SN65176B: SN65176B input impedance

Hi Marc,

No, the 16.8-kOhm resistance is present on each input (A and B) and is in series between the external pin and the rest of the internal circuitry. This resistance combines in series with the two 960-Ohm pull-up/pull-down resistances shown in the figure to give a nominal input impedance of 16.8 kOhm + (960 Ohm || 960 Ohm) = 17.28 kOhm.

Note, though, that the pull-up/pull-down resistances are placed in series with transistors. These paths will only conduct when the IC is powered, so if you try to measure the input impedance with the device unpowered you will measure something much higher. (Measuring the input impedance in a powered state can be tricky as well, since the active voltage biasing used by the IC may interfere with the resistance measurement of a multimeter. Another way to measure this would be to measure the voltage present at A or B when left open, then apply a known voltage to the pin through a known resistance and determine the current flowing into the pin.)

Yes, you are right that generally higher input impedance is better. Lower input impedance would mean that the driver would need to source more current to account for the additional leakage currents into each receiver.

Please let me know if this is unclear or if you have additional questions.

Regards,
Max