Eric Bogatin is an expert on signal integrity, with over thirty years of experience under his belt. He was kind enough to answer a few questions at this year’s DesignCon.
What is the difference between signal integrity discipline for analog circuits and digital circuits?
Analog and digital circuits serve different purposes—and have different data rates and power speeds. As such, the issues in relation to their circuits are also different. Frequency components and interconnect properties are becoming higher and faster than traditional analog electronics can handle. As Bogatin notes, we’re “pushing the envelope of analog signal propagation interconnects to a much higher end.”
What is the relation between signal integrity and EMI?
While all aspects of the PCB board are related in some way, signal integrity and EMI have a particularly tenuous relationship. Signal integrity expands to power integrity, and then to EMI. They are, however, all different disciplines. When signal integrity is bad, it leads to more EMI problems. It’s all about “the physical properties of the interconnects; layer count, spacing, materials, etc.” Electromagnetic interference, or EMI, must work alongside signal integrity, but has its own certification test. Unfortunately, stable signal integrity does not guarantee passing of the certification test.
What are best practices for designing ground and power distribution for good signal integrity?
Bogatin has five essential principles for good signal integrity; number four covers this topic. Best practices include the signal path return path. “The vast majority of problems today are because of poor return paths,” states Bogatin. Return paths should be well-planned. Impedance between power and ground layers should be as low as possible. Three ways to avoid return path issues: avoid the problem at the start; put a band-aid on it; and sandwich it between ground planes.
Are there any simple rules for estimating crosstalk between two coplanar conductors?
This is one of six problems addressed in his book, “Signal Integrity—Simplified.” The return path dramatically influences the amount of crosstalk, and a 2D field solver can estimate how much crosstalk your PCB will have. As a rule of thumb, keep the spacing at least two line widths apart in critical lines; this keeps the crosstalk below 2%—the maximum amount of crosstalk that Bogatin says is safe.
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