Single/Dual/Quad, Micropower, Single-Supply,

Rail-to-Rail Op Amps

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9

tance. If the output sources either more or less current,

stability is increased. These devices perform well with a

1000pF pure capacitive load (Figure 5). Figures 6a, 6b,

and 6c show the performance with a 500pF load in par-

allel with various load resistors.

To increase stability while driving large-capacitive

decrease the current the amplifier must source. If the

amplifier is made to sink current rather than source,

stability is further increased.

Frequency stability can be improved by adding an out-

(Figure 7). This resistor improves the phase margin of

the circuit by isolating the load capacitor from the op

amp’s output. Figure 8a shows the MAX4092 driving

≥ 100kΩ), while Figure 8b adds a 47Ω iso-

lation resistor.

Because the MAX4091/MAX4092/MAX4094 have excel-

lent stability, no isolation resistor is required, except in

the most demanding applications. This is beneficial

because an isolation resistor would degrade the low-

frequency performance of the circuit.

Power-Up Settling Time

The MAX4091/MAX4092/MAX4094 have a typical sup-

ply current of 130µA per op amp. Although supply cur-

rent is already low, it is sometimes desirable to reduce

it further by powering down the op amp and associated

ICs for periods of time. For example, when using a

MAX4092 to buffer the inputs of a multi-channel analog-

to-digital converter (ADC), much of the circuitry could

be powered down between data samples to increase

battery life. If samples are taken infrequently, the op

amps, along with the ADC, may be powered down

most of the time.

When power is reapplied to the MAX4091/MAX4092/

MAX4094, it takes some time for the voltages on the

supply pin and the output pin of the op amp to settle.

Supply settling time depends on the supply voltage, the

value of the bypass capacitor, the output impedance of

the incoming supply, and any lead resistance or induc-

tance between components. Op amp settling time

depends primarily on the output voltage and is slew-

rate limited. With the noninverting input to a voltage fol-

lower held at midsupply (Figure 9), when the supply

(Figure 10b).

Power Supplies and Layout

The MAX4091/MAX4092/MAX4094 operate from a sin-

gle 2.7V to 6V power supply, or from dual supplies of

±1.35V to ±3V. For single-supply operation, bypass the

power supply with a 0.1µF capacitor. If operating from

dual supplies, bypass each supply to ground.

Good layout improves performance by decreasing the

amount of stray capacitance at the op amp’s inputs

and output. To decrease stray capacitance, minimize

both trace lengths and resistor leads and place exter-

nal components close to the op amp’s pins.

Chip Information

MAX4091 TRANSISTOR COUNT: 168

MAX4092 TRANSISTOR COUNT: 336

MAX4094 TRANSISTOR COUNT: 670

PROCESS: Bipolar