GS2989

Final Data Sheet

Rev.6

GENDOC-052133

July 2015

14 of 20

Semtech

www.semtech.com

4.7 Output Return Loss Measurement

The GS2989 has a feature which allows users to measure ORL reliably while the device is

still powered. The device can be put into a Balance mode which prevents the outputs

from toggling while the device is powered on, allowing the ORL to be measured while

the device is still powered.

When EQ_EN is LOW while SD/HD is HIGH, the device goes into Balance mode. This

mode is used during ORL measurement, disabling the AC signal path of the device

without powering it down. When in Balance mode, the device produces equal

pull-down currents in both differential shoulders of both serial data differential outputs,

effectively stopping all outputs at the output common mode voltage level. Semtech

recommends using Balance mode when measuring ORL with 2.5V termination voltage.

5. Application Information

5.1 PCB Layout

Special attention must be paid to component layout when designing serial digital

interfaces for HDTV.

An FR-4 dielectric can be used, however, controlled impedance transmission lines are

required for PCB traces longer than approximately 1cm. Note the following PCB artwork

features used to optimize performance:

•

The PCB trace width for HD rate signals is closely matched to SMT component

width to minimize reflections due to changes in trace impedance

•

The PCB ground plane is removed under the GS2989 output components to

minimize parasitic capacitance (Note: care should be taken, as removing too much

of the plane will make the system susceptible to EMI)

•

The PCB ground plane is removed under the GS2989 RSET pin and resistor to

minimize parasitic capacitance. The RSET resistor should be directly connected to

the VCC plane

•

Input and output BNC connectors are surface mounted in-line to eliminate a

transmission line stub caused by a BNC mounting via high-speed traces

Table 4-5: Typical RSET Values

Output Swing (mV)

RSET (

Ω)

1800*

332

800

750

500

1210

*Note: In order to generate output swings greater than 1040mV, VCC_TERM must be

connected to a 5V supply.