Lowering Power Consumption in Digital Interfaces

Signal and Power Impedance
1

I know why we match the impedance while terminating a transmission line. However, digital interfaces use quite low impedance transmission lines like 100 Ohm in Ethernet communication.
Is it possible to lower the power consumption of digital interfaces by employing high impedance transmission lines with high impedance terminations? I haven’t come across any examples of this approach, leading me to question its viability.

2

I imagine so. Since it probably wouldn’t work you should save some current, Would be interesting to analyze, but if you haven’t seen examples of it there’s likely a reason.

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3

Sounds risky to me and should be simulated before implementation.

If this were a good idea, there would be people presenting that idea in seminars. I have never seen anybody suggest that to reduce power consumption.

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4

In theory, higher impedances would reduce power dissipation for the same voltage swing, but there are several practical issues to consider:

  1. Signal to Noise Ratio: It’s the power of a signal, not the voltage, that determines the signal to noise ratio. If you need to swing the full rail, higher impedance helps. However, if you’re dealing with a specific power, low impedance isn’t a problem—just reduce the swing.
  2. Manufacturability: Achieving impedances above 100 ohms on a board is impractical. The signal conductor would need to be extremely thin, and the spacing to the ground plane would be too large. Impedance increases with the log ratio of spacing to center, so you quickly reach a limit.
  3. Conductor Size: A thicker conductor is preferred for manufacturing and performance reasons. Copper losses decrease with larger conductor surface area, and 75 ohms offers the lowest loss geometry. For high power handling, around 35 ohms is ideal. The standard 50 ohms is a compromise between these criteria.
  4. High-Speed Detectors: In high-speed detectors, input impedance is critical. Lower impedance lines are easier to manage, and high impedance lines are difficult to achieve both on the board and within receiver ICs for similar geometrical reasons.

These practical considerations are why lower impedances are often used despite the theoretical benefits of higher impedances.