LT1204 LINER [Linear Technology], LT1204 Datasheet - Page 16

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

A
LT1204
improper cable termination due to feedback resistors
loading the cable.
The limit to the number of expanded inputs is set by the
acceptable error budget of the system.
16
PPLICATI
16-to-1 MUX Response Using Disable vs Shutdown
–100
–120
16-to-1 Multiplexer All Hostile Crosstalk
–20
–40
–60
–80
–2
–4
–6
4
2
0
1
1
V
R
R
V
R
R
R
S
L
F
S
L
F
S
O
= R
= 15V
= 100
= R
= 15V
= 100
= 0
G
G
U
= 1k
= 1k
S
FREQUENCY (MHz)
FREQUENCY (MHz)
SHUTDOWN
I FOR ATIO
DISABLE
CROSSTALK
SHUTDOWN
U
10
10
DISABLE
CROSSTALK
W
1204 AI15
1204 AI16
100
100
U
For a 64-to-1 MUX we need sixteen LT1204s. The
equivalent load resistance due to the feedback resistor
R
This voltage represents a 2.1% loading error. If the
shutdown feature is used instead of the disable feature,
then the LT1204 could expand to only an 8-to-1 MUX for
the same error.
As a practical matter the gain error at frequency is also
set by capacitive loading. The disabled output capaci-
tance of the LT1204 is about 8pF, and in the case of
sixteen LT1204s, it would represent a 128pF load. The
combination of 1.67k and 128pF correspond to about a
0.3dB roll-off at 5MHz.
EQ
V
O
in Disable is 25k/15 = 1.67k. See Figure 3.
=
75(75) + 150R
LT1204
LT1204
Figure 3. Equivalent Loading Schematic
OFF
ON
75R
1V
EQ
1V
75
EQ
75
75
, V
R
EQ
O
= 0.489V
V
OUT
CABLE
75
1204 F03
V
OUT
75