GS1560A/GS1561 Data Sheet
3.4 Serial Digital Loop-Through Output (GS1560A only)
The GS1560A contains an integrated current mode differential serial digital cable
driver with automatic slew rate control. When enabled, this serial digital output
provides an active loop-through of the input signal.
To enable the loop-through output, SDO_EN/DIS must be set HIGH by the
application layer. Setting the SDO_EN/DIS signal LOW will cause the SDO and
SDO output pins to become high impedance, resulting in reduced device power
consumption.
With suitable external return loss matching circuitry, the GS1560A's loop-through
outputs will provide a minimum output return loss of -15dB at SD rates. Gennum
recommends using the GS1528 SDI Dual Slew-Rate Cable Driver to meet output
return loss specifications at HD rates.
The integrated cable driver uses a separate power supply of +1.8V DC supplied via
the CD_VDD and CD_GND pins.
3.4.1 Output Swing
Nominally, the voltage swing of the serial digital loop-through output is 800mV
p-p
single-ended into a 75Ω load. This is set externally by connecting the RSET pin to
CD_VDD through 281Ω.
The loop-through output swing may be decreased by increasing the value of the
RSET resistor. The relationship is approximated by the curve shown in Figure 3-1.
Alternatively, the serial digital output can drive 800mVp-p into a 50Ω load. Since
the output swing is reduced by a factor of approximately one third when the smaller
load is used, the RSET resistor must be 187Ω to obtain 800mVp-p.
1000
900
800
700
600
500
400
300
300
600
250
450 500 550
650 700
350 400
750
RSET(Ω)
Figure 3-1: Serial Digital Loop-Through Output Swing
27360 - 8 September 2005
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GENNUM [ GENNUM CORPORATION ]
