Low-Cost, Low-Voltage, Rail-to-Rail,

Input/Output, SOT23 5MHz Op Amp

8

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Rail-to-Rail Output Stage

The minimum output voltage will be within millivolts of

ground for single-supply operation where the load is

voltage range and output voltage swing of a MAX4321

connected as a voltage follower. With a +3V supply

and the 100k

Ω load tied to ground, the output swings

from 0.02V to 2.97V. The maximum output voltage

swing depends on the load but will be within 300mV of

a +5V supply, even with the maximum load (250

Ω to

ground).

Driving a capacitive load can cause instability in most

high-speed op amps, especially those with low quies-

cent current. The MAX4321 has a high tolerance for

capacitive loads. It is stable with capacitive loads up to

500pF. Figure 4 gives the stable operating region for

capacitive loads. Figures 5 and 6 show the response

with capacitive loads and the results of adding an isola-

tion resistor in series with the output (Figure 7). The

resistor improves the circuit’s phase margin by isolating

the load capacitor from the op amp’s output.

Figure 3. Rail-to-Rail Input /Output Voltage Range

Figure 4. Capacitive-Load Stability

Figure 5. Small-Signal Transient Response with Capacitive

Load

Figure 6. Transient Response to Capacitive Load with Isolation

Resistor

IN

400ns/div

OUT

IN

400ns/div

OUT

Ω

IN

2

µs/div

OUT

Ω

10,000

100

100

1k

100k

10k

RESISTIVE LOAD (

Ω)

1000

UNSTABLE

REGION