EMC Question of the Week: January 2, 2023
Ideally, for EMC and signal integrity, digital signals should have matched terminations when the propagation delay is large compared to
- the transition time
- the bit width
- 1 nanosecond
- 1 picosecond (i.e., always)
Answer
The best answer is “a.” Although specific design advice varies, matched terminations and controlled impedance transmission lines are usually required when the propagation delay is greater than 1/6 to 1/2 the transition time of the digital signal. The bit width is not considered. (Although obviously, if the propagation delay is greater than the bit width, it is much greater than the transition time.)
One nanosecond is not a good rule of thumb, because it would require matching virtually all signals traveling more than 10-30 cm, even at slow data rates with controlled transition times.
It's also not correct to say that matching is always desirable from an EMC and signal integrity standpoint. Matched transmission lines generally consume more power, limit design options, and cost more. They may also convey noise more readily and be more difficult to filter.
In situations where the propagation delay is much greater than the transition time, the first option to consider is whether the transition time can be increased. This option reduces the bandwidth of the signal and can significantly reduce the power bus noise. Matching is the option of last resort, but it is often necessary when sending high-speed signals significant distances.
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