SGM9114 Datasheet, PDF (8/11 Pages) SG Micro Corp – Video Driver with 6dB Gain

SGM9114
APPLICATION INFORMATION
8MHz, 5th-Order Video Driver
with 6dB Gain
IN
RIN
V_CC = 3.0V to 5.5V
To External Power
Supply or Control I/O
0.1µF
CIN
10kΩ
VCC
5th-Order
Filter
Level
Shifter
DISABLE
OUT
6dB
SAG
SGM9114
GND
Figure 2. AC-Coupled Input/DC-Coupled Output
R_OUT
75Ω _{75Ω RL}
SAG Correction
The SGM9114 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 220µF. In SAG
configuration, the SGM9114 removes the need for large
coupling capacitors, and instead only requires one 22µF
and one 47µF capacitors (Figure 1) to achieve the same
performance as the large capacitor.
The Sallen Key Low Pass Filter
The Sallen Key in a classic low pass configuration is
illustrated in Figure 1. The filter provides a very stable
low pass function, and in the case of the SGM9114, a
5th-order roll-off at around 8MHz. The 5th-order 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 produce a nice 5th-order roll-off at around
8MHz.
Layout and Power-Supply Bypassing
The SGM9114 operates from single 3.0V to 5.5V supply.
Bypass the supply with a 0.1µF capacitor as close to the
pin as possible. It is recommended to use 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.
SG Micro Corp
8
www.sg-micro.com