__________Applications Information

Grounding, Bypassing,

and PC Board Layout

In order to obtain the MAX4178/MAX4278’s full 330MHz/

310MHz bandwidths, Micro-Strip and Stripline tech-

niques are recommended in most cases. To ensure

that the PC board does not degrade the amplifier’s per-

formance, it’s a good idea to design the board for a fre-

quency greater than 1GHz. Even with very short traces,

it’s good practice to use these techniques at critical

points, such as inputs and outputs. Whether you use a

constant-impedance board or not, observe the follow-

ing guidelines when designing the board:

• Do not use wire-wrap boards. They are too inductive.

• Do not use IC sockets. They increase parasitic

capacitance and inductance.

• In general, surface-mount components have shorter

leads and lower parasitic reactance, giving better

high-frequency performance than through-hole com-

ponents.

• The PC board should have at least two layers, with

one side a signal layer and the other a ground plane.

• Keep signal lines as short and straight as possible.

Do not make 90° turns; round all corners.

• The ground plane should be as free from voids as

possible.

On Maxim’s evaluation kit, the ground plane has been

removed from areas where keeping the trace capaci-

tance to a minimum is more important than maintaining

ground continuity.

Driving Capacitive Loads

The MAX4178/MAX4278 provide maximum AC perfor-

mance with no output load capacitance. This is the

case when the MAX4178/MAX4278 are driving a cor-

rectly terminated transmission line (e.g., a back-termi-

nated 75

Ω cable). However, the MAX4178/MAX4278

are capable of driving capacitive loads up to 100pF

without oscillations, but with reduced AC performance

Driving large capacitive loads increases the chance of

oscillations in most amplifier circuits. This is especially

true for circuits with high loop gains, such as voltage

followers. The amplifier’s output resistance and the load

capacitor combine to add a pole and excess phase to

the loop response. If the frequency of this pole is low

enough and if phase margin is degraded sufficiently,

oscillations may occur.

A second problem when driving capacitive loads

results from the amplifier’s output impedance, which

looks inductive at high frequency. This inductance

forms an L-C resonant circuit with the capacitive load,

which causes peaking in the frequency response and

degrades the amplifier’s gain margin.

The MAX4178/MAX4278 drive capacitive loads up to

100pF without oscillation. However, some peaking (in

the frequency domain) or ringing (in the time domain)

may occur. This is shown in Figures 2a and 2b and the

in the Small- and Large-Signal Pulse Response graphs

in the

Typical Operating Characteristics.

To drive larger-capacitance loads or to reduce ringing,

add an isolation resistor between the amplifier’s output

and the load, as shown in Figure 1.

capacitive load. Figures 3a and 3b show the Bode

plots that result when a 20

Ω isolation resistor is used

with a voltage follower driving a range of capacitive

loads. At the higher capacitor values, the bandwidth is

the bandwidth of the amplifier itself is much higher.

Note that adding an isolation resistor degrades gain

accuracy. The load and isolation resistor form a divider

that decreases the voltage delivered to the load.

330MHz, Gain of +1/Gain of +2

Closed-Loop Buffers

8

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MAX4178

MAX4278

Figure 1. Capacitive-Load Driving Circuit