MAX4547 Maxim, MAX4547 Datasheet - Page 11

• Manufacturer Part Number: MAX4547
• Description: The MAX4545/MAX4546/MAX4547 are low-voltage T-switches designed for switching RF and video signals from DC to 300MHz in 50Ω and 75Ω systems
• Manufacturer: Maxim

Figure 2. Impedance Matching On/Off Switch
On the MAX4545, GND5 and GND6 should always be
connected to the ground plane with solid copper to
improve isolation and crosstalk.
Keep all signal leads as short as possible. Separate all
signal leads from each other and other traces with the
ground plane on both sides of the board. Where possi-
ble, use coaxial cable instead of printed circuit board
traces.
LOGIC
SWITCHES SHOWN FOR LOGIC “0” INPUT
( ) ARE FOR 75 SYSTEMS.
IN/OUT
IN
50
(61
38
1
2
3
4
6
7
8
IN1
COM1
GND1
GND1
NO1
NC4
GND4
COM4
______________________________________________________________________________________
LOGIC
10nF
10nF
0
1
MAX4546
V+
V+
V-
V-
12
5
SWITCH
OFF
ON
COM2
COM3
GND2
GND3
NO2
NC3
IN2
Signal Routing
Bidirectional RF/Video Switches
16
15
14
13
11
10
9
38
(61
OUT/IN
50
82
(194
Quad/Dual, Low-Voltage,
IC sockets degrade high-frequency performance and
should not be used if signal bandwidth exceeds 5MHz.
Surface-mount parts, having shorter internal lead
frames, provide the best high-frequency performance.
Keep all bypass capacitors close to the device, and
separate all signal leads with ground planes. Such
grounds tend to be wedge-shaped as they get closer to
the device. Use vias to connect the ground planes on
each side of the board, and place the vias in the apex of
the wedge-shaped grounds that separate signal leads.
Logic-level signal lead placement is not critical.
The typical on-resistances of the switches in the
MAX4545/MAX4546/MAX4547 are 14Ω, but the off-
state impedances are approximately equal to a 6pF
capacitor. In coaxial systems, therefore, it is impossible
to match any impedance for both the on and off state. If
impedance matching is critical, the MAX4546 is best
suited, since its two sections can be configured as a
single on/off switch, as shown in Figure 2. This circuit
“wastes” switches and has higher losses, but has bet-
ter off isolation and maintains good impedance match-
ing in both the on and off states. The resistance values
shown in Figure 3 are optimized with ±5V supplies for
both 50Ω and 75Ω systems at room temperature.
With its excellent off isolation, the MAX4545 is ideal for
use in high-frequency video multiplexers. Figure 3
shows such an application for switching any one of four
video inputs to a single output. The same circuit may
be used as a demultiplexer by simply reversing the sig-
nal direction.
Stray capacitance of traces and the output capacitance
of switches placed in parallel reduces bandwidth, so the
outputs of no more than four individual switches should
be placed in parallel if high bandwidth is to be main-
tained. If more than four mux channels are needed, the
4-channel circuit should be duplicated and cascaded.
Impedance Matching
Board Layout
Multiplexer
11