Soo.. I've been testing my designing limits, trying to learn about Serial Digital Interface, and building a simple reclocker with TI IC's. And now I've run into a small speedbump with the output from the cable driver IC, LMH0307.

As you can see in the picture below (Taken from the LMH0307 datasheet), both SDO and it's complement !SDO are apparently being connected to coaxial cables and transmitted. This seems really weird to me, as SDI is designed for transmitting over a single cable. And as far as I know, it's not supposed to be connected to the shield, because that's straight-forward GND or floating on one side (again, AFAIK), and !SDO is not the same as a return path for SDO.

LMH0307 output

And when I look at the input of the cable equalizers in the same series (the LMH0394 etc, see below), I see that !SDI is connected to GND, through a cap and a resistor. In other words, this IC does not bother with the complement input at all.

LMH0394 input

So, what is the best practice on terminating the complement output from the cable driver, when I obviously can't/shouldn't do what's in the datasheet?

-- Thanks, Araho


2 Answers 2


I've used this driver (actually, the LMH0302). Terminate your unused output with 75ohms to Vcc and 4.7uF in series with 75 ohms to Gnd.

Like so:

enter image description here

Source: TI AN1972


If you do a single-ended "receive" with the LMH0394, your signal to noise ratio will be impaired - you're only potentially receiving half the signal because you have /SDI grounded via the capacitor and 37.4 ohm resistor. Also any noise "hitting" the cable will unbalance the single-ended transmission.

Whereas if you used SD and its inverse for tx and rx you get twice the signal and common mode noise is more likely to affect both paths equally and thus cancel out when received. This means using two coax cables or one shielded twisted pair cable.

  • \$\begingroup\$ SMPTE-signals like HD-SDI and 3G-SDI are designed to run broadcast signals over one cable only. Even though this means all non-PCB-trace transmissions will be single-ended, it's really the only way to do it if I want to follow the standards and be compatible with other products, unless I'm missing something crucial here? \$\endgroup\$
    – Araho
    Sep 21, 2013 at 16:01
  • \$\begingroup\$ @Araho I'm not saying you need to run them over two cables, I'm just pointing out what the improvement might be in a "noisy" environment and why some of the chip suppliers will offer this option. \$\endgroup\$
    – Andy aka
    Sep 21, 2013 at 16:05

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