margin (gain at loop phase = 360°) at frequencies of 70

= 5pF) results in a peak in the gain of 3 amplifier con-

figurations as shown in Figures 3 and 4.

Figure 3 shows a blow up of the open loop characteris-

tics in the 10MHz to 200MHz frequency range, as well

as the corresponding closed loop characteristics for a

gain of three non-inverting amplifier at similar load con-

ditions. It should be noted that the open loop character-

istic does not show the additional phase shift covered

by the input capacitance pole. This is why the closed

loop peaking at 30 to 40MHz is greater than what

would be expected from the 50 to 60 degrees of phase

margin indicated by the open loop characteristics.

Corresponding small signal step response characteris-

tics show well-behaved pulse waveforms with 16–33%

overshoot.

The input capacitive pole can be neutralized by adding

the sum of the common mode and differential input

capacitance

Figure 4 shows the results of this feedback capacitor

addition. Neutralizing the input capacitance demon-

strates the peaking that can result from the loss of gain

margin at 70 to 100MHz. As the load time constant

forms are as expected with a very strong 88MHz ring

Ω, CL = 5pF.

Layout Considerations

As with any high-speed wideband amplifier, certain lay-

out considerations are necessary to ensure stable opera-

tion. All connections to the amplifier should remain as

short as possible, and the power supplies bypassed with

0.1µF capacitors to signal ground. It is suggested that a

ground plane be considered as the best method for

ensuring stability because it minimizes stray inductance

and unwanted coupling in the ground signal paths.

To minimize capacitive effects, resistor values should be

kept as small as possible, consistent with the application.

MAX408 Offset Voltage Nulling

nulling range of ±15mV. If a smaller adjustment range is

desired, resistor values R1 and R2 can be increased

accordingly. For example, at R1 = 3.6k

Ω, the adjustment

range is ±5mV. Since pins 1 and 5 are not part of the sig-

nal path, AC characteristics are left undisturbed.

Single/Dual/Quad High-Speed, Fast-Settling,

High Output Current Operational Amplifier

8

_______________________________________________________________________________________

2

V+ BAL.

BAL.

5

6

1

3

Simplified Schematic. For MAX428/448 omit balance pins.