Fundamentals of Signal Integrity

Higher data rates, timing issues, and other challenges continue to make signal and power integrity a key concern for chip and board designers.

As the name suggests, signal integrity deals with the integrity of an electrical signal. It all stems from the fact that digital signals are not really binary values of “1” or “0” but are analog voltage (or current) waveforms. As such, these waveforms are subject to the real-world, physical effects of noise, distortion, and loss. If the distances are short and at low bit rates, then a simple conductor will transmit a waveform with acceptable fidelity. However, at high bit rates and over greater distances or through different mediums, then several effects can degrade the electrical signal to the point where errors occur, data is compromised, and devices fail.

In practice, signal integrity consists of a set of measurements that determine the quality of a signal as a way to analyze and mitigate the effects of noise, distortion and loss. It is a set of design practices and test that address how the electrical properties of almost any interconnect cab mess-up the (relatively) pristine signals that come from integrated circuit chip and how these problems can be fixed. There are two common signal integrity electrical design concerns, namely, the timing and the quality of the signal. Does the signal reach its destination when it is supposed to? Is it in good condition when it gets there?

Electronic and electrical packages are full of interconnects that can affect signal integrity within a chip and throughout a printed circuit board (PCB). For example, consider the changes that a signal may experience when traveling through even a short connector. If there are instantaneous impedance changes, then some of the signal will reflect and the rest will probably have some distortion. In simple terms, there may be ringing in the circuit, often due to multiple reflections between impedance discontinuities at various interface ends.

Image Source: Design News / John Blyler

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