SGM9110 Datasheet, PDF (8/10 Pages) SG Micro Corp – 8MHz Rail-to-Rail Composite Video Driver with 6dB Gain

+V_S
+V_S
+V_S
C₃
0.1µF
IN
IN
R_IN ^CIN
SYNC CLAMP
V_DC
R₁
R₂ R₃
C₁
C₂
SALLEN KEY LOW
PASS FILTER
DISABLE
GND
DISABLE = GND: SHUTDOWN I+ ~ 0
DISABLE = +VS: ACTIVE I+ ~ 2.65mA
OUT
R₄
SAG
R₅
R₆
R₇
SAG NETWORK
Figure 2. AC-Coupled Input/DC-Coupled Output
R_OUT
75Ω RL
75Ω
SAG Correction
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 Sallen Key Low Pass Filter
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.
Layout and Power-Supply Bypassing
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
and inductance.
• Do not use wire-wrap boards; they are too inductive.
• Use surface-mount instead of through-hole components for
better, high-frequency performance.
• Use a PC board with at least two layers; it should be as free
from voids as possible.
• Keep signal lines as short and as straight as possible. Do not
make 90° turns; round all corners.
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SGM9110