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19-4398; Rev 0; 2/09
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 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 400A 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 MAX(R) and a space-saving, 6-pin TDFN package, and is specified over the automotive operating temperature range (-40C to +125C).
Features
+4.75V to +38V Single-Supply Voltage Range 2.4V to 19V Dual-Supply Voltage Range Rail-to-Rail Output Voltage Swing 400A 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 -40C to +125C Available in Space-Saving, 6-Pin TDFN Package (3mm x 3mm)
MAX9945
Applications
Medical Pulse Oximetry Photodiode Sensor Interface Industrial Sensors and Instrumentation Chemical Sensor Interface High-Performance Audio Line Out Active Filters and Signal Processing
MAX is a registered trademark of Maxim Integrated Products, Inc.
PART MAX9945ATT+ MAX9945AUA+
Ordering Information
TEMP RANGE PINPACKAGE TOP MARK AUE --
-40C to +125C 6 TDFN-EP* -40C to +125C 8 MAX
+Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad.
Typical Operating Circuit
VCC PHOTODIODE INOUT MAX9945 IN+ VEE SIGNAL CONDITIONING/ FILTERS ADC
________________________________________________________________ Maxim Integrated Products
1
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38V, Low-Noise, MOS-Input, Low-Power Op Amp 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 = +70C) 6-Pin TDFN-EP (derate 23.8mW/C above +70C) Multilayer Board ....................................................1904.8mW Package Thermal Resistance (Note 1) JA ..............................................................................42C/W JC ................................................................................9C/W 8-Pin MAX (derate 4.8mW/C above +70C) Multilayer Board ......................................................387.8mW Package Thermal Resistance (Note 1) JA .........................................................................206.3C/W JC ..............................................................................42C/W Operating Temperature Range .........................-40C to +125C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer 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- = GND = 0, ROUT = 100k to GND, TA = -40C to +125C, typical values are at TA = +25C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX VCC 1.2 V TA = TMIN to TMAX Input Offset Voltage Input Offset Voltage Drift Input Bias Current (Note 3) VOS VOS - TC IB VCM = VEE to VCC - 1.2V, TA = +25C VCM = VEE to VCC - 1.4V, TA = TMIN to TMAX VEE + 0.3V VOUT VCC - 0.3V, ROUT = 100k to GND VEE + 0.75V VOUT VCC - 0.75V, ROUT = 10k to GND 78 78 110 110 TA = +25C TA = TMIN to TMAX 2 50 94 dB 94 130 dB 130 25 mA VEE 0.6 VCC 1.4 5 8 mV V/C fA UNITS
DC ELECTRICAL CHARACTERISTICS Guaranteed by CMRR TA = +25C VEE
Input Voltage Range
VIN+, VIN-
Common-Mode Rejection Ratio
CMRR
Open-Loop Gain
AOL
Output Short-Circuit Current
ISC
2
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +15V, VEE = -15V, VIN+ = VIN- = GND = 0, ROUT = 100k to GND, TA = -40C to +125C, typical values are at TA = +25C, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS ROUT = 10k to GND Output Voltage Low VOL ROUT = 100k to GND ROUT = 10k to GND Output Voltage High VOH ROUT = 100k to GND f = 1kHz f = 0.1Hz to 10Hz f = 100Hz Input Voltage-Noise Density Gain Bandwidth Slew Rate Capacitive Loading (Note 4) Total Harmonic Distortion VN GBW SR CLOAD THD No sustained oscillations VOUT = 4.5VP-P, AV = 1V/V, f = 10kHz, ROUT = 10k to GND Guaranteed by PSRR, VEE = 0 VCC - VEE = +4.75V to +38V TA = +25C TA = TMIN to TMAX +4.75 82 100 400 700 850 f = 1kHz f = 10kHz TA = TMIN to TMAX TA = TMIN to TMAX TA = TMIN to TMAX TA = TMIN to TMAX VCC 0.45 VCC 0.15 MIN TYP VEE + 0.26 VEE + 0.05 VCC 0.24 V VCC 0.03 1 2 25 16.5 15 3 2.2 120 97 MHz V/s pF dB nV/Hz fA/Hz VP-P MAX VEE + 0.45 V VEE + 0.15 UNITS
MAX9945
AC ELECTRICAL CHARACTERISTICS Input Current-Noise Density Input Voltage Noise IN VNP-P
POWER-SUPPLY ELECTRICAL CHARACTERISTICS Power-Supply Voltage Range Power-Supply Rejection Ratio Quiescent Supply Current VCC - VEE PSRR ICC +38 V dB A
Note 2: All devices are 100% production tested at TA = +25C. 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.
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38V, Low-Noise, MOS-Input, Low-Power Op Amp MAX9945
Typical Operating Characteristics
(VCC = +15V, VEE = -15V, VIN+ = VIN- = GND = 0, ROUT = 100k to GND, TA = -40C to +125C, typical values are at TA = +25C, unless otherwise noted.)
QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE AND TEMPERATURE
MAX9945 toc01
OUTPUT VOLTAGE SWING LOW vs. TEMPERATURE
MAX9945 toc02
OUTPUT VOLTAGE SWING HIGH vs. TEMPERATURE
MAX9945 toc03
600
0.25
0.25
SUPPLY CURRENT (A)
500 VOL - VEE (V) TA = +125C 400 TA = +25C 300 TA = -40C 200 5 10 15 20 25 30 35 SUPPLY VOLTAGE (V)
0.20
0.20 VCC - VOH (V)
0.15 ISINK = 1.0mA 0.10 ISINK = 0.1mA 0.05
0.15 ISOURCE = 1.0mA 0.10 ISOURCE = 0.1mA
0.05
0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
INPUT BIAS CURRENT vs. TEMPERATURE
MAX9945 toc04
INPUT VOLTAGE 0.1Hz TO 10Hz NOISE
MAX9945 toc05
INPUT VOLTAGE-NOISE DENSITY vs. FREQUENCY
INPUT VOLTAGE-NOISE DENSITY (nV/ Hz)
MAX9945 toc06
80 70 60 50 IBIAS (pA) 40 30 20 10 0 -10 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
1000
100
10 1s/div 1V/div 1 10 100 1000 10,000 100,000 FREQUENCY (Hz)
TOTAL HARMONIC DISTORTION vs. FREQUENCY
MAX9945 toc07
TOTAL HARMONIC DISTORTION + NOISE vs. FREQUENCY
VCC - VEE = 30V 4.5VP-P RL = 10k
MAX9945 toc08
-70 VCC - VEE = 30V, 4.5VP-P, RL = 10k -80
-50
-60 THD+N (dB)
THD (dB)
-70
-90
-80
-100
-90
-110 100 1000 10,000 100,000 FREQUENCY (Hz)
-100 10 100 1000 FREQUENCY (Hz) 10,000 100,000
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
Typical Operating Characteristics (continued)
(VCC = +15V, VEE = -15V, VIN+ = VIN- = GND = 0, ROUT = 100k to GND, TA = -40C to +125C, typical values are at TA = +25C, unless otherwise noted.)
INPUT OFFSET VOLTAGE vs. COMMON-MODE VOLTAGE
MAX9945 toc09
MAX9945
INPUT OFFSET VOLTAGE vs. TEMPERATURE
VCM = VCC - 1.2V INPUT OFFSET VOLTAGE (V) 800
MAX9945 toc10
1000
1000
INPUT OFFSET VOLTAGE (V)
800
600
600 VCM = 0
400
400
200
200 VCM = VEE
0 -15 -10 -5 0 5 10 COMMON-MODE VOLTAGE (V)
0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
OPEN-LOOP GAIN vs. FREQUENCY
MAX9945 toc11
COMMON-MODE REJECTION RATIO vs. FREQUENCY
-30 -40
MAX9945 toc12
-20
120 OPEN-LOOP GAIN (dB)
80 CMRR (dB) 1m 1 10 100 1k 10k 100k 1M 10M
-50 -60 -70
40
0
-80 -90
-40 FREQUENCY (Hz)
-100 10 100 1k 10k 100k 1M 10M FREQUENCY (Hz)
POWER-SUPPLY REJECTION RATIO vs. FREQUENCY
MAX9945 toc13
RESISTOR ISOLATION vs. CAPACITIVE LOAD
MAX9945 toc14
0 -20 -40 PSRR (dB)
10,000
UNSTABLE CLOAD (pF)
-60 -80 -100 -120 1
UNIPOLAR PSRR-
UNIPOLAR PSRR+
1000
BIPOLAR PSRR 100 10 100 1k 10k 100k 1M 10M 1
STABLE
10 RISO ()
100
FREQUENCY (Hz)
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38V, Low-Noise, MOS-Input, Low-Power Op Amp MAX9945
Typical Operating Characteristics (continued)
(VCC = +15V, VEE = -15V, VIN+ = VIN- = GND = 0, ROUT = 100k to GND, TA = -40C to +125C, typical values are at TA = +25C, unless otherwise noted.)
OP-AMP STABILITY vs. CAPACITIVE AND RESISTIVE LOADS
MAX9945 toc15
OUTPUT IMPEDANCE vs. FREQUENCY
MAX9945 toc16
10,000 PARALLEL LOAD CAPACITANCE (pF)
1000.00
1000
OUTPUT IMPEDANCE ()
UNSTABLE
100.00
10.00 ACL = 10 1.00 ACL = 1
100
STABLE
0.10
10 10 100 1000 10,000
PARALLEL LOAD RESISTANCE (k)
0.01 10 100 1k 10k 100k 1M 10M FREQUENCY (Hz)
LARGE-SIGNAL RESPONSE vs. FREQUENCY
MAX9945 toc17
LARGE SIGNAL-STEP RESPONSE
MAX9945 toc18
30 RLOAD = 100k 25 OUTPUT VOLTAGE (VP-P) 20 15 10
+5V
AV = 1V/V VIN = 10VP-P RL = 10k CL = 100pF
VOUT 2.5V/div
-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
+1V
AV = 1V/V VIN = 2VP-P RL = 10k CL = 100pF
AV = 1V/V VIN = 40mVP-P RL = 100k +20mV
VOUT 500mV/div
VOUT 10mV/div
-1V
-20mV
1s/div
2s/div
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
Pin Description
PIN TDFN-EP 1 2 3 4 5 6 -- MAX 6 4 3 2 1, 5, 8 7 -- NAME OUT VEE IN+ INN.C. VCC EP Amplifier Output Negative Power Supply. Bypass VEE with 0.1F ceramic and 4.7F electrolytic capacitors to quiet ground plane if different from VEE. Noninverting Amplifier Input Inverting Amplifier Input No Connection. Not internally connected. Positive Power Supply. Bypass VCC with 0.1F ceramic and 4.7F electrolytic capacitors to quiet ground plane or VEE. Exposed Pad. Connect to VEE externally. Connect to a large copper plane to maximize thermal performance. Not intended as an electrical connection (TDFN only). FUNCTION
MAX9945
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.
Rail-to-Rail Output Stage
The MAX9945 output stage swings to within 50mV (typ) of either power-supply rail with a 100k load and provides a 3MHz GBW with a 2.2V/s 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.
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 2VP-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.
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38V, Low-Noise, MOS-Input, Low-Power Op Amp MAX9945
+ 1 2 3 IN4 MAX9945 IN+ VOUT + 8 7 6 5
MAX9945 MAX
Figure 1. Shielding the Inverting Input to Reduce Leakage
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.
IN+ 10k
Power-Supply Decoupling
The MAX9945 operates from a +4.75V to +38V, VEE referenced power supply. Bypass the power-supply inputs VCC and VEE to a quiet copper ground plane, with a 0.1F ceramic capacitor in parallel with a 4.7F electrolytic capacitor, placed close to the leads.
MAX9945 10k IN-
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 MAX 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.
Figure 2. Input Differential Voltage Protection
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.
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
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. PROCESS: BiCMOS
Chip Information
MAX9945
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38V, Low-Noise, MOS-Input, Low-Power Op Amp MAX9945
Pin Configurations
TOP VIEW
+ N.C. 1 IN- 2 8 7 N.C. VCC VEE 6 5 OUT IN+ VEE 4 N.C. 3 *EP 2 OUT + 1 6 5 4 VCC N.C. IN-
MAX9945
IN+ 3
MAX9945
MAX
TDFN-EP
*EP = EXPOSED PAD
10
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 6 TDFN-EP 8 MAX PACKAGE CODE T633-2 U8-1 DOCUMENT NO. 21-0137 21-0036
MAX9945
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6, 8, &10L, DFN THIN.EPS
11
38V, Low-Noise, MOS-Input, Low-Power Op Amp MAX9945
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
PACKAGE VARIATIONS PKG. CODE T633-2 T833-2 T833-3 T1033-1 T1033-2 T1433-1 T1433-2 N 6 8 8 10 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEED-3 MO229 / WEED-3 ------b 0.400.05 0.300.05 0.300.05 0.250.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.95 REF 1.95 REF 2.00 REF 2.00 REF 2.40 REF 2.40 REF
0.25 MIN. 0.20 REF.
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38V, Low-Noise, MOS-Input, Low-Power Op Amp
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
MAX9945
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
(c) 2009 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
8LUMAXD.EPS

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