LMH0303, LMH0384 / about termination

1) The unused LMH0303 cable driver output should NOT be left open.  The unused output (/SDO) is the complement to the used output (SDO), and so it should be terminated similarly to the true output to ensure that the differential output driver is balanced.  To accomplish this, the unused output should have the 75 ohm pullup resistor, followed by the AC coupling capacitor, then followed by a 75 ohm resistor to ground.  The return loss compensation network (resistor in parallel with inductor) may or may not be included (it is not absolutely necessary on the unused output).

2)  If you use only the 75 ohm termination on the SDI input as shown in your PDF, the LMH0384 will still be able to equalize according to the datasheet; the equalizer performance will not be reduced.  However, this 75 ohm resistor in parallel with the 5.6 nH inductor that you are proposing to remove constitutes the return loss compensation network.  Without this network, this input will not be able to meet the SMPTE input return loss requirements.

3)  See the figure below for an example interface with the LMH0303 driving 100 ohm differential cable to the LMH0384.  Note that the LMH0303 R_REF resistor needs to be changed to 953 ohms in this example to configure the LMH0303 to drive 800 mV_P-P differential (400 mV_P-P single ended) into the 100 ohm differential cable.

Gary Melchior

I do not see your attached PDF, but I have attached the recommended circuit for the LMH0303 with a single-ended output configuration:

The 75Ω resistor in parallel with the 6.8 nH inductor in series with the LMH0303 output is the return loss compensation network, which is necessary in order to meet the SMPTE return loss requirements for input/output ports.  Essentially, to meet these return loss requirements, the impedance looking back into the circuit from the BNC must be close to 75Ω.  This network compensates for the output capacitance of the driver circuit to maintain this impedance across the required frequency range. 

It is not necessary to meet this return loss requirement for unused ports, which is why this network may be left out.  The return loss network may optionally be included on unused outputs in order to provide 100% symmetry between the true output (SDO) and its complement (SDO/).  It does not hurt to include it, but the effects on the active output (SDO) are negligible either way.

Gary Melchior