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

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

compensation capacitor, forms the dominant pole in the

frequency and the -3dB bandwidth, but also increases

peaking due to interaction with other nondominant

bandwidth.

Tables 1 and 2 show optimal values for the feedback

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

tain consistency over a wide range of production lots.

Table 1. MAX4158/MAX4159 Bandwidth

and Gain vs. Gain-Setting Resistors

200

110

110

2

6

110

5

14

80

12

40

6

32.5

130

14.5

10

20

130

-3dB BW

(MHz)

0.1dB BW

(MHz)

(

Ω)

(

Ω)

GAIN

350

100

∞

430

1

0

(dB)

(V/V)