MAX408 Datasheet, PDF (8/12 Pages) Maxim Integrated Products – Single/Dual/Quad High-Speed, Fast-Settling, High Output Current Operational Amplifier

Single/Dual/Quad High-Speed, Fast-Settling,
High Output Current Operational Amplifier
margin (gain at loop phase = 360°) at frequencies of 70
to 100MHz which at a gain margin of 5dB (R_L = 2k, C_L
= 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
a feedback capacitor to R₂. The value of capacitance
is selected according to R₁ C_IN = R₂C_FB, where C_IN is
the sum of the common mode and differential input
capacitance ≈5pF. For R₂ = 2R₁, C_FB = C_IN/2 ≈ 2.5pF.
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
(R_LC_L) increases the peaking gets progressively worse
≈6dB at R_L = 2K, C_L = 50pF. The step response wave-
forms are as expected with a very strong 88MHz ring
being exhibited at R_L = 2k, C_L = 50pF and no over-
shoot at R_L = 50Ω, 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
The configuration of Figure 2 will give a typical V_OS
nulling range of ±15mV. If a smaller adjustment range is
desired, resistor values R1 and R2 can be increased
accordingly. For example, at R1 = 3.6kΩ, the adjustment
range is ±5mV. Since pins 1 and 5 are not part of the sig-
nal path, AC characteristics are left undisturbed.
R_P = 10kΩ
R₁ = 1.3k R₂ = 1.3kΩ
2
1
V+ BAL.
3
BAL. ^{5 6}
Figure 2. V_OS Nulling Method for MAX408
Simplified Schematic. For MAX428/448 omit balance pins.
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