LT1204CSW datasheet(16/20 Pages) LINER

16 / 20 page

LT1204

APPLICATI

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

REQ in Disable is 25k/15 = 1.67k. See Figure 3.

VO =

75REQ

75(75) + 150REQ

, VO = 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)

–6

–4

–2

100

1204 AI15

VS = ±15V

RL = 100Ω

RF = RG = 1k

DISABLE

SHUTDOWN

FREQUENCY (MHz)

–120

–100

–80

–60

–40

100

1204 AI16

–20

VS = ±15V

RL = 100Ω

RF = RG = 1k

RS = 0

SHUTDOWN

CROSSTALK

DISABLE

CROSSTALK

Ω

LT1204

OFF

Ω

LT1204

Ω

VOUT

CABLE

Ω

VOUT

REQ

Ω

1204 F03

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