12

LT1203/LT1205

output impedance of only 25

Ω at 10MHz. This attenuator

is responsible for the outstanding All Hostile Crosstalk

Rejection of 90dB at 10MHz with 15 input signals.

Several suggestions to attain this high rejection include:

1. Mount the feedback resistors for the surface mount

LT1252 on the back side of the PC board.

2. Keep the feedback trace (pin 3) of the LT1252 as short

as possible.

(top) side and under the devices (between inputs and

outputs).

4. Use the backside of the PC board as a solid ground

plane. Connect the LT1205 device grounds and by-

pass capacitors grounds as vias to the backside

ground plane.

S

APPLICATI

I FOR ATIO

16-to-1 MUX, Switching LT1205 Enable Lines

1V

0V

0V

LT1203/05 • AI05

16-to-1 Multiplexer All Hostile Crosstalk Rejection

16-to-1 MUX Response

FREQUENCY (MHz)

1

–6

–4

–2

0

10

100

LT1203/05 • AI07

2

Each “off” switch has 2.8pF of output capacitance and 15

“off” switches tied together represent a 48pF load to the

one active switch. In this case the active device will peak

about 3dB at 50MHz. An attribute of current feedback

amplifiers is that the bandwidth can easily be adjusted by

changing the feedback resistors, and in this application

the LT1252’s bandwidth is reduced to about 60MHz using

1.6k feedback resistors. This has the effect of reducing the

peaking in the MUX to 0.25dB and flattening the response

to 0.05dB at 30MHz.

4

× 4 Crosspoint

The compact high performance 4

× 4 crosspoint shown in

Figure 6 uses four LT1205s to route any input to any or all

outputs. The complete crosspoint uses only six SO pack-

ages and less than six square inches of PC board space.

The LT1254 quad current feedback amplifier serves as a

cable driver with a gain of 2. A

±5Vsupplyisusedtoensure

that the maximum 150

°C junction temperature of the

LT1254 is not exceeded in the SO package. With this

supply voltage the crosspoint can operate at a 70

°C

ambient temperature and drive 2V (peak or DC) into a

double-terminated 75

Ω video cable. The feedback resis-

tors of these output amplifiers have been optimized for

this supply voltage. The – 3dB bandwidth of the crosspoint

is over 100MHz with only 0.8dB of peaking. All Hostile

Crosstalk Rejection is 85dB at 10MHz when a shorted

input is routed to all outputs. To obtain this level of

performance it is necessary to follow techniques similar to

SELECT

LINE C

FREQUENCY (MHz)

1

–120

–100

–80

–60

–40

10

100

LT1203/05 • AI06

–20

5V