LT1204 LINER [Linear Technology], LT1204 Datasheet - Page 12

• Manufacturer Part Number: LT1204
• Description: 4-Input Video Multiplexer with 75MHz Current Feedback Amplifier
• Manufacturer: LINER [Linear Technology]

LT1204
A
When the decoder turns off the tee switch (Q2 on) the
emitter base junctions of Q1 and Q3 become reverse-
biased while Q2 emitter absorbs current from I
do the reverse-biased emitter base junctions provide good
isolation, but any signal at V
further attenuated by the shunt impedance of Q2 emitter.
Current from I
Crosstalk performance is a strong function of the IC
package, the PC board layout as well as the IC design. The
die layout utilizes grounds between each input to isolate
adjacent channels, while the output and feedback pins are
on opposite sides of the die from the input. The layout of
a PC board that is capable of providing – 90dB all hostile
crosstalk at 10MHz is not trivial. That level corresponds to
a 30 V output below a 1V input at 10MHz. A demonstra-
tion board has been fabricated to show the component and
ground placement required to attain these crosstalk num-
12
PPLICATI
–100
–120
–20
–40
–60
–80
1
2
is routed to any on switch.
V
V
V
R
O
S
IN0
IN1,2,3
L
= 15V
= 100
= GND
All Hostile Crosstalk
U
= 0dBm
DEMO PCB #029
S
FREQUENCY (MHz)
I FOR ATIO
U
IN0
10
PDIP
coupling to Q1 emitter is
SO
DEMO PCB #028
W
1204 AI07
100
1
. Not only
U
bers. A graph of all hostile crosstalk for both the PDIP and
SO packages is shown. It has been found empirically from
these PC boards that capacitive coupling across the pack-
age of greater than 3fF (0.003pF) will diminish the rejec-
tion, and it is recommended that this proven layout be
copied into designs. The key to the success of the SO PC
board #028 is the use of a ground plane guard around Pin
13, the feedback pin.
PDIP PC Board #029, Component Side
VIN1
VIN2
VIN3
GND
VIN0
U1
R1
V–
C2
C4
+
DEMONSTRATION BOARD
R2
C1
+
R6
C3
LT1204 VIDEO MUX
V+
RF
[phone]
RO
VOUT
R3
ENABLE
S/D
REF
R1
R0
1204 AI09