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LMH0001 and LMH0074 for differential cable transmission.

Kawai
Guru 19785 points
Other Parts Discussed in Thread: LMH0001, LMH0074

I have been reading the similar design information listed in this forum post.
http://e2e.ti.com/support/interface/high_speed_interface/f/138/p/206106/730943.aspx#730943

As I still cannot understand, please tell me how to design LMH0001 and LMH0074 for differential cable transmission.


1) What is the recommendation for RREF resistor of LMH0001 to connect to LMH0074 through UTP Cable(100ohm differential) ?

I understand that LMH0001 output pull-up resistors should be changed to 50ohm and LMH0074 schematic should be changed to Figure4 in Application Note 2002 (SLNA123).


2) Regarding 1) question, should the Launch Amplitude of LMH0001  be set to 800mVp-p differential signal into the cable with 50 ohm output pullup resistors ?


3) Please tell us how we should design the return loss network for UTP Cable.

  • 0
    TI__Expert 4400 points

    1) The LMH0001 RREF resistor should be 953 ohms, and the LMH0001 output pullup resistors should be 50 ohms.  This will drive 800 mVP-P differential into the 100 ohm differential cable so that it can be properly recovered by the LMH0074 equalizer on the other end.  The LMH0074 input network should indeed be configured as shown in Figure 4 of SNLA123.

    2) Correct; the LMH0001 launch amplitude should be set to 800 mVP-P differential into the cable with 50 ohm output pullup resistors by using an RREF resistor of 953 ohms as explained above.

    3) Is return loss compensation necessary for your application?  The LMH0001 cable driver and LMH0074 cable equalizer are typically used in (75 ohm) SMPTE applications which have strict requirements for the return loss.  This often necessitates the use of a return loss compensation network (typically an inductor in parallel with a resistor) to compensate for the input or output capacitance of the receiver or driver device.  The typical values for this return loss compensation network are shown in the LMH0001 and LMH0074 datasheets for 75 ohm applications.  For your 100 ohm differential cable application (non-SMPTE?), I would recommend to not use the return loss compensation network at all (leave it out) as it is not required for normal operation.

    If your application has specific requirements for the return loss, then I would expect that you would want to use a variation of our recommended 75 ohm return loss compensation network and fine tune it for your 100 ohm differential application.  But this is not necessary, and normally we do not see return loss compensation networks when these devices are used in non-SMPTE applications.