8MHz, 5th-Order Video Driver

SGM9114

with 6dB Gain

8

SG Micro Corp

www.sg-micro.com

APPLICATION INFORMATION

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.

OUT

SAG

SGM9114

5th-Order

Filter

6dB

Level

Shifter

IN

GND

0.1µ

F

75

Ω

75

Ω

10k

Ω

To External Power

Supply or Control I/O

DISABLE

Figure 2. AC-Coupled Input/DC-Coupled Output