HD-SDI serial data output termination

Question

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.

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.

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

Answer 1

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:

Source: TI AN1972

Answer 2

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.