Datasheet

16
LT1204
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
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-to-1 MUX Response Using Disable vs Shutdown
For a 64-to-1 MUX we need sixteen LT1204s. The
equivalent load resistance due to the feedback resistor
R
EQ
in Disable is 25k/15 = 1.67k. See Figure 3.
V
O
=
75R
EQ
75(75) + 150R
EQ
, V
O
= 0.489V
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.
Figure 3. Equivalent Loading Schematic
16-to-1 Multiplexer All Hostile Crosstalk
FREQUENCY (MHz)
1
–6
GAIN (dB)
–4
–2
0
2
10 100
1204 AI15
4
V
S
= ±15V
R
L
= 100Ω
R
F
= R
G
= 1k
DISABLE
SHUTDOWN
FREQUENCY (MHz)
1
–120
ALL HOSTILE CROSSTALK (dB)
–100
–80
–60
–40
10 100
1204 AI16
–20
V
S
= ±15V
R
L
= 100Ω
R
F
= R
G
= 1k
R
S
= 0
SHUTDOWN
CROSSTALK
DISABLE
CROSSTALK
ON
75Ω
1V
LT1204
OFF
75Ω
LT1204
75Ω
V
OUT
CABLE
75Ω
V
OUT
R
EQ
75Ω
1V
1204 F03