3.4 OUTPUT AMPLITUDE ADJUSTMENT
The output amplitude of the GS9068 can be adjusted by
changing the value of the RSET resistor as shown in Figure 2
and Table 1 below. For an 800mVp-p output with a nominal
7% tolerance, a value of 750Ω is required. A 1% SMT
resistor should be used.
The RSET resistor is part of the high speed output circuit of
the GS9068. The resistor should be placed as close as
possible to the RSET pin. In addition, the PCB capacitance
should be minimized at this node by removing the PCB
groundplane beneath the RSET resistor and the RSET pin.
Table 1: RSET vs VOD
1100
1000
900
RSET (Ω)
995
OUTPUT SWING
608mV
824
734mV
750
800mV
800
600
884mV
700
600
573
1040mV
500
600
700
SET
800
900
1000
NOTE: For reliable operation of the GS9068 over the full
temperature range, do not use an RSET value below 573Ω.
R
(Ω)
Fig. 2 Output Amplitude Adjustment
4. APPLICATION REFERENCE DESIGN
4.1 PCB LAYOUT
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 ground plane is removed under the GS9068
SET pin and resistor to minimize parasitic capacitance.
R
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 which are curved
to minimize impedance variations due to change of PCB
trace width.
•
The PCB groundplane is removed under the GS9068
output components to minimize parasitic capacitance.
4.2 TYPICAL APPLICATION CIRCUIT
*
5.6n
BNC
75
4u7
4u7
GS9068
VCC
49.9
49.9
1
2
3
8
7
6
75
75
SDI
SDI
VEE
SDO
SDO
NC
10n
10n
DIFFERENTIAL
DATA INPUT
4
5
RSET
VCC
75
*
5.6n
BNC
4u7
4u7
TYPICAL VALUE VARIES WITH LAYOUT
750
VCC
VCC
*
10n
NOTE: All resistors in Ohms, capacitors in Farads,
and inductors in Henrys, unless otherwise noted.
6 of 8
GENNUM CORPORATION
22213 - 2
GENNUM [ GENNUM CORPORATION ]
