Sparkfun_QwiicBus_Endpoint Board
A question to the gurus in this forum:
The driver IC in the mentioned board is a PCA9615, driving the two pairs differential I2C extension lines, usually some twisted pair. The most common TP available on the market is the CAT5 -among some others-, with a line impedance around 100 Ohm. Now, if we look at the Sparkfun Schematic (rev. v11), we see a resistive line termination divider chain (390, 100, 390 Ohm) which results in an equivalent termination impedance of 66.1 Ohm, instead of the 100 Ohm nominal one.
Is there a special reason for this impedance mismatch? (Pulse reflection issues expected?) VSWR 1:1.5
The COM-16988 Description text is clear: …“” The differential signals are sent over an Ethernet cable, which attaches to the breakout through the on-board RJ-45 connectors. The differential signalling allows the I2C signals to reach distances of up to 100ft. while still maintaining their signal integrity!””…
I would change the 100 Ohm mid-resistors to 200 Ohm to get a closer value to 100 Ohm (~99) myself…
Regards.