MAX9637 Просмотр технического описания (PDF) - Maxim Integrated

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MAX9637

3V/5V Low-Power, Low-Noise, CMOS, Rail-to-Rail I/O Op Amps

Maxim Integrated 

MAX9636/MAX9637/

MAX9638

3V/5V Low-Power, Low-Noise, CMOS,

Rail-to-Rail I/O Op Amps

Driving Capacitive Loads

The ICs have a high tolerance for capacitive loads. In

unity-gain configuration, the op amps can typically drive

up to 300pF pure capacitive load. Increasing the gain

enhances the amplifier’s ability to drive greater capacitive

loads. In unity-gain configurations, capacitive load drive

can be improved by inserting a small (5Ω to 30Ω) isola-

tion resistor, RISO, in series with the output, as shown in

Figure 1. This significantly reduces ringing while maintain-

ing DC performance for purely capacitive loads. However,

if the load also has a resistive component then a voltage-

divider is created, introducing a direct current (DC) error

at the output. The error introduced is proportional to the

ratio RISO/RL, which is usually negligible in most cases.

Applications that cannot tolerate this slight DC error can

use an alternative approach of providing stability by plac-

ing a suitable resistance in parallel with the capacitive

load as shown in Figure 2 (see the Typical Operating

Characteristics section for graphs of the stable operating

region for various capacitive loads vs. resistive loads).

While this approach of adding a resistor parallel to the

load does not introduce DC error, it nevertheless reduces

the output swing proportionally.

High-Impedance

Sensor Front-Ends

The ICs interface to both current-output sensors, such

as photodiodes (Figure 3), and high-impedance voltage

sources, such as piezoelectric sensors. For current-

output sensors, a transimpedance amplifier is the most

noise-efficient method for converting the input signal to

a voltage. High-value feedback resistors are commonly

chosen to create large gains, while feedback capacitors

help stabilize the amplifier by cancelling any poles intro-

duced in the feedback function by the highly capacitive

sensor or cabling. A combination of low-current noise

and low-voltage noise is important for these applications.

Take care to calibrate out photodiode dark current if DC

accuracy is important. The high bandwidth and slew rate

also allows AC signal processing in certain medical photo-

diode sensor applications such as pulse oximetry.

For voltage-output sensors, a noninverting amplifier is

typically used to buffer and/or apply a small gain to the

input voltage signal. Due to the extremely high imped-

ance of the sensor output, a low input bias current with

minimal temperature variation is very important for these

applications.

-

MAX9636/

RISO

MAX9637/

MAX9638

+

AV =

RL

RL + RISO

≈ 1V/V

RL

CL

Figure 1. Using a Series Resistor to Isolate the Capacitive

Load from the Op Amp

-

MAX9636/

MAX9637/

MAX9638

+

RP

RL

CL

Figure 2. Using a Parallel Resistor to Degenerate the Effect of

the Capacitive Load and Increase Stability

PHOTODIODE

VDD

IN-

MAX9636 OUT

IN+

REF

Figure 3. The MAX9636 in a Sensor Preamp Configuration

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SIGNAL

CONDITIONING/

FILTERS

MAX1286

ADC

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