MAX4158/MAX4159/MAX4258/MAX4259
350MHz/250MHz, 2-Channel
Video Multiplexer-Amplifiers
______________________________________________________________________________________ 11
To realize the full AC performance of these high-speed
amplifiers, pay careful attention to power-supply
bypassing and board layout. The PC board should
have at least 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, with one exception: the feedback pin
(FB) should have as low a capacitance to ground as
possible. This means that there should be no ground
plane under FB or under the components (R
F
and R
G
)
connected to it. With multilayer boards, locate the
ground plane on a layer that incorporates no signal or
power traces.
Regardless of whether or not a constant-impedance
board is used, it is best to observe the following guide-
lines when designing the board:
1) Do not use wire-wrap boards (they are much too
inductive) or breadboards (they are much too
capacitive).
2) Do not use IC sockets. IC sockets increase reac-
tances.
3) Keep lines as short and as 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) Bear in mind that, in general, surface-mount compo-
nents have shorter bodies and lower parasitic reac-
tance, giving much better high-frequency
performance than through-hole components.
The bypass capacitors should include a 10nF ceramic
surface-mount capacitor between each supply pin and
the ground plane, located as close to the package as
possible. Optionally, place a 10µF tantalum capacitor at
the power-supply pins’ points of entry to the PC board
to ensure the integrity of incoming supplies. The power-
supply trace should lead directly from the tantalum
capacitor to the V+ and V- pins. To minimize parasitic
inductance, keep PC traces short and use surface-
mount components.
Ground pins have been placed between input channels
to minimize crosstalk between the two input channels.
(The grounds extend inside the package all the way to
the silicon.) These pins should be connected to a com-
mon ground plane on the PC board.
Input termination resistors and output back-termination
resistors, if used, should be surface-mount types, and
should be placed as close to the IC pins as possible.
Choosing Feedback
_________________and Gain Resistors
As with all current-mode feedback amplifiers, the fre-
quency response of the MAX4158/MAX4159/MAX4258/
MAX4259 is critically dependent on the value of the
feedback resistor R
F
. R
F
, in conjunction with an internal
compensation capacitor, forms the dominant pole in the
feedback loop. Reducing R
F
’s value increases the pole
frequency and the -3dB bandwidth, but also increases
peaking due to interaction with other nondominant
poles. Increasing R
F
’s value reduces peaking and
bandwidth.
Tables 1 and 2 show optimal values for the feedback
resistor (R
F
) and gain-setting resistor (R
G
) for all parts.
Note that the MAX4258/MAX4259 offer superior AC per-
formance for all gains except unity gain (0dB). These
values provide optimal AC response using surface-
mount resistors and good layout techniques. The
MAX4159/MAX4259 evaluation kit provides a practical
example of such layout techniques.
Stray capacitance at FB causes feedback resistor
decoupling and produces peaking in the frequency-
response curve. Keep the capacitance at FB as low as
possible by using surface-mount resistors, and avoid-
ing the use of a ground plane beneath or beside these
resistors and the FB pin. Some capacitance is unavoid-
able; if necessary, its effects can be counteracted by
adjusting R
F
. 1% resistors are recommended to main-
tain consistency over a wide range of production lots.
Table 1. MAX4158/MAX4159 Bandwidth
and Gain vs. Gain-Setting Resistors
200 1101102 6 110
5 14 80 12
40 6
32.5 130
14.510 20 130
-3dB BW
(MHz)
0.1dB BW
(MHz)
R
G
(Ω)
R
F
(Ω)
GAIN
350 100
∞
4301 0
(dB)(V/V)
