SGM9121_16 Datasheet, PDF (8/11 Pages) SG Micro Corp – 8MHz, 5th-Order Video Driver

SGM9121
APPLICATION INFORMATION
SAG Correction
The SGM9121 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 SGM9121 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 SGM9121, 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.
8MHz, 5th-Order Video Driver
with 6dB Gain
Layout and Power-Supply Bypassing
The SGM9121 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.
IN
R_IN
V_CC = 3.0V to 5.5V
To External Power
Supply or Control I/O
0.1µF
C_IN
10kΩ
V_CC
5th-Order
Filter
Level
Shifter
DISABLE
OUT
6dB
R_OUT
75Ω _{75Ω RL}
SGM9121
GND
SAG
Figure 2. AC-Coupled Input/DC-Coupled Output
SG Micro Corp
8
www.sg-micro.com