GS2978 Datasheet, PDF (8/15 Pages) Gennum Corporation – Dual coaxial cable driving outputs with selectable slew rate

4. Detailed Description
4.1 Input Interfacing
DDI/DDI are high impedance differential inputs. The equivalent input circuit is shown in
Figure 3-1.
Several conditions must be observed when interfacing to these inputs:
• The differential input signal amplitude must be between 100 and 2200mVpp.
• The common mode voltage range must be as specified in the DC Electrical
Characteristics on page 5.
• For input trace lengths longer than approximately 1cm, the inputs should be
terminated as shown in the Typical Application Circuit.
The GS2978 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.
SD/HD Input Pin
The GS2978 SDO rise and fall times can be set to comply with both SMPTE 259M/344M
and SMPTE 424M / SMPTE 292M. For all SMPTE 259M standards, or any data rate that
requires longer rise and fall time characteristics, the SD/HD pin must be set HIGH by the
application layer. For SMPTE 424M and SMPTE 292M standards and signals which
require faster rise and fall times, this pin should be set LOW.
4.2 Output Interfacing
The GS2978 outputs are current mode, and will drive typically 800mV into a 75Ω load.
These outputs are protected from accidental static damage with internal ESD protection
diodes.
In order for a DDI output circuit using the GS2978 to meet this specification, the output
application circuit shown in Typical Application Circuit on page 10 is recommended.
The value of L_COMP will vary depending on the PCB layout, with a typical value of 5.6nH.
A 4.7μF capacitor is used for AC coupling the output of the device. This value is chosen
to ensure that pathological signals can be coupled without a significant DC component
occurring. Please see Application Information on page 10 for more details.
GS2978
HD-LINX® III Multi-Rate Dual Slew-Rate Cable Driver
Data Sheet
37186 - 7
March 2009
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