Low-Cost, High-Speed, Single-Supply, Gain of +2

Buffers with Rail-to-Rail Outputs in SOT23

10

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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 1k

Ω 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 500

Ω.

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

MAX4014

MAX4017

MAX4019

MAX4022

500

Ω

500

Ω

OUT

IN-

IN+

Figure 5. Input Protection Circuit

6

-4

100k

10M

100M

1M

1G

-2

FREQUENCY (Hz)

0

2

4

5

-3

-1

1

3

Figure 6. Small-Signal Gain vs. Frequency with Load

Capacitance and No Isolation Resistor

500

Ω

500

Ω

50

Ω

MAX40_ _

Figure 7. Driving a Capacitive Load through an Isolation Resistor