10
LT1227
U
SA
O
PP
L
IC
AT
ITY
P
I
CA
L
MUX Amplifier
The shutdown function can be effectively used to con-
struct a MUX amplifier. A two-channel version is shown,
but more inputs could be added with suitable logic. By
configuring each amplifier as a unity-gain follower, there
is no loading by the feedback network when the amplifier
is off. The open drains of the 74C906 buffers are used to
interface the 5V logic to the shutdown pin. Feedthrough
from the unselected input to the output is –70dB at
10MHz. The differential voltage between MUX inputs V
IN1
and V
IN2
appears across the inputs of the shutdown
device, this voltage should be less than ±5V to avoid
turning on the clamp diodes discussed previously. If the
inputs are sinusoidal having a zero DC level, this implies
that the amplitude of each input should be less than
5V
P-P
. The output impedance of the off amplifier remains
high until the output level exceeds approximately 6V
P-P
at
10MHz, this sets the maximum usable output level. Switch-
ing time between inputs is about 4µs without an external
pullup. Adding a 10k pullup resistor from each shutdown
pin to V
+
will reduce the switching time to 2µs but will
increase the positive supply current in shutdown by 1.5mA.
MUX Amplifier
INPUT
SELECT
V
OUT
MUX Input Crosstalk vs Frequency
FREQUENCY (MHz)
1
–90
INPUT CROSSTALK (dB)
–80
–70
–60
–50
–40
10 100
LT1227 TA05
5V
74HC04
5V
74C906
–
+
LT1227
1227 TA04
INPUT
SELECT
V
OUT
S/D
–
+
LT1227
15V
–15V
1.5k
5V
74C906
V
IN1
15V
–15V
V
IN2
1.5k
S/D
V
OUT
V
IN
= 1
MUX Output
TA03
V
IN1
= 1V
P-P
, V
IN2
= 0V
