Termination choices
Compare series, parallel, Thevenin, AC (RC) and differential termination.
| Level | ●● onsite technical |
| Track | SI & high-speed |
| Really testing | Picking termination by topology and power. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
Worked example — power cost. A 3.3 V line held high with 50 Ω to ground: 66 mA and 0.22 W per line. Thevenin 100 Ω/100 Ω (50 Ω equivalent) draws 16.5 mA all the time and makes the driver source or sink 33 mA. A 50 Ω to VTT = 1.65 V takes ±33 mA — and the VTT supply must both source and sink (that is why DDR uses special VTT regulators).
Worked example — AC termination. For a 2 ns line, C ≈ 3·TD/Z0 = 120 pF. Smaller and the termination doesn't work; much larger and long runs of 1s or 0s shift the DC level.
Choosing, in one line each.
- Point-to-point CMOS → series.
- Multi-drop or bidirectional → end termination (to VTT if power matters).
- Clock with a long line → series, or AC if the edge must be clean everywhere.
Where people go wrong: series-terminating a multi-drop bus — the middle receivers see the half-step shelf.
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