Datasheet

12
LT1203/LT1205
output impedance of only 25Ω 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.
3. Route V
+
and V
–
for the LT1205s on the component
(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.
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
16-to-1 MUX, Switching LT1205 Enable Lines
1V
0V
0V
R
F
= R
G
= 1.6k
R
L
= 100Ω
V
IN4
= 0V
V
IN0
= 1V
LT1203/05 • AI05
16-to-1 Multiplexer All Hostile Crosstalk Rejection
16-to-1 MUX Response
FREQUENCY (MHz)
1
GAIN (dB)
–6
–4
–2
0
10 100
LT1203/05 • AI07
2
V
S
= ±15V
R
L
= 100Ω
R
F
= R
G
= 1.6k
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 ±5V supply is used to ensure
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 75Ω 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
HOSTILE CROSSTALK REJECTION (dB)
–100
–80
–60
–40
10 100
LT1203/05 • AI06
–20
V
S
= ±15V
R
S
= 10Ω
R
L
= 100Ω
5V