General Description
The MAX9945 operational amplifier features an excellent
combination of low operating power and low input voltage 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 current. The MAX9945 is unity-gain stable and is capable of
rail-to-rail output voltage swing.
The MAX9945 is ideal for portable medical and industrial applications that require low noise analog front-ends
for performance applications such as photodiode transimpedance 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).
Applications
Medical Pulse Oximetry
Photodiode Sensor Interface
Industrial Sensors and Instrumentation
Chemical Sensor Interface
High-Performance Audio Line Out
Active Filters and Signal Processing
Features
♦ +4.75V to +38V Single-Supply Voltage Range
♦ ±2.4V to ±19V Dual-Supply Voltage Range
♦ Rail-to-Rail Output Voltage Swing
♦ 400µA Low Quiescent Current
♦ 50fA Low Input Bias Current
♦ 1fA/
√
√
Hz Low Input Current Noise
♦ 15nV/
√
√
Hz Low Noise
♦ 3MHz Unity-Gain Bandwidth
♦ Wide Temperature Range from -40°C to +125°C
♦ Available in Space-Saving, 6-Pin TDFN Package
(3mm x 3mm)
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
________________________________________________________________
Maxim Integrated Products
1
19-4398; Rev 0; 2/09
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.
Ordering Information
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
µMAX is a registered trademark of Maxim Integrated Products, Inc.
IN-
IN+
OUT
V
EE
V
CC
PHOTODIODE
SIGNAL
CONDITIONING/
FILTERS
ADC
MAX9945
Typical Operating Circuit
PART TEMP RANGE
MAX9945ATT+ -40°C to +125°C 6 TDFN-EP* AUE
MAX9945AUA+ -40°C to +125°C 8 µMAX —
PINPACKAGE
TOP
MARK
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC= +15V, VEE= -15V, V
IN+
= V
IN-
= GND = 0, R
OUT
= 100kΩ to GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.) (Note 2)
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.
Supply Voltage (VCCto VEE) ..................................-0.3V to +40V
IN+, IN-, OUT Voltage......................(V
EE
- 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 (T
A
= +70°C)
6-Pin TDFN-EP (derate 23.8mW/°C above +70°C)
Multilayer Board ....................................................1904.8mW
Package Thermal Resistance (Note 1)
θ
JA
..............................................................................42°C/W
θ
JC
................................................................................9°C/W
8-Pin µMAX (derate 4.8mW/°C above +70°C)
Multilayer Board ......................................................387.8mW
Package Thermal Resistance (Note 1)
θ
JA
.........................................................................206.3°C/W
θ
JC
..............................................................................42°C/W
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
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
.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DC ELECTRICAL CHARACTERISTICS
Input Voltage Range V
Input Offset Voltage V
Input Offset Voltage Drift V
Input Bias Current (Note 3) I
Common-Mode Rejection Ratio CMRR
Open-Loop Gain A
Output Short-Circuit Current I
Guaranteed by
, V
IN+
IN-
CMRR
TA = +25°C ±0.6 ±5
OS
TA = T
- T
OS
C
B
VCM = VEE to VCC - 1.2V,
T
V
T
V
R
OL
V
R
SC
to T
MIN
= +25°C
A
= VEE to VCC - 1.4V,
CM
= T
A
EE
OUT
EE
OUT
to T
MIN
+ 0.3V ≤ V
= 100kΩ to GND
+ 0.75V ≤ V
= 10kΩ to GND
VCC -
1.2
VCC -
1.4
±8
MAX
TA = +25°C V
= T
MIN
to T
MAX
T
A
EE
V
EE
2 µV/°C
50 fA
78 94
MAX
≤ VCC - 0.3V,
OUT
≤ VCC - 0.75V,
OUT
78 94
110 130
110 130
25 mA
V
mV
dB
dB
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC= +15V, VEE= -15V, V
IN+
= V
IN-
= GND = 0, R
OUT
= 100kΩ to GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.) (Note 2)
Note 2: All devices are 100% production tested at T
A
= +25°C. All temperature limits are guaranteed by design.
Note 3: 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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Output Voltage Low V
Output Voltage High V
AC ELECTRICAL CHARACTERISTICS
Input Current-Noise Density I
Input Voltage Noise V
Gain Bandwidth GBW 3 MHz
Slew Rate SR 2.2 V/µs
Capacitive Loading (Note 4) C
Total Harmonic Distortion THD
POWER-SUPPLY ELECTRICAL CHARACTERISTICS
Power-Supply Voltage Range VCC - VEEGuaranteed by PSRR, V
Power-Supply Rejection Ratio PSRR VCC - VEE = +4.75V to +38V 82 100 dB
Quiescent Supply Current I
R
OL
R
GND
R
OH
NP-P
LOAD
CC
R
GND
f = 1kHz 1 fA/√Hz
N
f = 0.1Hz to 10Hz 2 µV
f = 100Hz 25
f = 1kHz 16.5Input Voltage-Noise Density V
N
f = 10kHz 15
No sustained oscillations 120 pF
V
f = 10kHz, R
TA = +25°C 400 700
TA = T
= 10kΩ to GND TA = T
OUT
= 100kΩ to
OUT
= 10kΩ to GND TA = T
OUT
= 100kΩ to
OUT
OUT
= 4.5V
MIN
P-P
OUT
to T
, AV = 1V/V,
MAX
= 10kΩ to GND
to T
MIN
MAX
T
= T
to T
MIN
MAX
to T
MIN
MAX
= T
to T
MIN
MAX
= 0 +4.75 +38 V
VCC -
0.45
VCC -
0.15
T
EE
A
A
VEE +
0.26
VEE +
0.05
VCC -
0.24
VCC -
0.03
VEE +
0.45
VEE +
0.15
97 dB
850
V
V
P-P
nV/√Hz
µA
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC= +15V, VEE= -15V, V
IN+
= V
IN-
= GND = 0, R
OUT
= 100kΩ to GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
INPUT BIAS CURRENT
vs. TEMPERATURE
MAX9945 toc04
TEMPERATURE (°C)
I
BIAS
(pA)
100806002040-20
0
10
20
30
40
50
60
70
80
-10
-40 120
µ
INPUT VOLTAGE-NOISE DENSITY
vs. FREQUENCY
MAX9945 toc06
FREQUENCY (Hz)
INPUT VOLTAGE-NOISE DENSITY (nV/ Hz)
10,000 100,000100010010
100
1000
10
1
TOTAL HARMONIC DISTORTION + NOISE
vs. FREQUENCY
MAX9945 toc08
FREQUENCY (Hz)
THD+N (dB)
-90
-80
-70
-60
-50
-100
100,00010,000100 100010
VCC - VEE = 30V
4.5V
P-P
R
L
= 10k
Ω
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE AND TEMPERATURE
600
0.25
OUTPUT VOLTAGE SWING LOW
vs. TEMPERATURE
OUTPUT VOLTAGE SWING HIGH
vs. TEMPERATURE
0.25
500
400
SUPPLY CURRENT (µA)
300
200
5
SUPPLY VOLTAGE (V)
TA = +125°C
TA = +25°C
TA = -40°C
MAX9945 toc01
0.20
0.15
(V)
EE
- V
OL
V
0.10
I
= 0.1mA
SINK
0.05
353025201510
0
-40
TEMPERATURE (°C)
I
SINK
= 1.0mA
MAX9945 toc02
0.20
(V)
0.15
OH
- V
CC
0.10
V
0.05
0
100 120806040200-20
-40
I
SOURCE
I
SOURCE
= 0.1mA
TEMPERATURE (°C)
= 1.0mA
MAX9945 toc03
100 120806040200-20
INPUT VOLTAGE
0.1Hz TO 10Hz NOISE
1s/div
V/div
1
MAX9945 toc05
-70
-80
-90
THD (dB)
-100
-110
TOTAL HARMONIC DISTORTION
vs. FREQUENCY
VCC - VEE = 30V,
, RL = 10kΩ
4.5V
P-P
100
FREQUENCY (Hz)
MAX9945 toc07
100,00010,0001000
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(VCC= +15V, VEE= -15V, V
IN+
= V
IN-
= GND = 0, R
OUT
= 100kΩ to GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
1000
800
600
400
INPUT OFFSET VOLTAGE (μV)
200
0
-15
120
80
40
OPEN-LOOP GAIN (dB)
0
-40
1m
vs. COMMON-MODE VOLTAGE
1
INPUT OFFSET VOLTAGE
COMMON-MODE VOLTAGE (V)
OPEN-LOOP GAIN
vs. FREQUENCY
100 1k 10k
10
FREQUENCY (Hz)
100k
INPUT OFFSET VOLTAGE
vs. TEMPERATURE
1000
VCM = VCC - 1.2V
MAX9945 toc09
1050-10 -5
800
600
400
INPUT OFFSET VOLTAGE (μV)
200
0
-40
VCM = 0
VCM = V
EE
TEMPERATURE (°C)
MAX9945 toc10
100 1208040 60-20 0 20
COMMON-MODE REJECTION RATIO
vs. FREQUENCY
-20
MAX9945 toc11
1M 10M
-30
-40
-50
-60
CMRR (dB)
-70
-80
-90
-100
10 10M
FREQUENCY (Hz)
MAX9945 toc12
1M100k100 1k 10k
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
0
-20
-40
-60
PSRR (dB)
-100
-120
UNIPOLAR
PSRR-
-80
1 10M
FREQUENCY (Hz)
UNIPOLAR
PSRR+
BIPOLAR
PSRR
1M100k10k1k10010
MAX9945 toc13
10,000
(pF)
1000
LOAD
C
100
RESISTOR ISOLATION
vs. CAPACITIVE LOAD
1
UNSTABLE
STABLE
R
ISO
MAX9945 toc14
10010
(Ω)
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VCC= +15V, VEE= -15V, V
IN+
= V
IN-
= GND = 0, R
OUT
= 100kΩ to GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
LARGE-SIGNAL RESPONSE
vs. FREQUENCY
MAX9945 toc17
FREQUENCY (kHz)
OUTPUT VOLTAGE (V
P-P
)
10,000100010010
5
10
15
25
20
30
0
1
R
LOAD
= 100kΩ
OUTPUT IMPEDANCE
vs. FREQUENCY
MAX9945 toc16
FREQUENCY (Hz)
OUTPUT IMPEDANCE (Ω)
1M100k10k1k100
0.10
1.00
10.00
100.00
1000.00
0.01
10 10M
ACL = 10
ACL = 1
10,000
OP-AMP STABILITY
vs. CAPACITIVE AND RESISTIVE LOADS
PARALLEL LOAD CAPACITANCE (pF)
1000
100
10
10
PARALLEL LOAD RESISTANCE (kΩ)
UNSTABLE
STABLE
MAX9945 toc15
10,000100 1000
+5V
V
OUT
2.5V/div
-5V
LARGE SIGNAL-STEP RESPONSE
MAX9945 toc18
AV = 1V/V
VIN = 10V
RL = 10kΩ
CL = 100pF
P-P
+1V
V
OUT
500mV/div
-1V
LARGE SIGNAL-STEP RESPONSE
AV = 1V/V
VIN = 2V
P-P
RL = 10kΩ
CL = 100pF
1μs/div
MAX9945 toc19
+20mV
V
OUT
10mV/div
-20mV
4μs/div
SMALL SIGNAL-STEP RESPONSE
AV = 1V/V
VIN = 40mV
P-P
RL = 100kΩ
2μs/div
MAX9945 toc20
Detailed Description
The MAX9945 features a combination of low input current 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 photodiode pulse oximeters, pH sensors, capacitive pressure 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 filters 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 2µV
P-P
(typ). The input stage
accepts a wide common-mode range, extending from
the negative supply, V
EE,
to within 1.2V of the positive
supply, VCC.
Rail-to-Rail Output Stage
The MAX9945 output stage swings to within 50mV (typ)
of either power-supply rail with a 100kΩ load and provides 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 capacitive 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 interfaces to both current-output sensors such as photodiodes 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 feedback capacitors help stabilize the amplifier by canceling 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 photodiode dark current if DC accuracy is important. The high
bandwidth and slew rate also allows AC signal processing in certain medical photodiode sensor applications such as pulse oximetry.
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
_______________________________________________________________________________________ 7
Pin Description
PIN
TDFN-EP µMAX
1 6 OUT Amplifier Output
24V
3 3 IN+ Noninverting Amplifier Input
4 2 IN- Inverting Amplifier Input
5 1, 5, 8 N.C. No Connection. Not internally connected.
67V
——EP
NAME FUNCTION
Negative Power Supply. Bypass VEE with 0.1µF ceramic and 4.7µF electrolytic
EE
capacitors to quiet ground plane if different from V
Positive Power Supply. Bypass VCC with 0.1µF ceramic and 4.7µF electrolytic capacitors
CC
to quiet ground plane or V
Exposed Pad. Connect to V
thermal performance. Not intended as an electrical connection (TDFN only).
EE.
.
EE
externally. Connect to a large copper plane to maximize
EE
MAX9945
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 impedance 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, VEEreferenced power supply. Bypass the power-supply
inputs VCCand VEEto 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 sensors. Use shielding techniques to guard against parasitic 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 voltage differentials that otherwise occur in the rest of the
circuit board. This reduces leakage and noise effects,
allowing sensitive measurements to be made accurately.
Take care to also decrease the amount of stray capacitance 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.
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
8 _______________________________________________________________________________________
Figure 1. Shielding the Inverting Input to Reduce Leakage
Figure 2. Input Differential Voltage Protection
+
1
8
IN-
MAX9945
IN+
2
3
4
+
V
OUT
-
10kΩ
10kΩ
MAX9945
μMAX
IN+
IN-
7
6
5
MAX9945
Input Differential Voltage Protection
During normal op-amp operation, the inverting and noninverting 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 presence of extremely fast input voltage transients or due to
certain application-specific fault conditions, use external 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.
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
_______________________________________________________________________________________ 9
Chip Information
PROCESS: BiCMOS
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
10 ______________________________________________________________________________________
Pin Configurations
TOP VIEW
N.C.
IN+
+
1
2
IN-
MAX9945
3
4
EE
μMAX
87N.C.
V
OUT
6
N.C.V
5
CC
+
16OUT V
25V
EE
MAX9945
34IN+ IN-
*EP
TDFN-EP
*EP = EXPOSED PAD
CC
N.C.
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
______________________________________________________________________________________ 11
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
6 TDFN-EP T633-2
21-0137
8 µMAX U8-1
21-0036
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
6, 8, &10L, DFN THIN.EPS
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
12 ______________________________________________________________________________________
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
COMMON DIMENSIONS
SYMBOL MIN. MAX.
A 0.70 0.80
D 2.90 3.10
E 2.90 3.10
A1
L 0.20 0.40
A2 0.20 REF.
0.00 0.05
0.25 MIN.k
PACKAGE VARIATIONS
PKG. CODE N D2 E2 e JEDEC SPEC b
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
1.50±0.10 MO229 / WEED-3
2.30±0.10 MO229 / WEED-3 2.00 REF0.25±0.050.50 BSC1.50±0.1010T1033-2
0.40 BSC - - - - 0.20±0.05 2.40 REFT1433-2 14 2.30±0.101.70±0.10
[(N/2)-1] x e
2.00 REF0.25±0.050.50 BSC2.30±0.1010T1033-1
2.40 REF0.20±0.05- - - - 0.40 BSC1.70±0.10 2.30±0.1014T1433-1
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
13
© 2009 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
α
α
8LUMAXD.EPS
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