10

LT1227

S

APPLICATI

TYPICAL

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

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

10MHz, this sets the maximum usable output level. Switch-

ing time between inputs is about 4

µs without an external

pullup. Adding a 10k pullup resistor from each shutdown

µs but will

increase the positive supply current in shutdown by 1.5mA.

MUX Amplifier

INPUT

SELECT

MUX Input Crosstalk vs Frequency

FREQUENCY (MHz)

1

–90

–80

–70

–60

–50

–40

10

100

LT1227 TA05

5V

74HC04

5V

74C906

–

+

LT1227

1227 TA04

INPUT

SELECT

S/D

–

+

LT1227

15V

–15V

1.5k

5V

74C906

15V

–15V

1.5k

S/D

= 1

MUX Output

TA03