19-4398; Rev 1; 12/10


38V, Low-Noise, MOS-Input, Low-Power Op Amp


MAX9945

General Description Features

The MAX9945 operational amplifier features an excellent combination of low operating power and low input volt- age noise. In addition, MOS inputs enable the MAX9945 to feature low input bias currents and low input current noise. The device accepts a wide supply voltage range from 4.75V to 38V and draws a low 400µA quiescent cur- rent. The MAX9945 is unity-gain stable and is capable of rail-to-rail output voltage swing.

The MAX9945 is ideal for portable medical and industri- al applications that require low noise analog front-ends for performance applications such as photodiode trans- impedance and chemical sensor interface circuits.

The MAX9945 is available in both an 8-pin µMAX® and a space-saving, 6-pin TDFN package, and is specified over the automotive operating temperature range (-40°C to +125°C).

+4.75V to +38V Single-Supply Voltage Range

Applications Ordering Information

PART

TEMP RANGE

PIN- PACKAGE

TOP MARK

MAX9945ATT+

-40°C to +125°C

6 TDFN-EP*

AUE

MAX9945AUA+

-40°C to +125°C

8 µMAX

Medical Pulse Oximetry Photodiode Sensor Interface

Industrial Sensors and Instrumentation

Chemical Sensor Interface

High-Performance Audio Line Out Active Filters and Signal Processing

+Denotes a lead(Pb)-free/RoHS-compliant package.

*EP = Exposed pad.


µMAX is a registered trademark of Maxim Integrated Products, Inc.


Typical Operating Circuit


PHOTODIODE

VCC

IN-


MAX9945

OUT

SIGNAL CONDITIONING/ FILTERS

ADC


IN+


VEE



Maxim Integrated Products 1

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.


MAX9945

ABSOLUTE MAXIMUM RATINGS

Supply Voltage (VCC to VEE) ..................................-0.3V to +40V

IN+, IN-, OUT Voltage......................(VEE - 0.3V) to (VCC + 0.3V) IN+ to IN- .............................................................................±12V

OUT Short Circuit to Ground Duration....................................10s

Continuous Input Current into Any Pin .............................±20mA Continuous Power Dissipation (TA = +70°C)

6-Pin TDFN-EP (derate 23.8mW/°C above +70°C)

Multilayer Board ....................................................1904.8mW


8-Pin µMAX (derate 4.8mW/°C above +70°C)

Multilayer Board ......................................................387.8mW

Operating Temperature Range .........................-40°C to +125°C Junction Temperature ......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C Soldering Temperature ....................................................+260°C

PACKAGE THERMAL CHARACTERISTICS (Note 1)

TDFN-EP

Junction-to-Ambient Thermal Resistance (JA) ............42°C/W Junction-to-Case Thermal Resistance (JC) ...................9°C/W

µMAX

Junction-to-Ambient Thermal Resistance (JA) .......206.3°C/W Junction-to-Case Thermal Resistance JC ...................42°C/W


Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four- layer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.


Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.


ELECTRICAL CHARACTERISTICS

(VCC = +15V, VEE = -15V, VIN+ = VIN- = VGND = 0V, ROUT = 100kΩ to GND, TA = -40°C to +125°C, typical values are at TA = +25°C,

unless otherwise noted.) (Note 2)


PARAMETER

SYMBOL

CONDITIONS

MIN TYP MAX

UNITS

DC ELECTRICAL CHARACTERISTICS


Input Voltage Range


VIN+, VIN-


Guaranteed by CMRR

TA = +25°C

VEE VCC -

1.2


V

TA = TMIN to TMAX

VEE VCC -

1.4

Input Offset Voltage

VOS

TA = +25°C

±0.6 ±5

mV

TA = TMIN to TMAX

±8

Input Offset Voltage Drift

VOS - TC


2

µV/°C


Input Bias Current (Note 3)


IB

-40°C TA +25°C

50 150

fA

-40°C TA +70°C

12

pA

-40°C TA +85°C

55

pA

-40°C TA +125°C

1.9

nA


Common-Mode Rejection Ratio


CMRR

VCM = VEE to VCC - 1.2V, TA = +25°C

78 94


dB

VCM = VEE to VCC - 1.4V, TA = TMIN to TMAX

78 94


Open-Loop Gain


AOL

VEE + 0.3V VOUT VCC - 0.3V, ROUT = 100kΩ to GND

110 130


dB

VEE + 0.75V VOUT VCC - 0.75V, ROUT = 10kΩ to GND

110 130

Output Short-Circuit Current

ISC


25

mA


MAX9945

ELECTRICAL CHARACTERISTICS (continued)

(VCC = +15V, VEE = -15V, VIN+ = VIN- = VGND = 0V, ROUT = 100kΩ to GND, TA = -40°C to +125°C, typical values are at TA = +25°C,

unless otherwise noted.) (Note 2)


PARAMETER

SYMBOL

CONDITIONS

MIN TYP MAX

UNITS


Output Voltage Low


VOL

ROUT = 10kΩ to GND

TA = TMIN to TMAX

VEE + VEE +

0.26 0.45


V

ROUT = 100kΩ to GND

TA = TMIN to TMAX

VEE + VEE +

0.05 0.15


Output Voltage High


VOH

ROUT = 10kΩ to GND

TA = TMIN to TMAX

VCC - VCC -

0.45 0.24


V

ROUT = 100kΩ to GND

TA = TMIN to TMAX

VCC - VCC -

0.15 0.03

AC ELECTRICAL CHARACTERISTICS

Input Current-Noise Density

IN

f = 1kHz

1

fA/Hz

Input Voltage Noise

VNP-P

f = 0.1Hz to 10Hz

2

µVP-P


Input Voltage-Noise Density


VN

f = 100Hz

25


nV/Hz

f = 1kHz

16.5

f = 10kHz

15

Gain Bandwidth

GBW


3

MHz

Slew Rate

SR


2.2

V/µs

Capacitive Loading (Note 4)

CLOAD

No sustained oscillations

120

pF

Total Harmonic Distortion

THD

VOUT = 4.5VP-P, AV = 1V/V,

f = 10kHz, ROUT = 10kΩ to GND

97

dB

POWER-SUPPLY ELECTRICAL CHARACTERISTICS

Power-Supply Voltage Range

VCC - VEE

Guaranteed by PSRR, VEE = 0V

+4.75 +38

V

Power-Supply Rejection Ratio

PSRR

VCC - VEE = +4.75V to +38V

82 100

dB

Quiescent Supply Current

ICC

TA = +25°C

400 700

µA

TA = TMIN to TMAX

850

Note 2: All devices are 100% production tested at TA = +25°C. All temperature limits are guaranteed by design.

Note 3: Guaranteed by design. IN+ and IN- are internally connected to the gates of CMOS transistors. CMOS GATE leakage is so small that it is impractical to test in production. Devices are screened during production testing to eliminate defective units.

Note 4: Specified over all temperatures and process variation by circuit simulation.


MAX9945

SUPPLY CURRENT (A)

Typical Operating Characteristics

(VCC = +15V, VEE = -15V, VIN+ = VIN- = VGND = 0V, ROUT = 100kΩ to GND, TA = -40°C to +125°C, typical values are at TA = +25°C,

unless otherwise noted.)


600

QUIESCENT SUPPLY CURRENT

MAX9945 toc03

vs. SUPPLY VOLTAGE AND TEMPERATURE


0.25

OUTPUT VOLTAGE SWING LOW vs. TEMPERATURE


MAX9945 toc02

0.25

OUTPUT VOLTAGE SWING HIGH vs. TEMPERATURE



500


400


300

0.20


MAX9945 toc01

VOL - VEE (V)















TA = +125C








TA = +25C








TA = -40C




0.15


0.10


0.05


ISINK


= 0.1mA


ISINK = 1.0mA

0.20


VCC - VOH (V)

0.15


0.10


0.05


ISOURCE = 1.0mA ISOURCE = 0.1mA


200

5


10 15


20 25 30 35

0

-40


-20 0


20 40 60


80 100 120

0

-40


-20 0


20 40 60


80 100 120


80

70

60

IBIAS (pA)

50

40

30

20

10

0

-10

SUPPLY VOLTAGE (V)


MAX9945 toc04

INPUT BIAS CURRENT vs. TEMPERATURE










































































TEMPERATURE (C)


INPUT VOLTAGE 0.1Hz TO 10Hz NOISE


MAX9945 toc05


INPUT VOLTAGE-NOISE DENSITY (nV/ Hz)

1000


100


10

TEMPERATURE (C)


MAX9945 toc06

INPUT VOLTAGE-NOISE DENSITY vs. FREQUENCY

-40

-20

0 20 40

60 80

100

120

1s/div

1 10

100

1000

10,000 100,000

TEMPERATURE (C)


TOTAL HARMONIC DISTORTION vs. FREQUENCY

-70

VCC - VEE = 30V, 4.5VP-P, RL = 10kΩ

-80

1V/div


-50


-60

FREQUENCY (Hz)


MAX9945 toc08

TOTAL HARMONIC DISTORTION + NOISE vs. FREQUENCY

VCC - VEE = 30V 4.5VP-P

RL = 10kΩ


THD (dB)

-90


MAX9945 toc07

-70


THD+N (dB)

-80


-100


-90


-110


100


1000


10,000


100,000

-100

10


100 1000


10,000


100,000

FREQUENCY (Hz) FREQUENCY (Hz)


MAX9945

Typical Operating Characteristics (continued)

(VCC = +15V, VEE = -15V, VIN+ = VIN- = VGND = 0V, ROUT = 100kΩ to GND, TA = -40°C to +125°C, typical values are at TA = +25°C,

unless otherwise noted.)


1000


INPUT OFFSET VOLTAGE (V)

800


600


400

INPUT OFFSET VOLTAGE vs. COMMON-MODE VOLTAGE


MAX9945 toc09

1000


INPUT OFFSET VOLTAGE (V)

800


600


400

INPUT OFFSET VOLTAGE vs. TEMPERATURE


MAX9945 toc10

VCM = VCC - 1.2V


VCM = 0V


200


0


-15


-10 -5 0


5 10

200
































0


-40


-20 0 20


VCM = VEE


40 60 80


100 120

COMMON-MODE VOLTAGE (V) TEMPERATURE (C)



120


OPEN-LOOP GAIN (dB)

80


40


0


MAX9945 toc12

-40

OPEN-LOOP GAIN vs. FREQUENCY


MAX9945 toc11









































































































































-20


-30


-40


CMRR (dB)

-50


-60


-70


-80


-90


-100

COMMON-MODE REJECTION RATIO vs. FREQUENCY

1m 1 10 100 1k 10k 100k 1M 10M

10 100 1k 10k

100k 1M

10M

FREQUENCY (Hz)

FREQUENCY (Hz)


POWER-SUPPLY REJECTION RATIO vs. FREQUENCY

0


10,000

RESISTOR ISOLATION vs. CAPACITIVE LOAD


-20


PSRR (dB)

-40


-60


-80


-100


-120


UNIPOLAR

PSRR-


UNIPOLAR PSRR+


BIPOLAR PSRR


1000


CLOAD (pF)

100


MAX9945 toc13

MAX9945 toc14

UNSTABLE


STABLE

1 10

100 1k

10k

100k 1M

10M

1 10

100

FREQUENCY (Hz)

RISO (Ω)


MAX9945

Typical Operating Characteristics (continued)

(VCC = +15V, VEE = -15V, VIN+ = VIN- = VGND = 0V, ROUT = 100kΩ to GND, TA = -40°C to +125°C, typical values are at TA = +25°C,

unless otherwise noted.)


10,000

OP-AMP STABILITY

vs. CAPACITIVE AND RESISTIVE LOADS


MAX9945 toc15

1000.00

OUTPUT IMPEDANCE vs. FREQUENCY



PARALLEL LOAD CAPACITANCE (pF)

1000

UNSTABLE

100.00


OUTPUT IMPEDANCE (Ω)

MAX9945 toc16

10.00


100


STABLE


1.00


0.10


ACL = 10


A

CL = 1


10

10 100 1000


10,000

0.01

10


100 1k


10k


100k 1M


10M

PARALLEL LOAD RESISTANCE (kΩ) FREQUENCY (Hz)


LARGE-SIGNAL RESPONSE vs. FREQUENCY


LARGE SIGNAL-STEP RESPONSE

MAX9945 toc17

MAX9945 toc18

30

RLOAD

OUTPUT VOLTAGE (VP-P)

25


20


15

= 100kΩ


+5V


VOUT

2.5V/div

AV = 1V/V VIN = 10VP-P

CL = 100pF

RL = 10kΩ


10

-5V

5


0

1 10


100

FREQUENCY (kHz)


1000


10,000


4s/div


LARGE SIGNAL-STEP RESPONSE

MAX9945 toc19

SMALL SIGNAL-STEP RESPONSE

MAX9945 toc20


AV = 1V/V

VIN = 40mVP-P RL = 100kΩ

AV = 1V/V VIN = 2VP-P RL = 10kΩ CL = 100pF

+1V +20mV


VOUT

500mV/div

VOUT

10mV/div


-1V -20mV


1s/div 2s/div


MAX9945

Pin Description


NAME

FUNCTION



1

6

OUT

Amplifier Output

2

4

VEE

Negative Power Supply. Bypass VEE with 0.1µF ceramic and 4.7µF electrolytic capacitors to quiet ground plane if different from VEE.

3

3

IN+

Noninverting Amplifier Input

4

2

IN-

Inverting Amplifier Input

5

1, 5, 8

N.C.

No Connection. Not internally connected.

6

7

VCC

Positive Power Supply. Bypass VCC with 0.1µF ceramic and 4.7µF electrolytic capacitors to quiet ground plane or VEE.

EP

Exposed Pad (TDFN Only). Connect to VEE externally. Connect to a large copper plane to maximize thermal performance. Not intended as an electrical connection (TDFN only).

PIN

TDFN-EP


µMAX


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

Rail-to-Rail Output Stage The MAX9945 output stage swings to within 50mV (typ) of either power-supply rail with a 100kΩ 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. The MAX9945 can interface with small Applications Information

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 voltage noise is a low 2µVP-P (typ). The input stage accepts a wide common-mode range, extending from the negative supply, VEE, to within 1.2V of the positive supply, VCC.

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.



1

+

8



-

VOUT

IN+

MAX9945

MAX

+

MAX9945

5

4

IN-

6

3

7

2

MAX9945

Figure 1. Shielding the Inverting Input to Reduce Leakage


10kΩ

IN+

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 a small temperature variation is very important for these applications.

Power-Supply Decoupling The MAX9945 operates from a +4.75V to +38V, VEE ref- erenced power supply. Bypass the power-supply inputs VCC and VEE to a quiet copper ground plane, with a 0.1µF ceramic capacitor in parallel with a 4.7µF electrolytic capacitor, placed close to the leads.

Layout Techniques A good layout is critical to obtaining high performance especially when interfacing with high-impedance sen- sors. Use shielding techniques to guard against para- sitic leakage paths. For transimpedance applications, for example, surround the inverting input, and the traces connecting to it, with a buffered version of its own voltage. A convenient source of this voltage is the noninverting input pin. Pins 1, 5, and 8 on the µMAX package are unconnected, and can be connected to an analog common potential, or to the driven guard potential, to reduce leakage on the inverting input.

A good layout guard rail isolates sensitive nodes, such as the inverting input of the MAX9945 and the traces connecting to it (see Figure 1), from varying or large volt- age differentials that otherwise occur in the rest of the circuit board. This reduces leakage and noise effects, allowing sensitive measurements to be made accurately.



MAX9945


10kΩ

IN-


Figure 2. Input Differential Voltage Protection


Take care to also decrease the amount of stray capaci- tance at the op amp’s inputs to improve stability. To achieve this, minimize trace lengths and resistor leads by placing external components as close as possible to the package. If the sensor is inherently capacitive, or is connected to the amplifier through a long cable, use a low-value feedback capacitor to control high-frequency gain and peaking to stabilize the feedback loop.


Input Differential Voltage Protection During normal op-amp operation, the inverting and non- inverting inputs of the MAX9945 are at approximately the same voltage. The ±12V absolute maximum input differential voltage rating offers sufficient protection for most applications. If there is a possibility of exceeding the input differential voltage specification, in the pres- ence of extremely fast input voltage transients or due to certain application-specific fault conditions, use exter- nal low-leakage pico-amp diodes and series resistors to protect the input stage of the amplifier (see Figure 2). The extremely low input bias current of the MAX9945 allows a wide range of input series resistors to be used. If low input voltage noise is critical to the application, size the input series resistors appropriately.

Chip Information

MAX9945

PROCESS: BiCMOS


OUT

IN+ 3 6

MAX9945

Pin Configurations


TOP VIEW


+

+


N.C. 1 8 N.C.

OUT 1

6 VCC


IN- 2

MAX9945

7 VCC

VEE 2

MAX9945

5 N.C.


IN+ 3

4 IN-

VEE 4

5 N.C.

EP


µMAX

TDFN


MAX9945

Package Information

For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.


PACKAGE TYPE

PACKAGE CODE

OUTLINE NO.

LAND PATTERN NO.

6 TDFN-EP

T633+2

21-0137

90-0058

8 µMAX

U8+1

21-0036

90-0092











MAX9945

Package Information (continued)

For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.



COMMON DIMENSIONS

SYMBOL

MIN.

MAX.

A

0.70

0.80

D

2.90

3.10

E

2.90

3.10

A1

0.00

0.05

L

0.20

0.40

k

0.25 MIN.

A2

0.20 REF.

PACKAGE VARIATIONS

PKG. CODE

N

D2

E2

e

JEDEC SPEC

b

[(N/2)-1] x e

T633-2

6

1.50±0.10

2.30±0.10

0.95 BSC

MO229 / WEEA

0.40±0.05

1.90 REF

T833-2

8

1.50±0.10

2.30±0.10

0.65 BSC

MO229 / WEEC

0.30±0.05

1.95 REF

T833-3

8

1.50±0.10

2.30±0.10

0.65 BSC

MO229 / WEEC

0.30±0.05

1.95 REF

T1033-1

10

1.50±0.10

2.30±0.10

0.50 BSC

MO229 / WEED-3

0.25±0.05

2.00 REF

T1033MK-1

10

1.50±0.10

2.30±0.10

0.50 BSC

MO229 / WEED-3

0.25±0.05

2.00 REF

T1033-2

10

1.50±0.10

2.30±0.10

0.50 BSC

MO229 / WEED-3

0.25±0.05

2.00 REF

T1433-1

14

1.70±0.10

2.30±0.10

0.40 BSC

- - - -

0.20±0.05

2.40 REF

T1433-2

14

1.70±0.10

2.30±0.10

0.40 BSC

- - - -

0.20±0.05

2.40 REF

T1433-3F

14

1.70±0.10

2.30±0.10

0.40 BSC

- - - -

0.20±0.05

2.40 REF


MAX9945

Package Information (continued)

For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status.





MAX9945

Revision History


REVISION NUMBER

REVISION DATE


DESCRIPTION

PAGES CHANGED

0

2/09

Initial release

1

12/10

Updated Input Bias Current spec in the Electrical Characteristics table and updated Note 3

2, 3


Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

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