MAX492 Maxim, MAX492 Datasheet - Page 15

• Manufacturer Part Number: MAX492
• Description: The dual MAX492, quad MAX494, and single MAX495 operational amplifiers combine excellent DC accuracy with rail-to-rail operation at the input and output
• Manufacturer: Maxim

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.
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
from 0V to V
for V
(Figure 11b).
Single/Dual/Quad, Micropower,
Single-Supply Rail-to-Rail Op Amps
Figure 11a. Power-Up Settling Time (V
14
______________________________________________________________________________________
CC
= +3V (Figure 11a) or 10µs for V
CC
, the output settles in approximately 4µs
5 s/div
Power-Up Settling Time
CC
= +3V)
CC
V
1V/div
V
500mV/div
CC
OUT
= +5V
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.
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.
Figure 11b. Power-Up Settling Time (V
Power Supplies and Layout
5 s/div
Rail-to-Rail Buffers
CC
= +5V)
V
2V/div
V
1V/div
CC
OUT