I'm using Ethernet (CAT5/5e/6) cables to communicate with the PCA9615 Differential I2C buffer. These cables roughly have a characteristic impedance of 100 ohms, but I have a few questions about choices of terminating resistors.
The datasheet for the IC gives an example of how to terminate one of the twisted pair ends. In there, they chose a combination of 600 ohms and 120 ohms for their resistors with the claim that the parallel combination yields a termination of 100 ohms at each end of the twisted pairs. Sparkfun also made a breakout board, but their choices were 390 and 100 ohms, so there's a slight difference. From what I've researched, the resistor between the two differential pins is the one that terminates the network while the other resistors act as pull-up resistors to VCC and/or ground. If we have the same terminating resistor on both sides of the cable, wouldn't the driving end see a parallel combination of the two (so in the image, the IC would see 60 ohms)? Wouldn't it be better to double it to match the impedance of the CAT5 cable?
Also, if some twisted pairs are not being used, it seems to be good practice to terminate them. However, if I'm using them for DC power and ground and not for signals, is it fine to not terminate them with a resistor?