit board.

The closeness of the termination from the receiver shorten the stub between the two, thus improving the overall signal integrity.

For wired connections, it is important to compare the interconnect flight time to the bit period to decide whether an impedance matched or unmatched connection is needed. The channel flight time delay of the interconnect is roughly 1 ns per 15 cm 6 in of FR-4 stripline the propagation velocity depends on the dielectric and the geometry [5].

Reflections of previous pulses at impedance mismatches die down after a few bounces up and down the line i. At low bit rates, the echoes die down on their own, and by midpulse, they are not a concern. Impedance matching is neither necessary nor desirable. There are many circuit board types other than FR-4, but usually they are more costly to manufacture.

The gentle trend to higher bit rates accelerated dramatically in , with the introduction by Intel of the PCI-Express standard. Examples of mitigation techniques for these impairments are a redesign of the via geometry to ensure an impedance match, use of differential signaling , and preemphasis filtering, respectively.

In communication engineering this is called intersymbol interference ISI.