I have a customer using the SN65LVDS1 to transmit a 12MHz clock along a backplane, one foot in length, that's terminated with 100 ohms. A parallel trace runs about half the distance along the backplane and is a single-ended 20Hz clock(3.3V). Significant coupling is observed between the 20Hz signal and the differential signals, resulting in a doubling of the amplitude on the differential signals during 20Hz transitions. An attached SN65LVDS2 receiver doesn't seem to subtract out this common voltage. Additionally, when testing the transmitter separately, the 12MHz square-wave input clock looks much more sinusoidal on the differential output when measured with the same high frequency probes. Is the behavior described above to be expected? Do we have any application guidelines or tips to insure they aree using these parts correctly? Do we have any tips, post-layout, that could help mitigate these effects?
Since coupling is dependent on the spacing between lines, we will need to know what the spacing is between the single-ended and differential signals in your customer's application.
If you can provide the layout files, as well as some more details about the backplane, such as the transmission line type (e.g. stripline, microstrip?), material (e.g. FR-4?), and geometry (e.g. trace width, trace spacing, etc.) on the backplane, we can review it in more detail.
We will also try to replicate your transmitter test in the lab and send you the results.
I have attached TI's HDMI design guide, which has a lot of guidelines that are relevant for any high speed application.
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