Because the MAX492/MAX494/MAX495 have excellent

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 MAX492/MAX494/MAX495 have a typical supply

current of 150µA per op amp. Although supply current 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 MAX494 to

buffer the inputs to a multi-channel analog-to-digital con-

verter (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 MAX492/MAX494/

MAX495, it takes some time for the voltages on the sup-

ply 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 follower

held at mid-supply (Figure 10), when the supply steps

(Figure 11b).

Power Supplies and Layout

The MAX492/MAX494/MAX495 operate from a single

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 1µF capacitor in parallel with a

0.1µF ceramic capacitor. If operating from dual sup-

plies, 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 external

components close to the op amp’s pins.

Rail-to-Rail Buffers

The

Typical Operating Circuit shows a MAX495 gain-of-

two buffer driving the analog input to a MAX187 12-bit

ADC. Both devices run from a single 5V supply, and the

converter’s internal reference is 4.096V. The MAX495’s

typical input offset voltage is 200µV. This results in an

error at the ADC input of 400µV, or less than half of one

least significant bit (LSB). Without offset trimming, the

op amp contributes negligible error to the conversion

result.

Single/Dual/Quad, Micropower,

Single-Supply Rail-to-Rail Op Amps

14

______________________________________________________________________________________

1V/div

500mV/div

5

µs/div

2V/div

1V/div

5

µs/div