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Low Power, Wide Supply Range, Low Cost Difference Amplifier, G = 1/2, 2 AD8278
FEATURES
Wide input range beyond supplies Rugged input overvoltage protection Low supply current: 200 A maximum Low power dissipation: 0.5 mW at VS = 2.5 V Bandwidth: 1 MHz (G = 1/2) CMRR: 80 dB minimum, dc to 20 kHz (G = 1/2) Low offset voltage drift: 2 V/C maximum (B Grade) Low gain drift: 1 ppm/C maximum (B Grade) Enhanced slew rate: 1.4 V/s Wide power supply range: Single supply: 2 V to 36 V Dual supplies: 2 V to 18 V 8-lead SOIC and MSOP packages
FUNCTIONAL BLOCK DIAGRAM
+VS
7
AD8278
-IN 2 40k 20k
5
SENSE
6
OUT
+IN 3
40k
20k
1
REF
08308-001
4
-VS
Figure 1.
Table 1. Difference Amplifiers by Category
Low Distortion AD8270 AD8271 AD8273 AD8274 AMP03
1
APPLICATIONS
Voltage measurement and monitoring Current measurement and monitoring Instrumentation amplifier building block Portable, battery-powered equipment Test and measurement
High Voltage AD628 AD629
Current Sensing 1 AD8202 (U) AD8203 (U) AD8205 (B) AD8206 (B) AD8216 (B)
Low Power AD8276 AD8277
U = unidirectional, B = bidirectional.
GENERAL DESCRIPTION
The AD8278 is a general-purpose difference amplifier intended for precision signal conditioning in power critical applications that require both high performance and low power. The AD8278 provides exceptional common-mode rejection ratio (80 dB) and high bandwidth while amplifying signals well beyond the supply rails. The on-chip resistors are laser-trimmed for excellent gain accuracy and high CMRR. They also have extremely low gain drift vs. temperature. The common-mode range of the amplifier extends to almost triple the supply voltage (for G = 1/2), making it ideal for singlesupply applications that require a high common-mode voltage range. The internal resistors and ESD circuitry at the inputs also provide overvoltage protection to the op amp. The AD8278 can be used as a difference amplifier with G = 1/2 or G = 2. It can also be connected in a high precision, singleended configuration for non-inverting and inverting gains of -1/2, -2, +3, +2, +11/2, +1, or +1/2. The AD8278 provides an integrated precision solution that has a smaller size, lower cost, and better performance than a discrete alternative. The AD8278 operates on single supplies (2.0 V to 36 V) or dual supplies (2 V to 18 V). The maximum quiescent supply current is 200 A, which makes it ideal for battery-operated and portable systems. The AD8278 is available in the space-saving 8-lead MSOP and SOIC packages. It is specified for performance over the industrial temperature range of -40C to +85C and is fully RoHS compliant.
Rev. 0
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)2009 Analog Devices, Inc. All rights reserved.
AD8278 TABLE OF CONTENTS
Features .............................................................................................. 1 Applications ....................................................................................... 1 Functional Block Diagram .............................................................. 1 General Description ......................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 7 Thermal Resistance ...................................................................... 7 Maximum Power Dissipation ..................................................... 7 Short-Circuit Current .................................................................. 7 ESD Caution .................................................................................. 7 Pin Configurations and Function Descriptions ........................... 8 Typical Performance Characteristics ..............................................9 Theory of Operation ...................................................................... 16 Circuit Information.................................................................... 16 Driving the AD8278................................................................... 16 Input Voltage Range ................................................................... 16 Power Supplies ............................................................................ 17 Applications Information .............................................................. 18 Configurations ............................................................................ 18 Instrumentation Amplifier........................................................ 19 Outline Dimensions ....................................................................... 20 Ordering Guide .......................................................................... 21
REVISION HISTORY
7/09--Revision 0: Initial Version
Rev. 0 | Page 2 of 24
AD8278 SPECIFICATIONS
VS = 5 V to 15 V, VREF = 0 V, TA = 25C, RL = 10 k connected to ground, G = 1/2 difference amplifier configuration, unless otherwise noted. Table 2.
G=1/2 Parameter INPUT CHARACTERISTICS System Offset1 vs. Temperature Average Temperature Coefficient vs. Power Supply Common-Mode Rejection Ratio (RTI) Input Voltage Range2 Impedance3 Differential Common Mode DYNAMIC PERFORMANCE Bandwidth Slew Rate Settling Time to 0.01% Settling Time to 0.001% GAIN Gain Error Gain Drift Gain Nonlinearity OUTPUT CHARACTERISTICS Output Voltage Swing4 Short-Circuit Current Limit Capacitive Load Drive NOISE5 Output Voltage Noise POWER SUPPLY Supply Current6 vs. Temperature Operating Voltage Range7 TEMPERATURE RANGE Operating Range
1 2
Conditions
Min
Grade B Typ Max 50 100 100 1 2.5
Min
Grade A Typ Max 50 250 250 5 5
Unit V V V/C V/V
TA = -40C to +85C TA = -40C to +85C VS = 5 V to 18 V VS = 15 V, VCM = 27 V, RS = 0 0.3
2
80 -3(VS + 0.1) 120 30 1 1.4
+3(VS - 1.5)
74 -3(VS + 0.1) 120 30 1 1.4
dB +3(VS - 1.5) V k k MHz V/s 9 10 0.01 0.05 5 10 s s % ppm/C ppm
1.1 10 V step on output, CL = 100 pF
1.1 9 10
0.005 TA = -40C to +85C VOUT = 20 V p-p VS = 15 V, RL = 10 k TA = -40C to +85C
0.02 1 5
-VS + 0.2 15 200
+VS - 0.2
-VS + 0.2 15 200 1.4 47
+VS - 0.2
V mA pF V p-p nV/Hz A A V C
f = 0.1 Hz to 10 Hz f = 1 kHz
1.4 47
50 200 250 18 +125
50 200 250 18 +125
TA = -40C to +85C 2 -40
2 -40
Includes input bias and offset current errors, RTO (referred to output) The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation for details. 3 Internal resistors are trimmed to be ratio matched and have 20% absolute accuracy. 4 Output voltage swing varies with supply voltage and temperature. See Figure 20 through Figure 23 for details. 5 Includes amplifier voltage and current noise, as well as noise from internal resistors. 6 Supply current varies with supply voltage and temperature. See Figure 24 and Figure 26 for details. 7 Unbalanced dual supplies can be used, such as -VS = -0.5 V and +VS = +2 V. The positive supply rail must be at least 2 V above the negative supply and reference voltage.
Rev. 0 | Page 3 of 24
AD8278
VS = 5 V to 15 V, VREF = 0 V, TA = 25C, RL = 10 k connected to ground, G = 2 difference amplifier configuration, unless otherwise noted. Table 3.
G=2 Parameter INPUT CHARACTERISTICS System Offset 1 vs. Temperature Average Temperature Coefficient vs. Power Supply Common-Mode Rejection Ratio (RTI) Input Voltage Range 2 Impedance 3 Differential Common Mode DYNAMIC PERFORMANCE Bandwidth Slew Rate Settling Time to 0.01% Settling Time to 0.001% GAIN Gain Error Gain Drift Gain Nonlinearity OUTPUT CHARACTERISTICS Output Voltage Swing 4 Short-Circuit Current Limit Capacitive Load Drive NOISE 5 Output Voltage Noise POWER SUPPLY Supply Current 6 vs. Temperature Operating Voltage Range 7 TEMPERATURE RANGE Operating Range
1 2
Conditions
Min
Grade B Typ Max 100 200 200 2 5
Min
Grade A Typ Max 100 500 500 5 10
Unit V V V/C V/V dB V k k kHz V/s
TA = -40C to +85C TA = -40C to +85C VS = 5 V to 18 V VS = 15 V, VCM = 27 V, RS = 0 0.6
2
86 -1.5(VS + 0.1) 120 30 550 1.4
80 +1.5(VS - 1.5) -1.5(VS + 0.1) 120 30 550 1.4
+1.5(VS - 1.5)
1.1 10 V step on output, CL = 100 pF
1.1 10 11
10 11 0.01 0.05 5 10
s s % ppm/ C ppm
0.005 TA = -40C to +85C VOUT = 20 V p-p VS = 15 V, RL = 10 k TA = -40C to +85C
0.02 1 5
-VS + 0.2 15 350
+VS - 0.2
-VS + 0.2 15 350 2.8 90
+VS - 0.2
V mA pF V p-p nV/Hz A A V
f = 0.1 Hz to 10 Hz f = 1 kHz
2.8 90
95 200 250 18
95 200 250 18
TA = -40C to +85C 2
2
-40
+125
-40
+125
C
Includes input bias and offset current errors, RTO (referred to output). The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation for details. 3 Internal resistors are trimmed to be ratio matched and have 20% absolute accuracy. 4 Output voltage swing varies with supply voltage and temperature. See Figure 20 through Figure 23 for details. 5 Includes amplifier voltage and current noise, as well as noise from internal resistors. 6 Supply current varies with supply voltage and temperature. See Figure 24 and Figure 26 for details. 7 Unbalanced dual supplies can be used, such as -VS = -0.5 V and +VS = +2 V. The positive supply rail must be at least 2 V above the negative supply and reference voltage.
Rev. 0 | Page 4 of 24
AD8278
VS = +2.7 V to <5 V, VREF = midsupply, TA = 25C, RL = 10 k connected to midsupply, G = 1/2 difference amplifier configuration, unless otherwise noted. Table 4.
G=1/2 Parameter INPUT CHARACTERISTICS System Offset 1 vs. Temperature Average Temperature Coefficient vs. Power Supply Common-Mode Rejection Ratio (RTI) Conditions Min Grade B Typ Max 75 TA = -40C to +85C TA = -40C to +85C VS = 5 V to 18 V VS = 2.7 V, VCM = 0 V to 2.4 V, RS = 0 VS = 5 V, VCM = -10 V to +7 V, RS = 0 0.3 150 150 1 2.5 74 74 -3(VS + 0.1) 120 30 870 1.3 7 0.02 1 0.01 0.05 5 Min Grade A Typ Max 75 250 250 5 5 Unit V V V/C V/V dB dB V k k kHz V/s s % ppm/C
2
80 80 -3(VS + 0.1) 120 30 870 1.3
Input Voltage Range 2 Impedance 3 Differential Common Mode DYNAMIC PERFORMANCE Bandwidth Slew Rate Settling Time to 0.01% GAIN Gain Error Gain Drift OUTPUT CHARACTERISTICS Output Swing 4 Short-Circuit Current Limit Capacitive Load Drive NOISE 5 Output Voltage Noise POWER SUPPLY Supply Current 6 Operating Voltage Range TEMPERATURE RANGE Operating Range
1 2
+3(VS - 1.5)
+3(VS - 1.5)
2 V step on output, CL = 100 pF, VS = 2.7 V
7 0.005
TA = -40C to +85C RL = 10 k , TA = -40C to +85C
-VS + 0.1 10 200
+VS - 0.15
-VS + 0.1 10 200 1.4 47
+VS - 0.15
V mA pF V p-p nV/Hz A V C
f = 0.1 Hz to 10 Hz f = 1 kHz TA = -40C to +85C 2.0 -40
1.4 47
50 200 36 +125
50 200 36 +125
2.0 -40
Includes input bias and offset current errors, RTO (referred to output). The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation section for details. 3 Internal resistors are trimmed to be ratio matched and have 20% absolute accuracy. 4 Output voltage swing varies with supply voltage and temperature. See Figure 20 through Figure 23 for details. 5 Includes amplifier voltage and current noise, as well as noise from internal resistors. 6 Supply current varies with supply voltage and temperature. See Figure 25 and Figure 26 for details.
Rev. 0 | Page 5 of 24
AD8278
VS = +2.7 V to <5 V, VREF = midsupply, TA = 25C, RL = 10 k connected to midsupply, G = 2 difference amplifier configuration, unless otherwise noted. Table 5.
G=2 Parameter INPUT CHARACTERISTICS System Offset 1 vs. Temperature Average Temperature Coefficient vs. Power Supply Common-Mode Rejection Ratio (RTI) Conditions Min Grade B Typ Max 150 TA = -40C to +85C TA = -40C to +85C VS = 5 V to 18 V VS = 2.7 V, VCM = 0 V to 2.4 V, RS = 0 VS = 5 V, VCM = -10 V to +7 V, RS = 0 0.6 300 300 2 5 80 80 +1.5(VS - 1.5) -1.5(VS + 0.1) 120 30 450 1.3 2 V step on output, CL = 100 pF, VS = 2.7 V 9 0.005 TA = -40C to +85C RL = 10 k, TA = -40C to +85C 0.02 1 120 30 450 1.3 9 0.01 0.05 5 Min Grade A Typ Max 150 500 500 5 10 Unit V V V/C V/V dB dB +1.5(VS - 1.5) V k k kHz V/s s % ppm/C
3
86 86 -1.5(VS + 0.1)
Input Voltage Range 2 Impedance 3 Differential Common Mode DYNAMIC PERFORMANCE Bandwidth Slew Rate Settling Time to 0.01% GAIN Gain Error Gain Drift OUTPUT CHARACTERISTICS Output Swing 4 Short-Circuit Current Limit Capacitive Load Drive NOISE 5 Output Voltage Noise POWER SUPPLY Supply Current 6 Operating Voltage Range TEMPERATURE RANGE Operating Range
1 2
-VS + 0.1 10 200
+VS - 0.15
-VS + 0.1 10 200 2.8 94
+VS - 0.15
V mA pF V p-p nV/Hz A V C
f = 0.1 Hz to 10 Hz f = 1 kHz TA = -40C to +85C 2.0 -40
2.8 94
100 200 36 +125
100 220 36 +125
2.0 -40
Includes input bias and offset current errors, RTO (referred to output). The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation section for details. 3 Internal resistors are trimmed to be ratio matched and have 20% absolute accuracy. 4 Output voltage swing varies with supply voltage and temperature. See Figure 20 through Figure 23 for details. 5 Includes amplifier voltage and current noise, as well as noise from internal resistors. 6 Supply current varies with supply voltage and temperature. See Figure 25 and Figure 26 for details.
Rev. 0 | Page 6 of 24
AD8278 ABSOLUTE MAXIMUM RATINGS
2.0
Table 6.
Parameter Supply Voltage Maximum Voltage at Any Input Pin Minimum Voltage at Any Input Pin Storage Temperature Range Specified Temperature Range Package Glass Transition Temperature (TG) Rating 18 V -VS + 40 V +VS - 40 V -65C to +150C -40C to +85C 150C
MAXIMUM POWER DISSIPATION (W)
1.6
TJ MAX = 150C
1.2
SOIC JA = 121C/W MSOP JA = 135C/W
0.8
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.
0.4
-25
0
25
50
75
100
125
AMBIENT TEMERATURE (C)
Figure 2. Maximum Power Dissipation vs. Ambient Temperature
SHORT-CIRCUIT CURRENT
The AD8278 has built-in, short-circuit protection that limits the output current (see Figure 27 for more information). While the short-circuit condition itself does not damage the part, the heat generated by the condition can cause the part to exceed its maximum junction temperature, with corresponding negative effects on reliability. Figure 2 and Figure 27, combined with knowledge of the supply voltages and ambient temperature of the part can be used to determine whether a short circuit will cause the part to exceed its maximum junction temperature.
THERMAL RESISTANCE
The JA values in Table 7 assume a 4-layer JEDEC standard board with zero airflow. Table 7. Thermal Resistance
Package Type 8-Lead MSOP 8-Lead SOIC JA 135 121 Unit C/W C/W
MAXIMUM POWER DISSIPATION
The maximum safe power dissipation for the AD8278 is limited by the associated rise in junction temperature (TJ) on the die. At approximately 150C, which is the glass transition temperature, the properties of the plastic change. Even temporarily exceeding this temperature limit may change the stresses that the package exerts on the die, permanently shifting the parametric performance of the amplifiers. Exceeding a temperature of 150C for an extended period may result in a loss of functionality.
ESD CAUTION
Rev. 0 | Page 7 of 24
08308-002
0 -50
AD8278 PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
REF 1 -IN 2 +IN 3 -VS 4
8
AD8278
TOP VIEW (Not to Scale)
NC +VS OUT
08308-003
REF 1 -IN 2
7 6 5
AD8278
8 7 6 5
NC +VS OUT
08308-004
SENSE
TOP VIEW +IN 3 (Not to Scale) -VS 4
SENSE
NC = NO CONNECT
NC = NO CONNECT
Figure 3. MSOP Pin Configuration
Figure 4. SOIC Pin Configuration
Table 8. Pin Function Descriptions
Pin No. 1 2 3 4 5 6 7 8 Mnemonic REF -IN +IN -VS SENSE OUT +VS NC Description Reference Voltage Input. Inverting Input. Noninverting Input. Negative Supply. Sense Terminal. Output. Positive Supply. No Connect.
Rev. 0 | Page 8 of 24
AD8278 TYPICAL PERFORMANCE CHARACTERISTICS
VS = 15 V, TA = 25C, RL = 10 k connected to ground, G = 1/2 difference amplifier configuration, unless otherwise noted.
600 N = 3840 MEAN = -16.8 SD = 41.7673 80 60 40
NUMBER OF HITS
500
SYSTEM OFFSET (V)
08308-005
400
20 0 -20 -40 -60
300
200
100 -80
08308-008 08308-010 08308-009
0 -150 -100 -50 0 50 100 150 SYSTEM OFFSET VOLTAGE (V)
REPRESENTATIVE DATA -100 -50 -35 -20 -5 10
25
40
55
70
85
TEMPERATURE (C)
Figure 5. Distribution of Typical System Offset Voltage, G = 2
800 700 600 500 400 300 200 N = 3837 MEAN = 7.78 SD = 13.569
Figure 8. System Offset vs. Temperature, Normalized at 25, G = 1/2
20 15 10
GAIN ERROR (V/V)
NUMBER OF HITS
5 0 -5 -10 -15 -20
100 0 -60 -40 -20 0 20 40 60 CMRR (V/V)
-25
08308-006
REPRESENTATIVE DATA -30 -50 -35 -20 -5 10
25
40
55
70
85
TEMPERATURE (C)
Figure 6. Distribution of Typical Common-Mode Rejection, G = 2
10
Figure 9. Gain Error vs. Temperature, Normalized at 25C, G = 1/2
30
5
COMMON-MODE VOLTAGE (V)
20
VS = 15V
0
CMRR (V/V)
10
-5
0 VS = 5V -10
-10
-15 REPRESENTATIVE DATA -20 -50 -35 -20 -5 10
-20
25
40
55
70
85
08308-007
-30 -20
-15
-10
-5
0
5
10
15
20
TEMPERATURE (C)
OUTPUT VOLTAGE (V)
Figure 7. CMRR vs. Temperature, Normalized at 25C, G = 1/2
Figure 10. Input Common-Mode Voltage vs. Output Voltage, 15 V and 5 V Supplies, G = 1/2
Rev. 0 | Page 9 of 24
AD8278
10 8
COMMON-MODE VOLTAGE (V)
5 VREF = MIDSUPPLY VS = 5V
COMMON-MODE VOLTAGE (V)
VS = 5V 4 3 2 1 0 -1 -2 -3 -0.5 VS = 2.7V
VREF = MIDSUPPLY
6 4 2 0 -2 -4 -6 -8 VS = 2.7V
08308-011
0.5
1.5
2.5
3.5
4.5
5.5
0.5
1.5
2.5
3.5
4.5
5.5
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
Figure 11. Input Common-Mode Voltage vs. Output Voltage, 5 V and 2.7 V Supplies, VREF = Midsupply, G = 1/2
12 10
COMMON-MODE VOLTAGE (V)
Figure 14. Input Common-Mode Voltage vs. Output Voltage, 5 V and 2.7 V Supplies, VREF = Midsupply, G = 2
6 5
COMMON-MODE VOLTAGE (V)
VREF = 0V VS = 5V
VREF = 0V VS = 5V
8 6 4 2 0 -2 -4
08308-012
4 3 2 1 VS = 2.7V 0 -1 -2 -0.5
VS = 2.7V
0.5
1.5
2.5
3.5
4.5
5.5
0.5
1.5
2.5
3.5
4.5
5.5
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
Figure 12. Input Common-Mode Voltage vs. Output Voltage, 5 V and 2.7 V Supplies, VREF = 0 V, G = 1/2
30 VS = 15V 20
COMMON-MODE VOLTAGE (V)
Figure 15. Input Common-Mode Voltage vs. Output Voltage, 5 V and 2.7 V Supplies, VREF = 0 V, G = 2
18 12 6 GAIN = 2
10
GAIN (dB)
0 -6 -12 -18 -24 GAIN = 1/2
0
VS = 5V
-10
-20 -30
08308-013 08308-016
-30 -20
-15
-10
-5
0
5
10
15
20
-36 100
1k
10k
100k
1M
10M
OUTPUT VOLTAGE (V)
FREQUENCY (Hz)
Figure 13. Input Common-Mode Voltage vs. Output Voltage, 15 V and 5 V Supplies, G = 2
Figure 16. Gain vs. Frequency, 15 V Supplies
Rev. 0 | Page 10 of 24
08308-015
-6 -0.5
08308-014
-10 -0.5
AD8278
18 12 6 0
GAIN (dB)
+VS -0.1
GAIN = 2
OUTPUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES
-0.2 -0.3 -0.4 TA = -40C TA = +25C TA = +85C TA = +125C
-6 -12 -18 -24 -30
GAIN = 1/2
+0.4 +0.3 +0.2 +0.1
08308-017
1k
10k
100k
1M
10M
2
4
6
8
10
12
14
16
18
FREQUENCY (Hz)
SUPPLY VOLTAGE (VS)
Figure 17. Gain vs. Frequency, +2.7 V Single Supply
120 GAIN = 2 100 GAIN = 1/2 80
CMRR (dB)
Figure 20. Output Voltage Swing vs. Supply Voltage and Temperature, RL = 10 k
+VS -0.2
OUTPUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES
-0.4 -0.6 -0.8 -1.0 -1.2 TA = -40C TA = +25C TA = +85C TA = +125C
60
+1.2 +1.0 +0.8 +0.6 +0.4 +0.2 -VS
40
20
08308-018
0 1 10 100 1k 10k 100k 1M FREQUENCY (Hz)
2
4
6
8
10
12
14
16
18
SUPPLY VOLTAGE (VS)
Figure 18. CMRR vs. Frequency
120
Figure 21. Output Voltage Swing vs. Supply Voltage and Temperature, RL = 2 k
+VS
OUTPUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES
100 -PSRR 80
PSRR (dB)
-4
-8 TA = -40C TA = +25C TA = +85C TA = +125C +8
60 +PSRR 40
20
+4
08308-019
1
10
100
1k
10k
100k
1M
10k LOAD RESISTANCE ()
100k
FREQUENCY (Hz)
Figure 19. PSRR vs. Frequency
Figure 22. Output Voltage Swing vs. RL and Temperature, VS = 15 V
Rev. 0 | Page 11 of 24
08308-022
0
-VS 1k
08308-021
08308-020
-36 100
-VS
AD8278
+VS -0.5
OUTPUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES
250
VREF = MIDSUPPLY
-1.0 -1.5 -2.0 TA = -40C TA = +25C TA = +85C TA = +125C
SUPPLY CURRENT (A)
200
150 VS = 15V 100 VS = +2.7V 50
+2.0 +1.5 +1.0 +0.5
08308-023
0
1
2
3
4
5
6
7
8
9
10
-30
-10
10
30
50
70
90
110
130
OUTPUT CURRENT (mA)
TEMPERATURE (C)
Figure 23. Output Voltage Swing vs. IOUT and Temperature, VS = 15 V
180 30 25
SHORT-CIRCUIT CURRENT (mA)
Figure 26. Supply Current vs. Temperature
170
SUPPLY CURRENT (A)
20 15 10 5 0 -5 -10 -15 ISHORT- ISHORT+
160
150
140
130
08308-024
0
2
4
6
8
10
12
14
16
18
-30
-10
10
30
50
70
90
110
130
SUPPLY VOLTAGE (V)
TEMPERATURE (C)
Figure 24. Supply Current vs. Dual-Supply Voltage, VIN = 0 V
180 2.0 1.8 170
SUPPLY CURRENT (A)
Figure 27. Short-Circuit Current vs. Temperature
-SLEW RATE
1.6
SLEW RATE (V/s)
160
1.4 1.2 1.0 0.8 0.6 0.4 0.2 +SLEW RATE
150
140
130
08308-025
0
5
10
15
20
25
30
35
40
-30
-10
10
30
50
70
90
110
130
SUPPLY VOLTAGE (V)
TEMPERATURE (C)
Figure 25. Supply Current vs. Single-Supply Voltage, VIN = 0 V, VREF = 0 V
Figure 28. Slew Rate vs. Temperature, VIN = 20 V p-p, 1 kHz
Rev. 0 | Page 12 of 24
08308-028
120
0 -50
08308-027
120
-20 -50
08308-026
-VS
0 -50
AD8278
8 6
NONLINEARITY (2ppm/DIV)
4
1V/DIV
2 0 -2 -4 -6 -8 -5
TIME (s) 3.64s TO 0.01% 4.12s TO 0.001%
0.002%/DIV
-4
-3
-2
-1
0
1
2
3
4
5
OUTPUT VOLTAGE (V)
Figure 29. Gain Nonlinearity, VS = 15 V, RL 2 k, G = 1/2
8 6
NONLINEARITY (2ppm/DIV)
08308-029
Figure 32. Large-Signal Pulse Response and Settling Time, 2 V Step, VS = 2.7 V, G = 1/2
4
5V/DIV
2 0 -2 -4 -6 -8 -10
TIME (s) 7.6s TO 0.01% 9.68s TO 0.001%
0.002%/DIV
-8
-6
-4
-2
0
2
4
6
8
10
OUTPUT VOLTAGE (V)
08308-030
Figure 30. Gain Nonlinearity, VS = 15 V, RL 2 k, G = 2
Figure 33. Large-Signal Pulse Response and Settling Time, 10 V Step, VS = 15 V, G = 2
5V/DIV 6.24s TO 0.01% 7.92s TO 0.001%
1V/DIV 4.34s TO 0.01% 5.12s TO 0.001%
0.002%/DIV
0.002%/DIV
08308-031
TIME (s)
TIME (s)
Figure 31. Large-Signal Pulse Response and Settling Time, 10 V Step, VS = 15 V, G = 1/2
Figure 34. Large-Signal Pulse Response and Settling Time, 2 V Step, VS = 2.7 V
Rev. 0 | Page 13 of 24
08308-034
40s/DIV
4s/DIV
08308-033
40s/DIV
08308-032
4s/DIV
AD8278
5.0 4.5 4.0 VS = 5V
OUTPUT VOLTAGE (V p-p)
3.5 3.0 2.5 2.0 1.5 1.0 VS = 2.5V
2V/DIV
08308-035
0.5 1k 10k FREQUENCY (Hz) 100k 1M
08308-038
10s/DIV
0 100
Figure 35. Large-Signal Step Response, G = 1/2
Figure 38. Maximum Output Voltage vs. Frequency, VS = 5 V, 2.7 V
20mV/DIV
5V/DIV
NO LOAD RL = 200pF RL = 147pF RL = 247pF 40s/DIV
08308-036
10s/DIV
Figure 36. Large-Signal Step Response, G = 2
30 VS = 15V 25
Figure 39. Small-Signal Step Response for Various Capacitive Loads, G = 1/2
OUTPUT VOLTAGE (V p-p)
20
20mV/DIV
15 VS = 5V
10
RL = 100pF RL = 200pF RL = 247pF
08308-040
5
RL = 347pF
1k 10k FREQUENCY (Hz) 100k 1M
08308-037
0 100
40s/DIV
Figure 37. Maximum Output Voltage vs. Frequency, VS = 15 V, 5 V
Figure 40. Small-Signal Step Response for Various Capacitive Loads, G = 2
Rev. 0 | Page 14 of 24
08308-039
AD8278
50 45 40 35
OVERSHOOT (%)
1k
2V
NOISE (nV/ Hz)
5V
30 25 20 15 10 5
08308-041
100
GAIN = 2 GAIN = 1/2
15V 18V
0
50
100
150
200
250
1
10
100 FREQUENCY (Hz)
1k
10k
100k
CAPACITIVE LOAD (pF)
Figure 41. Small-Signal Overshoot vs. Capacitive Load, RL 2 k, G = 1/2
35
Figure 43. Voltage Noise Density vs. Frequency
GAIN = 2 30 25
OVERSHOOT (%)
20 15 10
5V 15V 18V
1V/DIV
2V
GAIN = 1/2
5 0 0 50 100 150 200 250 300 350 CAPACITIVE LOAD (pF)
08308-044
Figure 42. Small-Signal Overshoot vs. Capacitive Load, RL 2 k, G = 2
08308-042
1s/DIV
Figure 44. 0.1 Hz to 10 Hz Voltage Noise
Rev. 0 | Page 15 of 24
08308-043
0
10 0.1
AD8278 THEORY OF OPERATION
CIRCUIT INFORMATION
The AD8278 consists of a low power, low noise op amp and four laser-trimmed on-chip resistors. These resistors can be externally connected to make a variety of amplifier configurations, including difference, noninverting, and inverting configurations. Taking advantage of the integrated resistors of the AD8278 provides the designer with several benefits over a discrete design, including smaller size, lower cost, and better ac and dc performance.
+VS
7
AC Performance
Component sizes and trace lengths are much smaller in an IC than on a PCB, so the corresponding parasitic elements are also smaller. This results in better ac performance of the AD8278. For example, the positive and negative input terminals of the AD8278 op amp are intentionally not pinned out. By not connecting these nodes to the traces on the PCB, their capacitance remains low and balanced, resulting in improved loop stability and excellent common-mode rejection over frequency.
DRIVING THE AD8278
AD8278
20k
5
-IN
2
40k
SENSE
6
OUT
+IN 3
40k
20k
1
REF
08308-045
4
Care should be taken to drive the AD8278 with a low impedance source: for example, another amplifier. Source resistance of even a few kilohms (k) can unbalance the resistor ratios and, therefore, significantly degrade the gain accuracy and commonmode rejection of the AD8278. Because all configurations present several kilohms (k) of input resistance, the AD8278 does not require a high current drive from the source and so is easy to drive.
-VS
INPUT VOLTAGE RANGE
The AD8278 is able to measure input voltages beyond the supply rails. The internal resistors divide down the voltage before it reaches the internal op amp, and provide protection to the op amp inputs. Figure 46 shows an example of how the voltage division works in a difference amplifier configuration. For the AD8278 to measure correctly, the input voltages at the input nodes of the internal op amp must stay below 1.5 V of the positive supply rail and can exceed the negative supply rail by 0.1 V. Refer to the Power Supplies section for more details.
R2 (V ) R1 + R2 IN+ R4 VIN- VIN+ R3 R1 R2 R2 (V ) R1 + R2 IN+
08308-046
Figure 45. Functional Block Diagram
DC Performance
Much of the dc performance of op amp circuits depends on the accuracy of the surrounding resistors. Using superposition to analyze a typical difference amplifier circuit, as is shown in Figure 46, the output voltage is found to be
R2 1 + R4 - V IN - R4 VOUT = V IN + R1 + R2 R3 R3 This equation demonstrates that the gain accuracy and commonmode rejection ratio of the AD8278 is determined primarily by the matching of resistor ratios. Even a 0.1% mismatch in one resistor degrades the CMRR to 69 dB for a G = 2 difference amplifier. The difference amplifier output voltage equation can be reduced to
VOUT R4 (VIN + - VIN - ) = R3
Figure 46. Voltage Division in the Difference Amplifier Configuration
as long as the following ratio of the resistors is tightly matched:
R2 R4 = R1 R3
The AD8278 has integrated ESD diodes at the inputs that provide overvoltage protection. This feature simplifies system design by eliminating the need for additional external protection circuitry, and enables a more robust system. The voltages at any of the inputs of the parts can safely range from +VS - 40 V up to -VS + 40 V. For example, on 10 V supplies, input voltages can go as high as 30 V. Care should be taken to not exceed the +VS - 40 V to -VS + 40 V input limits to avoid risking damage to the parts.
The resistors on the AD8278 are laser trimmed to match accurately. As a result, the AD8278 provides superior performance over a discrete solution, enabling better CMRR, gain accuracy, and gain drift, even over a wide temperature range.
Rev. 0 | Page 16 of 24
AD8278
POWER SUPPLIES
The AD8278 operates extremely well over a very wide range of supply voltages. It can operate on a single supply as low as 2 V and as high as 36 V, under appropriate setup conditions. For best performance, the user must exercise care that the setup conditions ensure that the internal op amp is biased correctly. The internal input terminals of the op amp must have sufficient voltage headroom to operate properly. Proper operation of the part requires at least 1.5 V between the positive supply rail and the op amp input terminals. This relationship is expressed in the following equation:
The AD8278 is typically specified at single- and dual-supplies, but it can be used with unbalanced supplies as well; for example, -VS = -5 V, +VS = 20 V. The difference between the two supplies must be kept below 36 V. The positive supply rail must be at least 2 V above the negative supply and reference voltage.
R1 (V ) R1 + R2 REF R4 R3 R1 R2 R1 (V ) R1 + R2 REF
08308-046
VREF
R1 V REF < + VS - 1.5 V R1 + R2 For example, when operating on a +VS= 2 V single supply and VREF = 0 V, it can be seen from Figure 47 that the op amps input terminals are biased at 0 V, allowing more than the required 1.5 V headroom. However, if VREF = 1 V under the same conditions, the input terminals of the op amp are biased at 0.66 V (G = 1/2). Now the op amp does not have the required 1.5 V headroom and can not function. Therefore, the user needs to increase the supply voltage or decrease VREF to restore proper operation.
Figure 47. Ensure Sufficient Voltage Headroom on the Internal Op Amp Inputs
Use a stable dc voltage to power the AD8278. Noise on the supply pins can adversely affect performance. Place a bypass capacitor of 0.1 F between each supply pin and ground, as close as possible to each supply pin. Use a tantalum capacitor of 10 F between each supply and ground. It can be farther away from the supply pins and, typically, it can be shared by other precision integrated circuits.
Rev. 0 | Page 17 of 24
AD8278 APPLICATIONS INFORMATION
CONFIGURATIONS
The AD8278 can be configured in several ways, as a difference amplifier or a single-ended amplifier (see Figure 48 to Figure 54). All of these configurations have excellent gain accuracy and gain drift because they rely on the internal matched resistors. Note that Figure 50 shows the AD8278 as a difference amplifier with a midsupply reference voltage at the noninverting input. This allows the AD8278 to be used as a level shifter, which is appropriate in single-supply applications that are referenced to midsupply. Table 9 lists several single-ended amplifier configurations that are not illustrated.
-IN 2 40k 20k 5 6 OUT
VOUT = -1/2VIN
-IN
5 20k
40k
2 6 OUT
+IN
1 20k
40k
3 VREF = MIDSUPPLY
08308-050
VOUT = 2(VIN+ - VIN-) + VREF
Figure 51. Difference Amplifier, Gain = 2, Referenced to Midsupply
IN 2 40k 20k 5 6 OUT
1
20k
3 40k
08308-051
+IN
3 40k
20k
1
08308-047
Figure 52. Inverting Amplifier, Gain = -1/2
2 40k 20k 5 6 1 20k IN
OUT
VOUT = 1/2(VIN+ - VIN-)
OUT
Figure 48. Difference Amplifier, Gain = 1/2
-IN 5 20k 40k 2 6
3 40k
08308-052
VOUT = 1.5VIN
Figure 53. Noninverting Amplifier, Gain = 1.5
+IN 1 20k 40k 3
08308-048
5 20k
40k
2 6 OUT
VOUT = 2(VIN+ - VIN-)
Figure 49. Difference Amplifier, Gain = 2
-IN 2 40k 20k 5 6 OUT
IN 1 20k 40k 3
08308-053
VOUT = 2VIN
Figure 54. Noninverting Amplifier, Gain = 2
+IN 3 40k 20k 1 VREF = MIDSUPPLY
08308-049
VOUT = 1/2(VIN+ - VIN-) + VREF
Figure 50. Difference Amplifier, Gain = 1/2, Referenced to Midsupply
Table 9. Difference and Single-Ended Amplifier Configurations
Amplifier Configuration Difference Amplifier Difference Amplifier Single-Ended Inverting Amplifier Single-Ended Inverting Amplifier Single-Ended Non Inverting Amplifier Single-Ended Non Inverting Amplifier Single-Ended Non Inverting Amplifier Single-Ended Non Inverting Amplifier Single-Ended Non Inverting Amplifier Single-Ended Non Inverting Amplifier Signal Gain +1/2 +2 -1/2 -2 +32 +3 +1/2 +1 +1 +2 Pin 1 (REF) GND IN+ GND GND IN IN GND IN GND IN Pin 2 (VIN-) IN- OUT IN OUT GND OUT GND GND OUT OUT Pin 3 (VIN+) IN+ GND GND GND IN IN IN GND IN GND Pin 5 (SENSE) OUT IN- OUT IN OUT GND OUT OUT GND GND
Rev. 0 | Page 18 of 24
AD8278
As with the other inputs, the reference must be driven with a low impedance source to maintain the internal resistor ratio. An example using the low power, low noise OP1177 as a reference is shown in Figure 55.
INCORRECT CORRECT
RF A2 +IN -IN A1 RF 20k RG 20k 40k REF VOUT = (1 + 2RF/RG) (VIN+ - VIN-) x 2 VOUT 40k
AD8278
08308-056
AD8278
REF V + V
AD8278
REF
Figure 56. Low Power Precision Instrumentation Amplifier
Table 10. Low Power Op Amps
OP1177
08308-054
-
Figure 55. Driving the Reference Pin
Op Amp (A1, A2) AD8506 AD8607 AD8617 AD8667
Features Dual micropower op amp Precision dual micropower op amp Low cost CMOS micropower op amp Dual precision CMOS micropower op amp
INSTRUMENTATION AMPLIFIER
The AD8278 can be used as a building block for a low power, low cost instrumentation amplifier. An instrumentation amplifier provides high impedance inputs and delivers high commonmode rejection. Combining the AD8278 with an Analog Devices, Inc., low power amplifier (see Table 10) creates a precise, power efficient voltage measurement solution suitable for power critical systems.
It is preferable to use dual op amps for the high impedance inputs, because they have better matched performance and track each other over temperature. The AD8278 difference amplifier cancels out common-mode errors from the input op amps, if they track each other. The differential gain accuracy of the in-amp is proportional to how well the input feedback resistors (RF) match each other. The CMRR of the in-amp increases as the differential gain is increased (1 + 2RF/RG), but a higher gain also reduces the common-mode voltage range. Note that dual supplies must be used for proper operation of this configuration. Refer to A Designer's Guide to Instrumentation Amplifiers for more design ideas and considerations.
Rev. 0 | Page 19 of 24
AD8278 OUTLINE DIMENSIONS
5.00 (0.1968) 4.80 (0.1890) 4.00 (0.1574) 3.80 (0.1497)
8 1 5 4
6.20 (0.2441) 5.80 (0.2284)
1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY 0.10 SEATING PLANE
1.75 (0.0688) 1.35 (0.0532)
0.50 (0.0196) 0.25 (0.0099) 8 0 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157)
45
0.51 (0.0201) 0.31 (0.0122)
COMPLIANT TO JEDEC STANDARDS MS-012-A A CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
Figure 57. 8-Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-8) Dimensions shown in millimeters and (inches)
3.20 3.00 2.80
3.20 3.00 2.80 PIN 1
8
5
1
5.15 4.90 4.65
4
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 58. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
Rev. 0 | Page 20 of 24
012407-A
AD8278
ORDERING GUIDE
Model AD8278ARZ 1 AD8278ARZ-R71 AD8278ARZ-RL1 AD8278BRZ1 AD8278BRZ-R71 AD8278BRZ-RL1 AD8278ARMZ1 AD8278ARMZ-R71 AD8278ARMZ-RL1 AD8278BRMZ1 AD8278BRMZ-R71 AD8278BRMZ-RL1
1
Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C
Package Description 8-Lead SOIC_N 8-Lead SOIC_N, 7" Tape and Reel 8-Lead SOIC_N, 13" Tape and Reel 8-Lead SOIC_N 8-Lead SOIC_N, 7" Tape and Reel 8-Lead SOIC_N, 13" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP, 13" Tape and Reel 8-Lead MSOP 8-Lead MSOP, 7" Tape and Reel 8-Lead MSOP, 13" Tape and Reel
Package Option R-8 R-8 R-8 R-8 R-8 R-8 RM-8 RM-8 RM-8 RM-8 RM-8 RM-8
Branding
Y21 Y21 Y21 Y22 Y22 Y22
Z = RoHS Compliant Part.
Rev. 0 | Page 21 of 24
AD8278 NOTES
Rev. 0 | Page 22 of 24
AD8278 NOTES
Rev. 0 | Page 23 of 24
AD8278 NOTES
(c)2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D08308-0-7/09(0)
Rev. 0 | Page 24 of 24

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