15
LT1204
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Multiplexer Expansion Pin 11 and Pin 12
To expand the number of MUX inputs, LT1204s can be
paralleled by shorting their outputs together. The multi-
plexer disable logic has been designed to prevent shoot-
through current when two or more amplifiers have their
outputs shorted together. (Shoot-through current is a
spike of power supply current caused by both amplifiers
being on at once.)
Monitoring Supply Current Spikes
The multiplexer uses a circuit to ensure the disabled
amplifiers do not load or alter the cable termination. When
the LT1204 is disabled (Pin 11 low) the output stage is
turned off and an active buffer senses the output and
drives the feedback pin to the CFA (Figure 2). This boot-
straps the feedback resistors and raises the true output
impedance of the circuit. For the condition where R
F
= R
G
= 1k, the Disable Output Resistance is typically raised to
25k and drops to 20k for A
V
= 10, R
F
= 2k and R
G
= 222Ω
due to loading of the feedback buffer. Operating the
Disable feature with R
G
< 100Ω is not recommended.
A shutdown feature (Pin 12 low) reduces the supply
current to 1.5mA and lowers the power dissipation
when the LT1204 is not in use. If the part is shut down,
the bootstrapping is inoperative and the feedback resis-
tors will load the output. If the CFA is operated at a gain
of +1, however, the feedback resistor will not load the
output even in shutdown because there is no resistive
path to ground, but there will be a –6dB loss through
the cable system.
A frequency response plot shows the effect of using the
disable feature versus using the shutdown feature. In
this example four LT1204s were connected together at
their outputs forming a 16-to-1 MUX. The plot shows the
effect of the bootstrapping circuit that eliminates the
–
+
+
+
+
LT1204
1
3
5
7
13
14
11
EN
V
–
16
V
+
TO SCOPE
15
75Ω
–
+
+
+
+
LT1204
1
3
5
7
13
14
11
EN
V
–
16
15
1k
1k
75Ω
V
+
74HC04
75Ω
TEK
CT-1
O
5V
OSCILLATOR
1204 AI13
1k
1k
Timing and Supply Current Waveforms
OSCILLATOR
5V/DIV
V
OUT
1V/DIV
I
S
10mA/DIV
74HC04
OUTPUT
5V/DIV
1204 AI14
TEE SWITCHV
IN0
TEE SWITCHV
IN1
TEE SWITCHV
IN2
TEE SWITCHV
IN3
V
–
LT1204
“ON”
–
+
CFA
“OFF”
FB
R
F
R
G
V
OUT
75Ω
75Ω
CABLE
1204 F02
A
V
= +1
75Ω
Figure 2. Active Buffer Drives FB Pin 13
