MAX4014/MAX4017/MAX4019/MAX4022
Low-Cost, High-Speed, Single-Supply, Gain of +2
Buffers with Rail-to-Rail Outputs in SOT23
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Disabled Output Resistance
The MAX4014/MAX4017/MAX4019/MAX4022 include
internal protection circuitry that prevents damage to the
precision input stage from large differential input volt-
ages, as shown in Figure 5. This protection circuitry con-
sists of five back-to-back Schottky diodes between IN_+
and IN_-. These diodes lower the disabled output resis-
tance from 1kΩ to 500Ω when the output voltage is 3V
greater or less than the voltage at IN_+. Under these
conditions, the input protection diodes will be forward
biased, lowering the disabled output resistance to 500Ω.
Output Capacitive Loading and Stability
The MAX4014/MAX4017/MAX4019/MAX4022 provide
maximum AC performance with no load capacitance.
This is the case when the load is a properly terminated
transmission line. However, they are designed to drive
up 25pF of load capacitance without oscillating, but
with reduced AC performance.
Driving large capacitive loads increases the chance of
oscillations occurring in most amplifier circuits. This is
especially true for circuits with high loop gains, such as
voltage followers. The buffer’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 to interfere with the loop response
and degrade phase margin sufficiently, oscillations can
occur.
A second problem when driving capacitive loads
results from the amplifier’s output impedance, which
looks inductive at high frequencies. 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.
Figure 6 shows the frequency response of the MAX4014/
MAX4017/MAX4019/MAX4022 under different capacitive
loads. To drive loads with greater than 25pF of capaci-
tance or to settle out some of the peaking, the output
requires an isolation resistor like the one shown in
