GS9068 SD SDI Cable Driver

Data Sheet

22213 - 4

August 2010

6 of 11

3. Detailed Description

3.1 Serial Digital Input

SDI/SDI are high-impedance differential inputs. Several conditions must be observed

when interfacing to these inputs:

1. The differential input signal amplitude must be between 300 and 2000mVpp.

2. For DC-coupling to the device, the common mode voltage must be between

3. For input trace lengths longer than approximately 1cm, the inputs should be

terminated as shown in the Typical Application Circuit.

The GS9068 inputs are self-biased, allowing for simple AC-coupling to the device. For

serial digital video, a minimum capacitor value of 4.7μF should be used to allow

coupling of pathological test signals. A tantalum capacitor is recommended.

3.2 Serial Digital Output

The GS9068 outputs are current mode, and will drive 800mV into a 75Ω load. These

outputs are protected from accidental static damage with internal static protection diodes.

The SMPTE 259M standard requires that the output of a cable driver have a source

impedance of 75Ω and a return loss of at least 15dB between 5MHz and 540MHz. In order

for an SDI output circuit using the GS9068 to meet this specification, the output circuit

shown in the Typical Application Circuit is recommended.

A 4.7μF capacitor is used for AC-coupling the output of the GS9068. This value is chosen

to ensure that pathological signals can be coupled without a significant DC component

occurring (see Section 4. Application Reference Design, for more details).

When measuring return loss at the GS9068 output, it is necessary to take the

measurement for both a logic high and a logic low output condition. This is because the

output protection diodes act as a varactor (voltage controlled capacitor) as shown in

Figure 3-1. Consequently, the output capacitance of the GS9068 is dependent on the

logic state of the output.

Figure 3-1: Static Protection Diodes

SDO

SDO