+VS
+VS
+VS
C3
VDC
0.1µF
IN
IN
OUT
ROUT = 75Ω
R2 R3
C1
R1
CIN
RIN
R4
RL = 75Ω
C2
R5
SALLEN KEY LOW
PASS FILTER
R6
R7
SYNC CLAMP
GND
SAG NETWORK
Figure 2. AC-Coupled Input/DC-Coupled Output
a frequency greater than 1GHz. Pay careful attention to inputs
and outputs to avoid large parasitic capacitance. Whether or
The Sallen Key Low Pass Filter
not you use
a constant-impedance board, observe the
following design guidelines:
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 SGM9111, 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.
• Do not use IC sockets; they increase parasitic capacitance
•aDnodninodt uucsteanwciree.-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
•fKroemepvsoiigdnsaalslinpeosssaisblseh.ort and as straight as possible. Do not
make 90° turns; round all corners.
Layout and Power-Supply Bypassing
The SGM9111 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
７
SGM9111
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
