MAX4028 データシートの表示（PDF） - Maxim Integrated
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MAX4028 Datasheet PDF : 12 Pages
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MAX4028/MAX4029
Triple/Quad, 2:1 Video
Multiplexer-Amplifiers with Input Clamps
SMALL-SIGNAL BANDWIDTH
vs. FREQUENCY
11
10
CLOAD = 15pF
9
8
CLOAD = 10pF
7
6
5
4
CLOAD = 5pF
3
2
1
100k
1M
10M
100M
1G
FREQUENCY (Hz)
Figure 4. Small-Signal Gain vs. Frequency with Capacitive
Load and No Isolation Resistor
A/B
75Ω CABLE 0.1F
IN_A
RT
75Ω
DISABLE
OUT_
RISO
75Ω CABLE 0.1µF
IN_B
RT
75Ω
CLAMP
CL
RL
RKEYREF
MAX4028
MAX4029
Figure 5. Using an Isolation Resistor (RISO) for a High-
Capacitive Load
amplifier’s output and the load (Figure 5). The value of RISO
depends on the circuit’s gain (+2V/V) and the capacitive
load (Figure 6). Also note that the isolation resistor forms a
divider that decreases the voltage delivered to the load.
Layout and Power-Supply Bypassing
The MAX4028/MAX4029 have high bandwidths and
consequently require careful board layout, including the
possible use of constant-impedance microstrip or stripline
techniques.
To realize the full AC performance of these high-speed
amplifiers, pay careful attention to power-supply bypass-
ing and board layout. The PC board should have at least
OPTIMAL ISOLATION RESISTANCE
vs. CAPACITIVE LOAD
30
25
20
15
10
5
0
0
50 100 150 200 250
CLOAD (pF)
Figure 6. Optimal Isolation Resistance vs. Capacitive Load
two layers: a signal and power layer on one side, and a
large, low-impedance ground plane on the other side.
The ground plane should be as free of voids as possible.
Whether or not a constant-impedance board is used, it is
best to observe the following guidelines when designing
the board:
1) Do not use wire-wrapped boards or breadboards.
2) Do not use IC sockets; they increase parasitic capaci-
tance and inductance.
3) Keep signal lines as short and straight as possible. Do
not make 90° turns; round all corners.
4) Observe high-frequency bypassing techniques to
maintain the amplifier’s accuracy and stability.
5) Use surface-mount components. They generally have
shorter bodies and lower parasitic reactance, yielding
better high-frequency performance than through-hole
components.
The bypass capacitors should include a 0.1µF, ceramic
surface-mount capacitor between VCC and the
ground plane, located as close to the package as
possible. Optionally, place a 10µF capacitor at the power
supply’s point-of-entry to the PCB to ensure the integrity of
incoming supplies. The power-supply traces should lead
directly from the capacitor to the VCC pin. To minimize
parasitic inductance, keep PC traces short and use
surface-mount components.
If input termination resistors and output back-termination
resistors are used, they should be surface-mount types,
and should be placed as close as possible to the IC pins.
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