[k0stas]

> Their point about diff pairs being more resistant to noise is correct, but it’s not the primary reason for using diff pairs.

The article is correct. This comment is wrong.

> Differential signals, at a physical layer level, are faster than single ended IO. It’s because you have double the current drive/sinking capability with two drivers.

The load is also double in a diff pair compared to a single wire, so the net effect is a wash compared to a single wire.

> You’re also getting capacitive coupling between each leg of the pair working in your favor, which keeps your edge transition nice and fast.

The opposite is true. Differential capacitance effectively appears 2X higher than the nominal capacitance to differential signals, making it a drawback of differential signaling rather than a benefit.

[CamperBob2]

When you see a differential pair, simply imagine two independent lines that are each referenced to ground. (That's close to what happens on the PCB anyway, if a ground plane is present. Most of the return current ends up ground-referenced.) Two lines that would have exhibited 50-ohm characteristic impedance in a single-ended circuit will form a ~100-ohm diff pair.

In other words, the capacitance isn't doubled, since the capacitance is split by the imaginary ground between the two lines. It looks like two caps in series, not in parallel. Same is true for the load resistance.

[k0stas]

I have designed PCIe compatible transceivers and this will be the last comment I make about it because correcting hardware nonsense on HN is only of transient interest to me.

Everything I wrote is 100% correct and in fact incontrovertible. Whether there is a little or a lot of differential capacitance does not change the fact that the differential portion of the capacitance has a 2X effect on the differential signal (as opposed to the common-mode signal, which it has no effect on). This is supported by basic math.

If capacitance is to ground then it is not differential capacitance so it is not relevant to this discussion. It may be true that differential capacitance is not a significant contributor to the impedance of PCB differential traces but that does not change the fundamental result (similarly, the principle of photovoltaic conversion still holds true in the dark even though there is little light to convert). And PCB traces are not the only kinds of differential pairs. Diff pairs exist inside the integrated circuits that drive the PCBs where they operate less like transmission lines and more like lumped capacitances due to the frequencies of interest compared to the dimensions of the conductors. In these circuit and conductor structures, differential capacitance can be significant and this is what OP was talking about since he was talking about the legs of the driver (transistors). OP was just wrong about the differential capacitance being good for speed. It's bad for speed.