SN65HVD78: Impedance of "R" pin and capacitance of "D" pin

Ted Dager

Part Number: SN65HVD78
Other Parts Discussed in Thread: THVD2450

the SN65HVD78 and similar parts like (THVD2450) have the following structure on the Receive pin (datasheet page 16)

I would like to know the impedance of the output stage ? or how to calculate it from rise time and fall time.

I would also like to know the capacitance of the Input pin "D" or how to calculate from the datasheet parameters.

The reason I need this information is that i want to place a series resistor on this "R" pin to match it to the pcb track impedance simlar to the following figure:

best regards

over 3 years ago

0 Eric Hackett over 3 years ago

TI__Guru 60242 points

Ted,

The RS485 expert in our group has been notified of this post and will respond accordingly.

Regards,

Eric Hackett

+1 Hao L over 3 years ago

TI__Mastermind 25705 points

The receiver output is a rail to rail buffer, like in the diagram you showed. Therefore the impedance varies with the output voltage. You only treat the trace as transmission line if the signal propagation time is 2 to 6 times longer than the rise time. To calculate the impedance, you can use (Vcc-Voh)/I or (Vol-GND)/I.

For the same reason, the D pin capacitance may not be critical to signal integrity either. BTW, which device do you want to know the D pin capacitance, SN65HVD78 or THVD2450?

Regards,

0 Ted Dager over 3 years ago in reply to Hao L

Prodigy 60 points

Thank u , this has cleared the issue . So at 3.3v impedence is (3.3-0.2)/8mA = 37.5 ohm

As for your question : I wanted to know the capacitance of D pin on THVD2450 .

0 Hao L over 3 years ago in reply to Ted Dager

TI__Mastermind 25705 points

The capacitance of the D pin is about 220pF to ground.

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SN65HVD78: Impedance of "R" pin and capacitance of "D" pin