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20 A Maximum, Rail-to-Rail I/O, Zero Input Crossover Distortion Amplifiers AD8506/AD8508
FEATURES
PSRR: 100 dB minimum CMRR: 105 dB typical Very low supply current: 20 A per amp maximum 1.8 V to 5 V single-supply or 0.9 V to 2.5 V dual-supply operation Rail-to-rail input and output Low noise: 45 nV/Hz @ 1 kHz 2.5 mV offset voltage maximum Very low input bias current: 1 pA typical
PIN CONFIGURATIONS
OUT A 1 -IN A 2 +IN A 3 V- 4
8
AD8506
TOP VIEW (Not to Scale)
V+ OUT B +IN B
06900-002
06900-045
7 6 5
-IN B
Figure 1. 8-Lead MSOP (RM-8)
OUT A -IN A +IN A V+ +IN B -IN B OUT B
1 2 3 4 5 6 7
14 13
OUT D -IN D +IN D
APPLICATIONS
Pressure and position sensors Remote security Bio sensors IR thermometers Battery-powered consumer equipment Hazard detectors
AD8508
12
TOP VIEW 11 V- (Not to Scale) 10 +IN C
9 8
-IN C OUT C
Figure 2. 14-Lead TSSOP (RU-14)
GENERAL DESCRIPTION
The AD8506/AD8508 are dual and quad micropower amplifiers featuring rail-to-rail input and output swings while operating from a 1.8 V to 5 V single or from 0.9 V to 2.5 V dual power supply. Using a novel circuit technology, these low cost amplifiers offer zero crossover distortion (excellent PSRR and CMRR performance) and very low bias current, while operating with a supply current of less than 20 A per amplifier. This amplifier family offers the lowest noise in its power class. This combination of features makes the AD8506/AD8508 amplifiers ideal choices for battery-powered applications because they minimize errors due to power supply voltage variations over the lifetime of the battery and maintain high CMRR even for a rail-to-rail input op amp. Remote battery-powered sensors, handheld instrumentation and consumer equipment, hazard detection (for example, smoke, fire, and gas), and patient monitors can benefit from the features of the AD8506/AD8508 amplifiers. The AD8506/AD8508 are specified for both the industrial temperature range of -40C to +85C and the extended industrial temperature range of -40C to +125C. The AD8506 dual amplifiers are available in an 8-lead MSOP package. The AD8508 quad amplifiers are available in the 14-lead TSSOP package.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2007-2008 Analog Devices, Inc. All rights reserved.
AD8506/AD8508 TABLE OF CONTENTS
Features .............................................................................................. 1 Applications ....................................................................................... 1 Pin Configurations ........................................................................... 1 General Description ......................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Electrical Characteristics--5 V Operation................................ 3 Electrical Characteristics--1.8 V Operation ............................ 4 Absolute Maximum Ratings............................................................ 5 Thermal Resistance ...................................................................... 5 ESD Caution...................................................................................5 Typical Performance Characteristics ..............................................6 Theory of Operation ...................................................................... 13 Applications Information .............................................................. 15 Pulse Oximeter Current Source ............................................... 15 Four-Pole Low-Pass Butterworth Filter for Glucose Monitor ......................................................................... 16 Outline Dimensions ....................................................................... 17 Ordering Guide .......................................................................... 17
REVISION HISTORY
7/08--Rev. 0 to Rev. A Added AD8508 ................................................................... Universal Added TSSOP Package ...................................................... Universal Changes to Features Section and General Description Section . 1 Added Figure 2; Renumbered Sequentially .................................. 1 Changed Electrical Characteristics Heading to Electrical Characteristics--5 V Operation ..................................................... 3 Changes to Table 1 ............................................................................ 3 Added Electrical Characteristics--1.8 V Operation Heading .... 4 Changes to Table 2 ............................................................................ 4 Changes to Table 3, Thermal Resistance Section, and Table 4 ... 5 Added TA = 25C Condition to Typical Performance Characteristics Section..................................................................... 6 Changes to Figure 3, Figure 4, Figure 6, and Figure 7 ................. 6 Added Figure 11 and Figure 14....................................................... 7 Changes to Figure 17 Through Figure 20.......................................8 Changes to Figure 21 Through Figure 26.......................................9 Changes to Figure 27, Figure 28, Figure 30, and Figure 31....... 10 Changes to Figure 34, Figure 37, and Figure 38 ......................... 11 Added Figure 39 and Figure 40 .................................................... 12 Added Theory of Operation Section, Figure 41, and Figure 42 .......................................................................................... 13 Added Figure 43 and Figure 44 .................................................... 14 Added Applications Information Section and Figure 45 .......... 15 Added Figure 46 ............................................................................. 16 Updated Outline Dimensions ....................................................... 17 Added Figure 48 ............................................................................. 17 Changes to Ordering Guide .......................................................... 17 11/07--Revision 0: Initial Version
Rev. A | Page 2 of 20
AD8506/AD8508 SPECIFICATIONS
ELECTRICAL CHARACTERISTICS--5 V OPERATION
VSY = 5 V, VCM = VSY/2, TA = 25C, RL = 100 k to GND, unless otherwise noted. Table 1.
Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Symbol VOS IB -40C TA +85C -40C TA +125C Input Offset Current IOS -40C TA +85C -40C TA +125C -40C TA +125C 0 V VCM 5 V -40C TA +85C -40C TA +125C 0.05 V VOUT 4.95 V -40C TA +125C -40C TA +125C 0.5 Conditions 0 V VCM 5 V -40C TA +125C Min Typ 0.5 1 Max 2.5 3.5 10 100 600 5 50 130 5 Unit mV mV pA pA pA pA pA pA V dB dB dB dB dB V/C pF pF V V V V mV mV mV mV mA dB dB dB A A mV/s kHz Degrees V p-p nV/Hz fA/Hz
Input Voltage Range Common-Mode Rejection Ratio
CMRR
Large Signal Voltage Gain Offset Voltage Drift Input Capacitance Differential Mode Input Capacitance Common Mode OUTPUT CHARACTERISTICS Output Voltage High
AVO VOS/T CDIFF CCM VOH
0 90 90 85 105 100
105
120 2 3 4.2
Output Voltage Low
VOL
Short-Circuit Limit POWER SUPPLY Power Supply Rejection Ratio
ISC PSRR
RL = 100 k to GND -40C TA +125C RL = 10 k to GND -40C TA +125C RL = 100 k to VSY -40C TA +125C RL = 10 k to VSY -40C TA +125C VOUT = VSY or GND VSY = 1.8 V to 5 V -40C TA +85C -40C TA +125C VOUT = VSY/2 -40C TA +125C RL = 100 k, CL = 10 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 f = 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz
4.98 4.98 4.9 4.9
4.99 4.95 2 10 45 5 5 25 30
100 100 95
110
Supply Current per Amplifier DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density
ISY
15
20 25
SR GBP M en p-p en in
13 95 60 2.8 45 15
Rev. A | Page 3 of 20
AD8506/AD8508
ELECTRICAL CHARACTERISTICS--1.8 V OPERATION
VSY = 1.8 V, VCM = VSY/2, TA = 25C, RL = 100 k to GND, unless otherwise noted. Table 2.
Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Symbol VOS IB -40C TA +85C -40C TA +125C Input Offset Current IOS -40C TA +85C -40C TA +125C -40C TA +125C 0 V VCM 1.8 V -40C TA +85C -40C TA +125C 0.05 V VOUT 1.75 V -40C TA +125C -40C TA +125C 0.5 Conditions 0 V VCM 1.8 V -40C TA +125C Min Typ 0.5 1 Max 2.5 3.5 10 100 600 5 50 100 1.8 Unit mV mV pA pA pA pA pA pA V dB dB dB dB dB V/C pF pF V V V V mV mV mV mV mA dB dB dB A A mV/s kHz Degrees V p-p nV/Hz fA/Hz
Input Voltage Range Common-Mode Rejection Ratio
CMRR
Large Signal Voltage Gain Offset Voltage Drift Input Capacitance Differential Mode Input Capacitance Common Mode OUTPUT CHARACTERISTICS Output Voltage High
AVO VOS/T CDIFF CCM VOH
0 85 85 80 95 95
100
115 2.5 3 4.2
Output Voltage Low
VOL
Short-Circuit Limit POWER SUPPLY Power Supply Rejection Ratio
ISC PSRR
RL = 100 k to GND -40C TA +125C RL = 10 k to GND -40C TA +125C RL = 100 k to VSY -40C TA +125C RL = 10 k to VSY -40C TA +125C VOUT = VSY or GND VSY = 1.8 V to 5 V -40C TA +85C -40C TA +125C VOUT = VSY/2 -40C TA +125C RL = 100 k, CL = 10 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 f = 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz
1.78 1.78 1.65 1.65
1.79 1.75 2 12 4.5 5 5 25 25
100 100 95
110
Supply Current per Amplifier DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density
ISY
16.5
20 25
SR GBP M en p-p en in
13 95 60 2.8 45 15
Rev. A | Page 4 of 20
AD8506/AD8508 ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Supply Voltage Input Voltage Input Current1 Differential Input Voltage2 Output Short-Circuit Duration to GND Storage Temperature Range Operating Temperature Range Junction Temperature Range Lead Temperature (Soldering, 60 sec)
1
THERMAL RESISTANCE
Rating 5.5 V VSY 0.1 V 10 mA VSY Indefinite -65C to +150C -40C to +125C -65C to +150C 300C
JA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. This was measured using a standard two-layer board. Table 4. Thermal Resistance
Package Type 8-Lead MSOP (RM-8) 14-Lead TSSOP (RU-14) JA 190 180 JC 44 35 Unit C/W C/W
ESD CAUTION
Input pins have clamp diodes to the supply pins. Input current should be limited to 10 mA or less whenever the input signal exceeds the power supply rail by 0.5 V. 2 Differential input voltage is limited to 5 V or the supply voltage, whichever is less.
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Rev. A | Page 5 of 20
AD8506/AD8508 TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25C, unless otherwise noted.
250 VSY = 1.8V VCM = VSY/2 200
250 VSY = 5V VCM = VSY/2 200 NUMBER OF AMPLIFIERS
NUMBER OF AMPLIFIERS
150
150
100
100
50
50
06900-003
-3
-2
-1
0 VOS (mV)
1
2
3
4
-3
-2
-1
0 VOS (mV)
1
2
3
4
Figure 3. Input Offset Voltage Distribution
16 14
NUMBER OF AMPLIFIERS
Figure 6. Input Offset Voltage Distribution
12 VSY = 5V -40C TA +125C 10
NUMBER OF AMPLIFIERS
VSY = 1.8V -40C TA +125C
12 10 8 6 4 2 0
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
0
1
2
3
4
5
6
7
8
9
10
11
12
13
TCVOS (V/C)
TCVOS (V/C)
Figure 4. Input Offset Voltage Drift Distribution
2000 1500 1000 500
VOS (V)
Figure 7. Input Offset Voltage Drift Distribution
2000 1500 1000 500
VOS (V)
VSY = 1.8V
VSY = 5V
0 -500 -1000 -1500 -2000
0 -500 -1000 -1500 -2000
06900-005
06900-007
06900-004
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
1
2 VCM (V)
3
4
5
VCM (V)
Figure 5. Input Offset Voltage vs. Input Common-Mode Voltage
Figure 8. Input Offset Voltage vs. Input Common-Mode Voltage
Rev. A | Page 6 of 20
06900-008
06900-006
0 -4
0 -4
AD8506/AD8508
TA = 25C, unless otherwise noted.
-115 VSY = 1.8V -120 VSY = 5V
-120
-125
-130
VOS (V)
VOS (V)
06900-037
-125
-135
-130
-140 -135
-145
0
0.2
0.4
0.6
0.8 1.0 VCM (V)
1.2
1.4
1.6
1.8
0
1
2 VCM (V)
3
4
5
Figure 9. Input Offset Voltage vs. Input Common-Mode Voltage
600 550 500 450 IB (pA) 400 350 300 250 200 VSY = 1.8V
Figure 12. Input Offset Voltage vs. Input Common-Mode Voltage
600 550 500 450 VSY = 5V
IB (pA)
400 350 300 250 200
VCM (V)
VCM (V)
Figure 10. Input Bias Current vs. Common-Mode Voltage at 125C
1000 VSY = 1.8V VCM = VSY/2 100
Figure 13. Input Bias Current vs. Common-Mode Voltage at 125C
1000 VSY = 5V VCM = VSY/2 100
10
IB (pA)
10
1
IB (pA)
1
0.1
0.1
06900-018
35
45
55
65 75 85 95 TEMPERATURE (C)
105
115
125
Figure 11. Input Bias Current vs. Temperature
Figure 14. Input Bias Current vs. Temperature
Rev. A | Page 7 of 20
06900-019
0.01 25
35
45
55
65 75 85 95 TEMPERATURE (C)
105
115
125
0.01 25
06900-012
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
06900-009
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
06900-038
-140
-150
AD8506/AD8508
TA = 25C, unless otherwise noted.
10k VSY = 1.8V 10k
OUTPUT VOLTAGE TO SUPPLY RAIL (mV)
OUTPUT VOLTAGE TO SUPPLY RAIL (mV)
VSY = 5V
1k
1k VDD - VOH
100
100 VDD - VOH VOL
10
VOL
10
1
1
0.1
06900-010
0.01
0.1 LOAD CURRENT (mA)
1
10
0.01
0.1 1 LOAD CURRENT (mA)
10
100
Figure 15. Output Voltage to Supply Rail vs. Load Current
14
OUTPUT VOLTAGE TO SUPPLY RAIL (mV)
Figure 18. Output Voltage to Supply Rail vs. Load Current
14
OUTPUT VOLTAGE TO SUPPLY RAIL (mV)
VSY = 1.8V 12 VDD - VOH @ RL = 10k 10 8 6 4 2 0 -40 VDD - VOH @ RL = 100k VOL @ RL = 100k
06900-011
VSY = 5V 12 VDD - VOH @ RL = 10k 10 8 6 4 2 0 -40 VDD - VOH @ RL = 100k VOL @ RL = 100k -25 -10 5 20 35 50 65 80 95 110 125
06900-014 06900-024
VOL @ RL = 10k
VOL @ RL = 10k
-25
-10
5
20
35
50
65
80
95
110
125
TEMPERATURE (C)
TEMPERATURE (C)
Figure 16. Output Voltage to Supply Rail vs. Temperature
90 80 VCM = VSY/2 AD8506 AD8508
Figure 19. Output Voltage to Supply Rail vs. Temperature
90 80
TOTAL SUPPLY CURRENT (A)
VSY = 1.8V AND 5V VCM = VSY/2
TOTAL SUPPLY CURRENT (A)
70 60 50 40 30 20 10
06900-021
70 60 50 40 30 20 10 0 -40 AD8508, AD8508, AD8506, AD8506, -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 1.8V 5V 1.8V 5V 125
0
0
0.5
1.0
1.5 2.0 2.5 3.0 3.5 SUPPLY VOLTAGE (V)
4.0
4.5
5.0
110
Figure 17. Total Supply Current vs. Supply Voltage
Figure 20. Total Supply Current vs. Temperature
Rev. A | Page 8 of 20
06900-013
0.1 0.001
0.01 0.001
AD8506/AD8508
TA = 25C, unless otherwise noted.
120 100 80 60 40 GAIN PHASE VSY = 1.8V 120 100 80 60
120 100 80 60 PHASE
VSY = 5V
120 100 80 60
PHASE (Degrees)
40 GAIN (dB) 20 0 -20 -40 -60 -80
06900-022
GAIN (dB)
20 0 -20 -40 -60 -80 GAIN, CL = 0pF PHASE, CL = 0pF GAIN, CL = 50pF PHASE, CL = 50pF GAIN, CL = 100pF PHASE, CL = 100pF 1k 10k FREQUENCY (Hz) 100k
20 0 -20 -40 -60 -80
GAIN
40 20 0
-100 -120 100
-100 -120 1M
GAIN, CL = 0pF PHASE, CL = 0pF GAIN, CL = 50pF PHASE, CL = 50pF GAIN, CL = 100pF PHASE, CL = 100pF 1k 10k FREQUENCY (Hz) 100k
-20 -40 -60 -80
Figure 21. Open-Loop Gain and Phase vs. Frequency
50 40 30
Figure 24. Open-Loop Gain and Phase vs. Frequency
50 40 30
CLOSED-LOOP GAIN (dB)
G = -100
VSY = 1.8V
G = -100
VSY = 5V
CLOSED-LOOP GAIN (dB)
20 10 0 -10 -20 -30 -40
G = -10
20 10 0 -10 -20 -30 -40
G = -10
G = -1
G = -1
06900-017
1k
10k FREQUENCY (Hz)
100k
1M
1k
10k FREQUENCY (Hz)
100k
1M
Figure 22. Closed-Loop Gain vs. Frequency
10k VSY = 1.8V 1k G = 100 G = 10
ZOUT () ZOUT ()
Figure 25. Closed-Loop Gain vs. Frequency
10k VSY = 5V
1k G = 100 100 G=1 10 G = 10 G=1
100
10
1 1
0.1
06900-028
100
1k 10k FREQUENCY (Hz)
100k
1M
100
1k 10k FREQUENCY (Hz)
100k
1M
Figure 23. ZOUT vs. Frequency
Figure 26. ZOUT vs. Frequency
Rev. A | Page 9 of 20
06900-031
0.1 10
0.01 10
06900-020
-50 100
-50 100
06900-025
-100 100
-100 1M
PHASE (Degrees)
40
AD8506/AD8508
TA = 25C, unless otherwise noted.
100 VSY = 1.8V
100
VSY = 5V
90
90
80 CMRR (dB)
80
CMRR (dB)
70
70
60
60
50
50
100
1k 10k FREQUENCY (Hz)
100k
1M
100
1k 10k FREQUENCY (Hz)
100k
1M
Figure 27. CMRR vs. Frequency
100 90 80 70
PSRR (dB)
Figure 30. CMRR vs. Frequency
100 VSY = 1.8V 90 80 70
PSRR (dB)
VSY = 5V
60 50 40 30 20 10 PSRR-
06900-023
60 50 40 30
PSRR+
20 10 PSRR-
PSRR+
100
1k
10k
100k
1M
100
1k
10k
100k
1M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 28. PSRR vs. Frequency
80 70 60
OVERSHOOT (%)
Figure 31. PSRR vs. Frequency
80 VSY = 5V RL = 100k
VSY = 1.8V RL = 100k
70 60
OVERSHOOT (%)
50 40 30 -OVERSHOOT 20 +OVERSHOOT 10
06900-027
50 40 30 -OVERSHOOT 20 +OVERSHOOT 10
06900-030
0 10
100 LOAD CAPACITANCE (pF)
600
0 10
100 LOAD CAPACITANCE (pF)
600
Figure 29. Small Signal Overshoot vs. Load Capacitance
Figure 32. Small Signal Overshoot vs. Load Capacitance
Rev. A | Page 10 of 20
06900-026
0 10
0 10
06900-032
06900-029
40 10
40 10
AD8506/AD8508
TA = 25C, unless otherwise noted.
VSY = 1.8V RL = 100k CL = 200pF G=1
VSY = 5V RL = 100k CL = 200pF G=1
VOLTAGE (1V/DIV)
VOLTAGE (500mV/DIV)
TIME (100s/DIV)
TIME (100s/DIV)
Figure 33. Large Signal Transient Response
VSY = 1.8V RL = 100k CL = 200pF G=1
VOLTAGE (5mV/DIV)
Figure 36. Large Signal Transient Response
VSY = 5V RL = 100k CL = 200pF G=1
VOLTAGE (5mV/DIV)
TIME (100s/DIV)
TIME (100s/DIV)
Figure 34. Small Signal Transient Response
VSY = 1.8V AND 5V 2.78V p-p
Figure 37. Small Signal Transient Response
1k VSY = 1.8V AND 5V
VOLTAGE NOISE DENSITY (nV/Hz)
VOLTAGE (0.5V/DIV)
100
10
06900-034
TIME (4s/DIV)
1
10
Figure 35. Voltage Noise 0.1 Hz to 10 Hz
100 FREQUENCY (Hz)
1k
10k
Figure 38. Voltage Noise Density vs. Frequency
Rev. A | Page 11 of 20
06900-047
1
06900-046
06900-036
06900-035
06900-033
AD8506/AD8508
TA = 25C, unless otherwise noted.
-40 100k -50
CHANNEL SEPARATION (dB)
10k
VSY = 1.8V VIN = 1.5V p-p
CHANNEL SEPARATION (dB)
-40 100k -50 -60 -70 -80 -90 -100 -110
06900-049 06900-048
10k
VSY = 5V VIN = 4V p-p
-60 -70 -80 -90 -100 -110 -120 100
1k FREQUENCY (Hz)
10k
100k
-120 100
1k FREQUENCY (Hz)
10k
100k
Figure 39. Channel Separation vs. Frequency
Figure 40. Channel Separation vs. Frequency
Rev. A | Page 12 of 20
AD8506/AD8508 THEORY OF OPERATION
The AD8506/AD8508 are unity-gain stable CMOS rail-to-rail input/output operational amplifiers designed to optimize performance in current consumption, PSRR, CMRR, and zero crossover distortion, all embedded in a small package. The typical offset voltage is 500 V, with a low peak-to-peak voltage noise of 2.8 V p-p from 0.1 Hz to 10 Hz and a voltage noise density of 45 nV/Hz at 1 kHz. The AD8506/AD8508 are designed to solve two key problems in low voltage battery-powered applications: battery voltage decrease over time and rail-to-rail input stage distortion. In battery-powered applications, the supply voltage available to the IC is the voltage of the battery. Unfortunately, the voltage of a battery decreases as it discharges itself through the load. This voltage drop over the lifetime of the battery causes an error in the output of the op amps. Some applications requiring precision measurements during the entire lifetime of the battery use voltage regulators to power up the op amps as a solution. If a design uses standard battery cells, the op amps experience a supply voltage change from roughly 3.2 V to 1.8 V during the lifetime of the battery. This means that for a PSRR of 70 dB minimum in a typical op amp, the input-referred offset error is approximately 440 V. If the same application uses the AD8506/AD8508 with a 100 dB minimum PSRR, the error is only 14 V. It is possible to calibrate out this error or to use an external voltage regulator to power the op amp, but these solutions can increase system cost and complexity. The AD8506/AD8508 solve the impasse with no additional cost or error-nullifying circuitry. The second problem with battery-powered applications is the distortion caused by the standard rail-to-rail input stage. Using a CMOS non-rail-to-rail input stage (that is, a single differential pair) limits the input voltage to approximately one VGS (gatesource voltage) away from one of the supply lines. Because VGS for normal operation is commonly over 1 V, a single differential pair input stage op amp greatly restricts the allowable input voltage range when using a low supply voltage. This limitation restricts the number of applications where the non-rail-to-rail input op amp was originally intended to be used. To solve this problem, a dual differential pair input stage is usually implemented (see Figure 41); however, this technique has its own drawbacks. One differential pair amplifies the input signal when the commonmode voltage is on the high end, whereas the other pair amplifies the input signal when the common-mode voltage is on the low end. This method also requires a control circuitry to operate the two differential pairs appropriately. Unfortunately, this topology leads to a very noticeable and undesirable problem: if the signal level moves through the range where one input stage turns off and the other one turns on, noticeable distortion occurs (see Figure 42).
VDD VBIAS
VIN+ IB
Q3
Q1
Q2
Q4
VIN- IB
VSS
Figure 41. A Typical Dual Differential Pair Input Stage Op Amp (Dual PMOS Q1 and Q2 Transistors Form the Lower End of the Input Voltage Range Whereas Dual NMOS Q3 and Q4 Compose the Upper End)
300 250 200 150 100 50 VSY = 5V TA = 25C
VOS (V)
0 -50 -100 -150 -200 -250 0 0.5 1.0 1.5 2.0 2.5 3.0 VCM (V) 3.5 4.0 4.5 5.0
06900-040
-300
Figure 42. Typical Input Offset Voltage vs. Common-Mode Voltage Response in a Dual Differential Pair Input Stage Op Amp (Powered by 5 V Supply; Results of Approximately 100 Units per Graph Are Displayed)
This distortion forces the designer to come up with impractical ways to avoid the crossover distortion areas, therefore narrowing the common-mode dynamic range of the operational amplifier. The AD8506/AD8508 solve this crossover distortion problem by using an on-chip charge pump to power the input differential pair. The charge pump creates a supply voltage higher than the voltage of the battery, allowing the input stage to handle a wide range of input signal voltages without using a second differential pair. With this solution, the input voltage can vary from one supply extreme to the other with no distortion, thereby restoring the op amp full common-mode dynamic range.
Rev. A | Page 13 of 20
06900-039
AD8506/AD8508
The charge pump has been carefully designed so that switching noise components at any frequency, both within and beyond the amplifier bandwidth, are much lower than the thermal noise floor. Therefore, the spurious-free dynamic range (SFDR) is limited only by the input signal and the thermal or flicker noise. There is no intermodulation between input signal and switching noise. Figure 43 displays a typical front-end section of an operational amplifier with an on-chip charge pump.
VPP = POSITIVE PUMPED VOLTAGE = VDD + 1.8V VPP VB CASCODE STAGE AND RAIL-TO-RAIL OUTPUT STAGE VDD
Figure 44, input offset voltage vs. input common-mode voltage response, shows the typical response of 12 devices from Figure 8. Figure 44 has been expanded so that it is easier to compare with Figure 42, typical input offset voltage vs. common-mode voltage response in a dual differential pair input stage op amp.
300 250 200 150 100 50 VOS (V) 0 -50 -100 VSY = 5V, TA = 25C
+IN
Q1
Q2
-IN
OUT
-150 -200 -250
VSS
06900-041
0
0.5
1.0
1.5
2.0
2.5 3.0 VCM (V)
3.5
4.0
4.5
5.0
Figure 43. Typical Front-End Section of an Op Amp with Embedded Charge Pump
Figure 44. Input Offset Voltage vs. Input Common-Mode Voltage Response (Powered by a 5 V Supply; Results of 12 Units Are Displayed)
This solution improves the CMRR performance tremendously. For instance, if the input varies from rail-to-rail on a 2.5 V supply rail, using a part with a CMRR of 70 dB minimum, an input-referred error of 790 V is introduced. Another part with a CMRR of 52 dB minimum generates a 6.3 mV error. The AD8506/AD8508 CMRR of 90 dB minimum causes only a 79 V error. As with the PSRR error, there are complex ways to minimize this error, but the AD8506/AD8508 solve this problem without incurring unnecessary circuitry complexity or increased cost.
Rev. A | Page 14 of 20
06900-042
-300
AD8506/AD8508 APPLICATIONS INFORMATION
PULSE OXIMETER CURRENT SOURCE
A pulse oximeter is a noninvasive medical device used for measuring continuously the percentage of hemoglobin (Hb) saturated with oxygen and the pulse rate of a patient. Hemoglobin that is carrying oxygen (oxyhemoglobin) absorbs light in the infrared (IR) region of the spectrum; hemoglobin that is not carrying oxygen (deoxyhemoglobin) absorbs visible red (R) light. In pulse oximetry, a clip containing two LEDs (sometimes more, depending on the complexity of the measurement algorithm) and the light sensor (photodiode) is placed on the finger or earlobe of the patient. One LED emits red light (600 nm to 700 nm) and the other emits light in the near IR (800 nm to 900 nm) region. The clip is connected by a cable to a processor unit. The LEDs are rapidly and sequentially excited by two current sources (one for each LED), whose dc levels depend on the LED being driven, based on manufacturer requirements, and the detector is synchronized to capture the light from each LED as it is transmitted through the tissue. An example design of a dc current source driving the red and infrared LEDs is shown in Figure 45. These dc current sources allow 62.5 mA and 101 mA to flow through the red and infrared LEDs, respectively. First, to prolong battery life, the LEDs are driven only when needed. One-third of the ADG733 SPDT analog switch is used to disconnect/connect the 1.25 V voltage reference from/to each current circuit. When driving the LEDs, the ADR1581 1.25 V voltage reference is buffered by 1/2 of the AD8506; the presence of this voltage on the noninverting input forces the output of the op amp (due to the negative feedback) to maintain a level that makes its inverting input-to-track the noninverting pin. Therefore, the 1.25 V appears in parallel with the 20 R1 or 12.4 R5 current source resistor, creating the flow of the 62.5 mA or 101 mA current through the red or infrared LED as the output of the op amp turns on the Q1 or Q2 N-MOSFET IRLMS2002. The maximum total quiescent currents for the 1/2 AD8506, ADR1581, and ADG733 are 25 A, 70 A, and 1 A, respectively, making a total of 96 A current consumption (480 W power consumption) per circuit, which is good for a system powered by a battery. If the accuracy and temperature drift of the total design need to be improved, then a more accurate and low temperature coefficient drift voltage reference and current source resistor should be utilized. C3 and C4 are used to improve stabilization of U1; R3 and R7 are used to provide some current limit into the U1 inverting pin; and R2 and R6 are used to slow down the rise time of the N-MOSFET when it turns on. These elements may not be needed, or some bench adjustments may be required.
+5V CONNECT TO RED LED +5V C1 0.1F 62.5mA R2 V 22 OUT1 Q1 IRLMS2002 R3 1k R1 20 0.1% 1/8W MIN
8
C2 0.1F U1 1/2 U2 ADG733 +5V R4 53.6k VREF = 1.25V U3 ADR1581
AD8506
5
16 VDD 14 D1
S1A 12 S1B 13
7
V+ V-
4
6
15 D2
S2A 2 S2B 1
C3 22pF 4 D3
S3A 5 S3B 3
RED CURRENT SOURCE
8 GND VSS
9 A2 10 A1 11 A0 6 EN 7
CONNECT TO INFRARED LED U1 1/2 +5V
101mA R6 22 VOUT2 Q2 IRLMS2002 R7 1k
AD8506
8 1
V+ V-
4
3
2
I_BIT2 I_BIT1 I_BIT0 I_ENA
C4 22pF
Figure 45. Pulse Oximeter Red and Infrared Current Sources Using the AD8506 as a Buffer to the Voltage Reference Device
Rev. A | Page 15 of 20
06900-043
R5 INFRARED CURRENT 12.4 SOURCE 0.1% 1/4W MIN
AD8506/AD8508
FOUR-POLE LOW-PASS BUTTERWORTH FILTER FOR GLUCOSE MONITOR
There are several methods of glucose monitoring: spectroscopic absorption of infrared light in the 2 m to 2.5 m range, reflectance spectrophotometry, and the amperometric type using electrochemical strips with glucose oxidase enzymes. The amperometric type generally uses three electrodes: a reference electrode, a control electrode, and a working electrode. Although this is a very old technique and widely used, signal-to-noise ratio and repeatability can be improved using the AD8506 with its low peak-to-peak voltage noise of 2.8 V p-p from 0.1 Hz to 10 Hz and voltage noise density of 45 nV/Hz at 1 kHz. Another consideration is operation from a 3.3 V battery. Glucose signal currents are usually less than 3 A full scale, so the I-to-V
C1 1000pF R1 5M +3.3V WORKING CONTROL
3 8
converter requires low input bias current. The AD8506 is an excellent choice because it provides 1 pA typical and 10 pA maximum of input bias current at ambient temperature. A low-pass filter with a cutoff frequency of 80 Hz to 100 Hz is desirable in a glucose meter device to remove extraneous noise; this can be a simple two- or four-pole Butterworth. Low power op amps with bandwidths of 50 kHz to 500 kHz should be adequate. The AD8506 with its 95 kHz GBP and 15 A typical of current consumption meets these requirements. A circuit design of a four-pole Butterworth filter (preceded by a one-pole low-pass filter) is shown in Figure 46. With a 3.3 V battery, the total power consumption of this design is 297 W typical at ambient temperature.
+3.3V V+
1
R2 22.6k
R3 22.6k C3 0.047F
AD8506
5 8
U1 1/2
+3.3V R4 22.6k R5 22.6k C5 0.047F
2
REFERENCE
V-
2 4
AD8506
C2 0.1F
U1 1/2
V+
7
AD8506
3 8
U2 1/2
V-
6 4
V+
1
V-
4
VOUT
C4 0.1F
DUPLICATE OF CIRCUIT ABOVE
06900-044
Figure 46. A Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter
Rev. A | Page 16 of 20
AD8506/AD8508 OUTLINE DIMENSIONS
3.20 3.00 2.80
3.20 3.00 2.80
8
5
1
5.15 4.90 4.65
4
PIN 1 0.65 BSC 0.95 0.85 0.75 0.15 0.00 0.38 0.22 SEATING PLANE 1.10 MAX 8 0 0.80 0.60 0.40
0.23 0.08
COPLANARITY 0.10
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 47. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
5.10 5.00 4.90
14
8
4.50 4.40 4.30
1 7
6.40 BSC
PIN 1 1.05 1.00 0.80 0.65 BSC 1.20 MAX 0.15 0.05 0.30 0.19
0.20 0.09
SEATING COPLANARITY PLANE 0.10
8 0
0.75 0.60 0.45
COMPLIANT TO JEDEC STANDARDS MO-153-AB-1
Figure 48. 14-Lead Thin Shrink Small Outline Package [TSSOP] (RU-14) Dimensions shown in millimeters
ORDERING GUIDE
Model AD8506ARMZ-R21 AD8506ARMZ-REEL1 AD8508ARUZ1 AD8508ARUZ-REEL1
1
Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package Description 8-Lead Mini Small Outline Package [MSOP] 8-Lead Mini Small Outline Package [MSOP] 14-Lead Thin Shrink Small Outline Package [TSSOP] 14-Lead Thin Shrink Small Outline Package [TSSOP]
Package Option RM-8 RM-8 RU-14 RU-14
Branding A1X A1X
Z = RoHS Compliant Part.
Rev. A | Page 17 of 20
AD8506/AD8508 NOTES
Rev. A | Page 18 of 20
AD8506/AD8508 NOTES
Rev. A | Page 19 of 20
AD8506/AD8508 NOTES
(c)2007-2008 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06900-0-7/08(A)
Rev. A | Page 20 of 20

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