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I am designing a 4-layer PCB with 16 channels of LVDS (differential signalling, 480MHz). My layer stackup is Signal-GND-Power-Signal. A microstrip design for the LVDS channels is usually traces over a ground plane. However, I was wondering if it was possible to also have a controlled impedance differential pair on the OTHER signal layer, in which the adjacent copper plane is NOT ground, but in fact a power plane (in this case 3.3V).

If this is possible, would it be a good idea for some LVDS channels to run above this power plane, but then on the subsequent connector (That connects these LVDS channels to another board), that connector is referenced to ground (ground lines distributed throughout cable to minimize ground loops, instead of the power rail)?

cksa361
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1 Answers1

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Yes, it certainly is possible. What you do is have some decoupling caps between the two planes, near any place that the LVDS signals cross from one side to another. The idea here is to give a place for the AC signal return path to cross over from one plane to another.

Of course, being LVDS, most of the AC signal return current is on the other side of the differential pair-- but not all of it. LVDS signals are only "mostly balanced". The less balanced they are, the more return current that will flow on the power/gnd plane.

  • Spot on my advice, with only the note of, "try to avoid changing which plane you are on with signals, if one is going to be on one plane, run it on that plan as far as possible, even if it is to vias to the other chip pins with termination at that point also with its own vias. – Kortuk Feb 14 '12 at 05:46