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MAX9945ATT+TN/AN/a2500avai38V, Low-Noise, MOS-Input, Low-Power Op Amp


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MAX9945ATT+T
38V, Low-Noise, MOS-Input, Low-Power Op Amp
General Description
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).
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 RangeRail-to-Rail Output Voltage Swing400µA Low Quiescent Current 50fA Low Input Bias Current 1fA/√√HzLow Input Current Noise 15nV/√√Hz Low Noise3MHz Unity-Gain BandwidthWide Temperature Range from -40°C to +125°CAvailable in Space-Saving, 6-Pin TDFN Package
(3mm x 3mm)
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp

19-4398; Rev 1; 12/10
Ordering Information
PARTTEMP RANGEPIN-
PACKAGE
TOP
MARK

MAX9945ATT+-40°C to +125°C6 TDFN-EP*AUE
MAX9945AUA+-40°C to +125°C8 µMAX—
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
µMAX is a registered trademark of Maxim Integrated Products, Inc.
IN-
IN+
OUT
VEE
VCC
PHOTODIODE
SIGNAL
CONDITIONING/
FILTERSADCMAX9945
Typical Operating Circuit
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS

(VCC= +15V, VEE= -15V, VIN+= VIN-= VGND= 0V, ROUT= 100kΩto 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......................(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
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
DC ELECTRICAL CHARACTERISTICS

TA = +25°CVEEVCC -
Input Voltage RangeVIN+, VIN-Guaranteed by
CMRR
TA = TMIN to TMAXVEEVCC -
TA = +25°C±0.6±5Input Offset VoltageVOSTA = TMIN to TMAX±8mV
Input Offset Voltage DriftVOS - TC2µV/°C
-40°C ≤ TA ≤ +25°C50150fA
-40°C ≤ TA ≤ +70°C12pA
-40°C ≤ TA ≤ +85°C55pAInput Bias Current (Note 3)IB
-40°C ≤ TA ≤ +125°C 1.9 nA
VCM = VEE to VCC - 1.2V,
TA = +25°C7894
Common-Mode Rejection RatioCMRR
VCM = VEE to VCC - 1.4V,
TA = TMIN to TMAX7894
VEE + 0.3V ≤ VOUT ≤ VCC - 0.3V,
ROUT = 100kΩ to GND110130
Open-Loop GainAOL
VEE + 0.75V ≤ VOUT ≤ VCC - 0.75V,
ROUT = 10kΩ to GND110130
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 /thermal-tutorial.
PACKAGE THERMAL CHARACTERISTICS (Note 1)

TDFN-EP
Junction-to-Ambient Thermal Resistance (θJA)............42°C/W
Junction-to-Case Thermal Resistance (θJC)...................9°C/W
µMAX
Junction-to-Ambient Thermal Resistance (θJA).......206.3°C/W
Junction-to-Case Thermal Resistance θJC...................42°C/W
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS

ROUT = 10kΩ to GNDTA = TMIN to TMAXVEE +
VEE +
Output Voltage LowVOL
ROUT = 100kΩ to
GNDTA = TMIN to TMAXVEE +
VEE +
ROUT = 10kΩ to GNDTA = TMIN to TMAXVCC -
VCC -
Output Voltage HighVOH
ROUT = 100kΩ to
GNDTA = TMIN to TMAXVCC -
VCC -
AC ELECTRICAL CHARACTERISTICS
Input Current-Noise DensityINf = 1kHz1fA/√Hz
Input Voltage NoiseVNP-Pf = 0.1Hz to 10Hz2µVP-P
f = 100Hz25
f = 1kHz16.5Input Voltage-Noise DensityVN
f = 10kHz15
nV/√Hz
Gain BandwidthGBW3MHz
Slew RateSR2.2V/µs
Capacitive Loading (Note 4)CLOADNo sustained oscillations120pF
Total Harmonic DistortionTHDVOUT = 4.5VP-P, AV = 1V/V,
f = 10kHz, ROUT = 10kΩ to GND97dB
POWER-SUPPLY ELECTRICAL CHARACTERISTICS

Power-Supply Voltage RangeVCC - VEEGuaranteed by PSRR, VEE = 0V+4.75+38V
Power-Supply Rejection RatioPSRRVCC - VEE = +4.75V to +38V82100dB
TA = +25°C400700Quiescent Supply CurrentICCTA = TMIN to TMAX850µA
ELECTRICAL CHARACTERISTICS (continued)

(VCC= +15V, VEE= -15V, VIN+= VIN-= VGND= 0V, ROUT= 100kΩto 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 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
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
Typical Operating Characteristics

(VCC= +15V, VEE= -15V, VIN+= VIN-= VGND= 0V, ROUT= 100kΩto GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE AND TEMPERATURE

MAX9945 toc01
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (3025201510
TA = +125°C
TA = +25°C
TA = -40°C
OUTPUT VOLTAGE SWING LOW
vs. TEMPERATURE

MAX9945 toc02
TEMPERATURE (°C)
- V
(V)
ISINK = 1.0mA
ISINK = 0.1mA
OUTPUT VOLTAGE SWING HIGH
vs. TEMPERATURE

MAX9945 toc03
TEMPERATURE (°C)
- V
(V)
ISOURCE = 1.0mA
ISOURCE = 0.1mA
INPUT BIAS CURRENT
vs. TEMPERATURE

MAX9945 toc04
TEMPERATURE (°C)
IBIAS
(pA)
INPUT VOLTAGE
0.1Hz TO 10Hz NOISE
MAX9945 toc05
1s/div
1µV/div
INPUT VOLTAGE-NOISE DENSITY
vs. FREQUENCY

MAX9945 toc06
FREQUENCY (Hz)
INPUT VOLTAGE-NOISE DENSITY (nV/ Hz)
10,000100,000100010010
TOTAL HARMONIC DISTORTION
vs. FREQUENCY
MAX9945 toc07
FREQUENCY (Hz)
THD (dB)
100,00010,0001000
VCC - VEE = 30V,
4.5VP-P, RL = 10kΩ
TOTAL HARMONIC DISTORTION + NOISE
vs. FREQUENCY

MAX9945 toc08
FREQUENCY (Hz)
THD+N (dB)
100,00010,000100100010
VCC - VEE = 30V
4.5VP-P
RL = 10kΩ
INPUT OFFSET VOLTAGE
vs. COMMON-MODE VOLTAGE

MAX9945 toc09
COMMON-MODE VOLTAGE (V)
INPUT OFFSET VOLTAGE (50-10-5
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
INPUT OFFSET VOLTAGE
vs. TEMPERATURE
MAX9945 toc10
TEMPERATURE (°C)
INPUT OFFSET VOLTAGE (
VCM = 0V
VCM = VEE
VCM = VCC - 1.2V
OPEN-LOOP GAIN
vs. FREQUENCY

MAX9945 toc11
FREQUENCY (Hz)
OPEN-LOOP GAIN (dB)
-401101001k10k100k1M10M
COMMON-MODE REJECTION RATIO
vs. FREQUENCY

MAX9945 toc12
FREQUENCY (Hz)
CMRR (dB)100k1001k10k
-10010M
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY

MAX9945 toc13
PSRR (dB)100k10k1k10010
110M
UNIPOLAR
PSRR-UNIPOLAR
PSRR+
BIPOLAR
PSRR
RESISTOR ISOLATION
vs. CAPACITIVE LOAD

MAX9945 toc14
LOAD
(pF)
10,000
STABLE
UNSTABLE
Typical Operating Characteristics (continued)

(VCC= +15V, VEE= -15V, VIN+= VIN-= VGND= 0V, ROUT= 100kΩto GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
Typical Operating Characteristics (continued)

(VCC= +15V, VEE= -15V, VIN+= VIN-= VGND= 0V, ROUT= 100kΩto GND, TA= -40°C to +125°C, typical values are at TA= +25°C,
unless otherwise noted.)
LARGE SIGNAL-STEP RESPONSE

MAX9945 toc19
1µs/div
+1V
-1V
VOUT
500mV/div
AV = 1V/V
VIN = 2VP-P
RL = 10kΩ
CL = 100pF
SMALL SIGNAL-STEP RESPONSE

MAX9945 toc20
2µs/div
+20mV
-20mV
VOUT
10mV/div
AV = 1V/V
VIN = 40mVP-P
RL = 100kΩ
LARGE-SIGNAL RESPONSE
vs. FREQUENCY

MAX9945 toc17
FREQUENCY (kHz)
OUTPUT VOLTAGE (V
P-P
10,000100010010
RLOAD = 100kΩ
LARGE SIGNAL-STEP RESPONSE

MAX9945 toc18
4µs/div
+5V
-5V
VOUT
2.5V/div
AV = 1V/V
VIN = 10VP-PRL = 10kΩ
CL = 100pF
OUTPUT IMPEDANCE
vs. FREQUENCY

MAX9945 toc16
FREQUENCY (Hz)
OUTPUT IMPEDANCE (100k10k1k100
0.0110M
ACL = 10
ACL = 1
OP-AMP STABILITY
vs. CAPACITIVE AND RESISTIVE LOADS

MAX9945 toc15
PARALLEL LOAD RESISTANCE (kΩ)
PARALLEL LOAD CAPACITANCE (pF)
10,0001001000
10,000
STABLE
UNSTABLE
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
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.
Rail-to-Rail Output Stage

The MAX9945 output stage swings to within 50mV (typ)
of either power-supply rail with a 100kΩload 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.
MAX9945
38V, Low-Noise, MOS-Input,
Low-Power Op Amp
Pin Description
PIN
TDFN-EPµMAXNAMEFUNCTION
6OUTAmplifier OutputVEENegative Power Supply. Bypass VEE with 0.1µF ceramic and 4.7µF electrolytic
capacitors to quiet ground plane if different from VEE.3IN+Noninverting Amplifier Input2IN-Inverting Amplifier Input1, 5, 8N.C.No Connection. Not internally connected.VCCPositive Power Supply. Bypass VCC with 0.1µF ceramic and 4.7µF electrolytic capacitors
to quiet ground plane or VEE.EPExposed 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).
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