+VS
+VS
+VS
C3
VDC
0.1µF
IN
IN
ROUT
OUT
SAG
R2 R3
C1
R1
CIN
RIN
RL
75Ω
R4
C2
75Ω
R5
SALLEN KEY LOW
PASS FILTER
R6
R7
SYNC CLAMP
= GND: SHUTDOWN I+ ~ 0
= +VS: ACTIVE I+ ~ 2.65mA
DISABLE
DISABLE
DISABLE
GND
SAG NETWORK
Figure 2. AC-Coupled Input/DC-Coupled Output
SAG Correction
Layout and Power-Supply Bypassing
The SGM9110 can use the SAG configuration if an
AC-coupled output video signal is needed. SAG correction
refers to the low-frequency compensation for the high pass
filter formed by the 150Ω load and the output capacitor. In
video applications, the cutoff frequency must be low enough to
pass the vertical sync interval to avoid field tilt. This cutoff
frequency should be less than 5Hz, and the coupling capacitor
must be very large in normal configuration, typically is 220µF.
In SAG configuration, the SGM9110 removes the need for
large coupling capacitors, and instead needs one 22µF and
one 47uF capacitors (Figure 1) to reach the same performance
as the large capacitor.
The SGM9110 operates from single 2.5V to 5.5V supply.
Bypass the supply with a 0.1µF capacitor as close to the pin as
possible. SGMC recommends using microstrip and stripline
techniques to obtain full bandwidth. To ensure that the PC
board does not degrade the device’s performance, design it for
a frequency greater than 1GHz. Pay careful attention to inputs
and outputs to avoid large parasitic capacitance. Whether or
not you use
a constant-impedance board, observe the
following design guidelines:
• Do not use IC sockets; they increase parasitic capacitance
•aDnodninodt uucsteanwciree.-wrap boards; they are too inductive.
The Sallen Key Low Pass Filter
• Use surface-mount instead of through-hole components for
better, high-frequency performance.
The Sallen Key in a classic low pass configuration illustrated in
Figure 1. The filter provides a very stable low pass function,
and in the case of the SGM9110, a 3-pole roll-off at around
8MHz. The 3-pole function is accomplished with an RC low
pass network placed in series with and before the Sallen Key.
One pole provided by the RC network and poles two and three
provided by the Sallen Key for a nice 3-pole roll-off at around
8MHz.
• Use a PC board with at least two layers; it should be as free
•fKroemepvsoiigdnsaalslinpeosssaisblseh.ort and as straight as possible. Do not
make 90° turns; round all corners.
８
SGM9110
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
