GS1508 Gennum Corporation, GS1508 Datasheet - Page 4

• Manufacturer Part Number: GS1508
• Description: Hd-linx(tm) HDTV Cable Driver
• Manufacturer: Gennum Corporation

DETAILED DESCRIPTION
DETAILED DESCRIPTION
DETAILED DESCRIPTION
DETAILED DESCRIPTION
INPUT INTERFACING
SDI/SDI are high impedance differential inputs. (See
Figure 1 for equivalent input circuit).
Several conditions must be observed when interfacing to
these inputs:
1. The differential input signal amplitude must be between
2. The common mode voltage range must be as specified
3. For input trace lengths longer than approximately 1cm,
The GS1508 inputs are self biased, allowing for simple AC
coupling to the device. For serial digital HDTV, a minimum
capacitor value of 4.7µF should be used to allow coupling
of pathological test signals. A tantalum capacitor is
recommended.
OUTPUT INTERFACING
The GS1508 outputs are current mode, and will drive
800mV into a 75 ohm load. These outputs are protected
from accidental static damage with internal static protection
diodes.
The SMPTE 292M standard requires that the output of a
cable driver have a source impedance of 75
loss of at least 15dB between 5Mhz and 1.485GHz.
In order for an HD SDI output circuit using the GS1508 to
meet this specification, the output circuit shown in the
Typical Application Circuit is recommended.
The values of L
layout, but typical values are 12nH and 0.5pF respectively.
The small value of C
parasitic PCB capacitance, so it is recommended that the
ground plane be removed under the GS1508 output
circuitry (see the Application Information section in this data
sheet for further details). A 4.7µF capacitor is used for AC
coupling the output of the GS1508. This value is chosen to
ensure that pathological signals can be coupled without a
significant DC component occurring.
522 - 02 - 04
150 and 800mVpp.
in the DC Characteristics table. For a 400mV input from
the GS1504 HD cable equalizer, this corresponds to a
common mode voltage range of between 2.7 and 4.8
volts.
the inputs should be terminated as shown in the Typical
Application Circuit.
COMP
and C
COMP
COMP
can be easily swamped by
will vary depending on PCB
and a return
4
Figures 3 and 4 show the eye diagram of the GS1508
output for two different output compensation conditions.
Figure 4 shows the GS1508 output optimized for wideband
return loss of 18dB. Figure 3 shows the GS1508 output
optimized for output waveform symmetry. In this case the
output may not exhibit a return loss of 15dB.
When measuring return loss at the GS1508 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 5.
Consequently, the output capacitance of the GS1508 is
dependent on the logic state of the output.
Fig. 3 Output Optimized for Waveform Symmetry
L=12nH, C=0.5pF, R=75 (Typical ORL=18dB)
Fig. 4 Output Optimized for Return Loss
L=8.2nH, R=75 (Typical ORL=13.5dB)