Driving Capacitive Loads

The MAX4291/MAX4292/MAX4294 are unity-gain stable

for loads up to 100pF (see the Load Resistor vs.

Capacitive Load graph in the Typical Operating

Characteristics). Applications that require greater

capacitive drive capability should use an isolation

resistor between the output and the capacitive load

(Figure 7). Note that this alternative results in a loss of

with the load resistor.

Power-Supply Bypassing and Layout

The MAX4291/MAX4292/MAX4294 family operates from

either a single +1.8V to +5.5V supply or dual ±0.9V to

±2.75V supplies. For single-supply operation, bypass

with separate 100nF capacitors.

Good PC board layout techniques optimize perfor-

mance by decreasing the amount of stray capacitance

at the op amp’s inputs and output. To decrease stray

capacitance, minimize trace lengths and widths by

placing external components as close as possible to

the op amp. Surface-mount components are an excel-

lent choice.

Using the MAX4291/MAX4292/MAX4294

as Comparators

Although optimized for use as operational amplifiers,

the MAX4291/MAX4292/MAX4294 can also be used as

rail-to-rail I/O comparators. Typical propagation delay

depends on the input overdrive voltage, as shown in

Figure 8. External hysteresis can be used to minimize

the risk of output oscillation. The positive feedback cir-

cuit, shown in Figure 9, causes the input threshold to

change when the output voltage changes state. The

two thresholds create a hysteresis band that can be

calculated by the following equations:

When the output of the comparator is low, the supply

current increases. The output stage has biasing circuit-

ry to monitor the output current. When the amplifier is

Ultra-Small, +1.8V, µPower,

Rail-to-Rail I/O Op Amps

______________________________________________________________________________________

11

MAX4240

MAX4241

MAX4242

MAX4243

MAX4244

≈ 1

OUT

IN

Figure 7a. Using a Resistor to Isolate a Capacitive Load from

the Op Amp

OUT

IN

0

100mV

0

100mV

10

µs/div

Figure 7b. Pulse Response Without Isolating Resistor

OUT

IN

0

100mV

0

100mV

10

µs/div

Figure 7c. Pulse Response with Isolating Resistor (100

Ω)

V

V

V

V

1

R1

R2

R1

R

V

V

V

R1

R

V

HYST

HI

LO

HI

HYST

REF

LO

HI

HYST

CC

=−

=+

+

















=−











