LT1204CSW datasheet(15/20 Pages) LINER

15 / 20 page

LT1204

APPLICATI

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 RF = RG

= 1k, the Disable Output Resistance is typically raised to

25k and drops to 20k for AV = 10, RF = 2k and RG = 222Ω

due to loading of the feedback buffer. Operating the

Disable feature with RG < 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

V–

V+

TO SCOPE

Ω

–

LT1204

V–

Ω

V+

74HC04

Ω

TEK

CT-1

OSCILLATOR

1204 AI13

Timing and Supply Current Waveforms

OSCILLATOR

5V/DIV

VOUT

1V/DIV

10mA/DIV

74HC04

OUTPUT

5V/DIV

1204 AI14

TEE SWITCH

VIN0

TEE SWITCH

VIN1

TEE SWITCH

VIN2

TEE SWITCH

VIN3

V–

LT1204

“ON”

–

CFA

“OFF”

VOUT

Ω

CABLE

1204 F02

AV = +1

Ω

Figure 2. Active Buffer Drives FB Pin 13

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