Detailed Description

The MAX9945 features a combination of low input cur-

rent and voltage noise, rail-to-rail output voltage swing,

wide supply voltage range, and low-power operation.

The MOS inputs on the MAX9945 make it ideal for use

as transimpedance amplifiers and high-impedance

sensor interface front-ends in medical and industrial

applications. The MAX9945 can interface with small

signals from either current-sources or high-output

impedance voltage sources. Applications include pho-

todiode pulse oximeters, pH sensors, capacitive pres-

sure sensors, chemical analysis equipment, smoke

detectors, and humidity sensors.

A high 130dB open-loop gain (typ) and a wide supply

voltage range, allow high signal-gain implementations

prior to signal conditioning circuitry. Low quiescent

supply current makes the MAX9945 compatible with

portable systems and applications that operate under

tight power budgets. The combination of excellent THD,

low voltage noise, and MOS inputs also make the

MAX9945 ideal for use in high-performance active fil-

ters for data acquisition systems and audio equipment.

Low-Current, Low-Noise Input Stage

The MAX9945 features a MOS-input stage with only

50fA (typ) of input bias current and a low 1fA/√Hz (typ)

input current-noise density. The low-frequency input

accepts a wide common-mode range, extending from

Rail-to-Rail Output Stage

The MAX9945 output stage swings to within 50mV (typ)

of either power-supply rail with a 100k

Ω load and pro-

vides a 3MHz GBW with a 2.2V/µs slew rate. The

device is unity-gain stable, and unlike other devices

with a low quiescent current, can drive a 120pF capaci-

tive load without compromising stability.

Applications Information

High-Impedance Sensor Front Ends

High-impedance sensors can output signals of interest

in either current or voltage form. The MAX9945 inter-

faces to both current-output sensors such as photo-

diodes and potentiostat sensors, and high-impedance

voltage sources such as pH 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 feed-

back capacitors help stabilize the amplifier by cancel-

ing any zeros in the transfer function created by a

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 photodi-

ode dark current if DC accuracy is important. The high

bandwidth and slew rate also allows AC signal pro-

cessing in certain medical photodiode sensor applica-

tions such as pulse oximetry.

38V, Low-Noise, MOS-Input,

Low-Power Op Amp

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7

Pin Description

PIN

TDFN-EP

µMAX

NAME

FUNCTION

1

6

OUT

Amplifier Output

24

3

3

IN+

Noninverting Amplifier Input

4

2

IN-

Inverting Amplifier Input

5

1, 5, 8

N.C.

No Connection. Not internally connected.

67

——

EP

thermal performance. Not intended as an electrical connection (TDFN only).