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Why are differential traces so important in LVDS?
Due to the high speed of LVDS, impedance matching is very important, even for very short runs. Any discontinuities in the differential LVDS traces will cause signal reflections, thereby degrading the signal quality. These discontinuities also increase the common mode noise and will be radiated as EMI.
What is the series impedance of a LVDS?
LVDS uses a 100ohms balanced termination resistance at the receiver, which for a differential signal is equivalent to each line having a 50ohm unbalanced load. The series impedance of C139x is completely negligible, and the shunt 500ohm impedance of two R115x in parallel is only 10% of the 50ohm load, so fairly negligible.
Can a PECL resistor reduce the LVDS swing?
If you do need to reduce the swing, small resistors in series with the PECL output is the right way to do it, they will pot down the signal into the transmission line (if any) that’s terminated with the LVDS termination resistor.
How is the differential impedance of a PCB controlled?
If there is more than 2-cm distance between the connector and the receiver input pins, the PCB must beconstructed to maintain a controlled differential impedance near 100 Ω. The physical length of each trace between the connector and the receiver inputs should be matched towithin 5 mm of each other. This may require mitering of the traces.
Where does the impedance discontinuity in Toradex come from?
The Toradex source you cite mentioned an impedance discontinuity, and EMC compliance. The impedance discontinuity comes from the fact that, if there is a via between traces, the traces are at first have a capacitive coupling with each other, then that coupling is removed and replaced with the via, then they couple together again.
Is it bad to have 6 layers of differential signal?
The short answer is that I’d argue that a spacing change near the start or end point of a differential signal is not that bad. I’d also argue that 6 layers is not that many. But at high speeds, definitely keep all noise sources away from the clock. For the longer answer, let’s look at the reasons given.
Can you couple two signals into a differential pair?
To go a bit more into coupling, in the ideal case, if you couple the same signal into a differential pair, you would prefer to couple into both. Doing that would bump them both by a few mV, and the differential signal (Vp – Vn) would be unaffected. As long as the absolute voltages of each signal are within spec, you should be fine.