GS1508-CKA Просмотр технического описания (PDF) - Gennum -> Semtech

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GS1508-CKA

HDTV Cable Driver

Gennum -> Semtech 

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

150 and 800mVpp.

2. The common mode voltage range must be as specified

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.

3. For input trace lengths longer than approximately 1cm,

the inputs should be terminated as shown in the Typical

Application Circuit.

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.

Fig. 3 Output Optimized for Waveform Symmetry

L=8.2nH, R=75 (Typical ORL=13.5dB)

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 75Ω and a return

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 LCOMP and CCOMP will vary depending on PCB

layout, but typical values are 12nH and 0.5pF respectively.

The small value of CCOMP can be easily swamped by

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.

Fig. 4 Output Optimized for Return Loss

L=12nH, C=0.5pF, R=75 (Typical ORL=18dB)

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.

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