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Zero Drift, Bidirectional Current Shunt Monitor AD8218
FEATURES
High common-mode voltage range 4 V to 80 V operating -0.3 V to 85 V survival Buffered output voltage Gain = 20 V/V Wide operating temperature range: -40C to +125C Excellent ac and dc performance 100 nV/C typical offset drift 50 V typical offset 5 ppm/C typical gain drift 110 dB typical CMRR at dc
FUNCTIONAL BLOCK DIAGRAM
VS
AD8218
R1
R4
-IN +IN
OUT R2 ENB GND
09592-001
LDO
R3
REF
Figure 1.
APPLICATIONS
High-side current sensing 48 V telecom Power management Base stations Bidirectional motor control Precision high voltage current sources
GENERAL DESCRIPTION
The AD8218 is a high voltage, high resolution current shunt amplifier. It features a set gain of 20 V/V, with a maximum 0.35% gain error over the entire temperature range. The buffered output voltage directly interfaces with any typical converter. The AD8218 offers excellent input common-mode rejection from 4 V to 80 V. The AD8218 performs bidirectional current measurements across a shunt resistor in a variety of industrial and telecom applications, including motor control, battery management, and base station power amplifier bias control. The AD8218 offers breakthrough performance throughout the -40C to +125C temperature range. It features a zero-drift core, which leads to a typical offset drift of 100 nV/C throughout the operating temperature range and the common-mode voltage range. Special attention is devoted to output linearity being maintained throughout the input differential voltage range of 0 mV to ~250 mV. The AD8218 also includes an internal 80 mV reference that can be enabled for optimal dynamic range in unidirectional current sense applications. The typical input offset voltage is 50 V. The AD8218 is offered in an 8-lead MSOP 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)2011 Analog Devices, Inc. All rights reserved.
AD8218 TABLE OF CONTENTS
Features .............................................................................................. 1 Applications ....................................................................................... 1 Functional Block Diagram .............................................................. 1 General Description ......................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 4 ESD Caution .................................................................................. 4 Pin Configuration and Function Descriptions ............................. 5 Typical Performance Characteristics ............................................. 6 Theory of Operation ...................................................................... 10 Amplifier Core ............................................................................ 10 Output Clamping ....................................................................... 10 Application Notes ........................................................................... 11 Supply (VS) Connections ........................................................... 11 Enable Pin (ENB) Operation .................................................... 11 Applications Information .............................................................. 12 Unidirectional High-Side Current Sensing ............................ 12 Bidirectional High-Side Current Sensing ............................... 12 Motor Control Current Sensing ............................................... 12 Outline Dimensions ....................................................................... 13 Ordering Guide .......................................................................... 13
REVISION HISTORY
2/11--Rev. 0 to Rev. A Changes to Features.......................................................................... 1 1/11--Revision 0: Initial Version
Rev. A | Page 2 of 16
AD8218 SPECIFICATIONS
TOPR = -40C to +125C, TA = 25C, RL = 25 k (RL is the output load resistor), input common-mode voltage (VCM) = 4 V, unless otherwise noted. Table 1.
Parameter GAIN Initial Accuracy Accuracy over Temperature Gain vs. Temperature VOLTAGE OFFSET Offset Voltage (RTI 1) Over Temperature (RTI1) Offset Drift INPUT Bias Current2 Common-Mode Input Voltage Range Differential Input Voltage Range3 Common-Mode Rejection (CMRR) OUTPUT Output Voltage Range Low Output Voltage Range High Output Impedance INTERNAL REFERENCE (ENB PIN CONNECTED TO GND) Initial Value Offset (RTI1) Offset Drift (RTO4) REFERENCE INPUT (REF, PIN 7) Input Impedance Input Current Input Voltage Range Input-to-Output Gain DYNAMIC RESPONSE Small-Signal -3 dB Bandwidth Slew Rate NOISE 0.1 Hz to 10 Hz (RTI1) Spectral Density, 1 kHz (RTI1) POWER SUPPLY Operating Range (Pin 2 Floating) VS Range (Pin 2) Quiescent Current over Temperature Power Supply Rejection Ratio (PSRR) TEMPERATURE RANGE For Specified Performance
1 2
Min
Typ 20 0.1
Max
Unit V/V % % ppm/C V V nV/C A A V mV dB V V mV
Test Conditions/Comments
0.35 5 200 300 100 130 4 0 90 0.01 VS - 0.1 2 80 -150 10 1.5 3 0 1 0.0001 450 1 2.3 110 4 4 80 5.5 800 90 -40 110 +125 60 5 +150 220 80 250 110
VO 0.1 V dc, TA TOPR TOPR 25C TOPR TOPR TA, input common mode = 4 V, VS = 4 V TOPR, input common mode = 4 V, VS = 4 V Common-mode continuous Differential input voltage TOPR
TA
Voltage at OUT with a differential input of 0 V and a common-mode input of 4 V VS = NC or VS = 5 V
V V/C M A V V/V kHz V/s V p-p nV/Hz V V A dB C
Dependent on VREF/1.5 M ENB not connected to GND
Power regulated from common mode, VS pin floating VS must be less than 5.5 V if standalone supply is used Throughout input common mode TOPR
RTI = referred to input. Refer to Figure 8 for more information on the input bias current. This current varies based on the input common-mode voltage. The input bias current flowing to the +IN pin is also the supply current to the internal LDO. 3 The differential input voltage is specified as 250 mV because the output is internally clamped to 5.2 V. This ensures that the output voltage does not exceed the typical ADC input range, preventing damage. The AD8218 can survive up to 5 V differentially but will only amplify ~250 mV correctly due to the output clamping function. 4 RTO = referred to output. Rev. A | Page 3 of 16
AD8218 ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Maximum Input Voltage ( +IN, -IN to GND) Differential Input Voltage (+IN to -IN) Human Body Model (HBM) ESD Rating Operating Temperature Range (TOPR) Storage Temperature Range Output Short-Circuit Duration Rating -0.3 V to 85 V 5 V 2000 V -40C to +125C -65C to +150C Indefinite
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.
ESD CAUTION
Rev. A | Page 4 of 16
AD8218 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
+IN 1 VS 2 ENB 3 GND 4
8
-IN REF NC OUT
09592-002
AD8218
TOP VIEW (Not to Scale)
7 6 5
NC = NO CONNECT. DO NOT CONNECT TO THIS PIN.
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. 1 2 3 4 5 6 7 8 Mnemonic +IN VS ENB GND OUT NC REF -IN Description Noninverting Input. Supply Pin. Bypass with a standard 0.1 F capacitor. Connect to GND to enable the internal 80 mV reference. Ground. Output. Do not connect to this pin. Reference Input. Connect to a low impedance voltage. Inverting Input.
Rev. A | Page 5 of 16
AD8218 TYPICAL PERFORMANCE CHARACTERISTICS
40 38 36
30 27 24 21
34
MAGNITUDE (dB)
VOSI (V)
18 15 12 9
32 30 28
6
26 24 -40
3
09592-003 09592-006
0
1k 10k 100k FREQUENCY (Hz) 1M
-20
0
20
40
60
80
100
120
140
TEMPERATURE (C)
Figure 3. Typical Input Offset vs. Temperature
Figure 6. Typical Small-Signal Bandwidth (VOUT = 200 mV p-p)
140 130
10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4
09592-004
110
100 90 80 70 60 50 100 -40C +25C +125C
TOTAL OUTPUT ERROR (%)
120
CMRR (dB)
1000
10k FREQUENCY (Hz)
100k
1M
0
5
10
15 20 25 30 35 DIFFERENTIAL INPUT (mV)
40
45
50
Figure 4. Typical CMRR vs. Frequency
Figure 7. Total Output Error vs. Differential Input Voltage
500 450
800 700
+IN
INPUT BIAS CURRENT (A)
400
600 500 400 300 200 100 -IN
GAIN ERROR (ppm)
350 300 250 200 150
09592-005
-20
0
20 40 60 TEMPERATURE (C)
80
100
120
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
INPUT COMMON-MODE VOLTAGE (V)
Figure 5. Typical Gain Error vs. Temperature
Figure 8. Input Bias Current vs. Input Common-Mode Voltage (Differential Input Voltage = 5 mV, VS = NC)
Rev. A | Page 6 of 16
09592-008
100 -40
0
09592-007
-5
AD8218
500
INPUT 5mV/DIV
450
SUPPLY CURRENT (A)
400
OUTPUT
350
100mV/DIV
300
250
09592-011 09592-013
09592-012
09592-109
200 -40
-20
0
20
40
60
80
100
120
1s/DIV
TEMPERATURE (C)
Figure 9. Supply Current vs. Temperature (VS = 5 V, VCM = 12 V)
Figure 12. Fall Time (Differential Input = 10 mV)
INPUT INPUT 5mV/DIV 100mV/DIV
OUTPUT 2V/DIV OUTPUT
09592-009
100mV/DIV 1s/DIV
5s/DIV
Figure 10. Rise Time (Differential Input = 10 mV)
Figure 13. Fall Time (Differential Input = 200 mV)
INPUT INPUT 100mV/DIV 200mV/DIV
OUTPUT
09592-010
OUTPUT 2V/DIV 5s/DIV
2V/DIV 5s/DIV
Figure 11. Rise Time (Differential Input = 200 mV)
Figure 14. Differential Overload Recovery, Rising
Rev. A | Page 7 of 16
AD8218
9.5
MAXIMUM OUTPUT SOURCE CURRENT (mA)
INPUT 200mV/DIV
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0
OUTPUT 2V/DIV
0
10
20
30
40
50
60
70
80
90
-40
-30
-20
-10
100
110
120
130
140
150
5.0
5.0
5s/DIV
09592-014
TEMPERATURE (C)
Figure 15. Differential Overload Recovery, Falling
Figure 18. Maximum Output Source Current vs. Temperature
82.0
5.010
81.5
OUTPUT VOLTAGE SWING FROM RAIL (V)
5.000 4.990 4.980 4.970 4.960 4.950 4.940 4.930 4.920 4.910 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
09592-017
09592-018
REFERENCE RTO (mV)
81.0
80.5
80.0
79.5
-20
0
20 40 60 80 TEMPERATURE (C)
100
120
09592-116
79.0 -40
4.900
OUTPUT SOURCE CURRENT (mA)
Figure 16. Internal Reference Voltage vs. Temperature (VS = 5 V, VS = NC, VCM = 12 V, Pin 1 (+IN) and Pin 8 (-IN) Shorted, Pin 3 (ENB) Shorted to Pin 4 (GND))
Figure 19. Output Voltage Swing from Rail vs. Output Source Current
12.0
250
11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5
09592-015
OUTPUT VOLTAGE RANGE FROM GND (V)
MAXIMUM OUTPUT SINK CURRENT (mA)
11.5
200
150
100
50
5.0 -40 -30 -20 -10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 OUTPUT SINK CURRENT (mA) 4.0 4.5
0
10 20 30 40 50 60 70 80 90 100 110 120 TEMPERATURE (C)
Figure 17. Maximum Output Sink Current vs. Temperature
Figure 20. Output Voltage Range from GND vs. Output Sink Current
Rev. A | Page 8 of 16
09592-016
AD8218
500
INPUT
400
50V/DIV
OUTPUT 1V/DIV
COUNT
300
200
100
09592-019
500ns/DIV
0 -4
-3
-2
-1
0
1
2
3
4
GAIN DRIFT (ppm/C)
Figure 21. Common-Mode Step Response, Rising
Figure 24. Gain Drift Distribution
140
INPUT 50V/DIV OUTPUT 1V/DIV
120 100
COUNT
80 60 40 20
09592-020
1s/DIV
0 -0.6
-0.4
-0.2
0
0.2
0.4
0.6
OFFSET DRIFT (V/C)
Figure 22. Common-Mode Step Response, Falling
Figure 25. Input Offset Drift Distribution
180
250
150
200
120
COUNT
COUNT
150
90
100
60
50
09592-021 09592-024
30
0 -200
0 -5 0 5 10 15 INTERNAL REF OFFSET DRIFT (V/C)
-100
0 VOSI (V)
100
200
Figure 23. Input Offset Distribution
Figure 26. Internal REF Offset Drift Distribution, Referred to Output (RTO)
Rev. A | Page 9 of 16
09592-023
09592-022
AD8218 THEORY OF OPERATION
AMPLIFIER CORE
In typical applications, the AD8218 amplifies a small differential input voltage generated by the load current flowing through a shunt resistor. The AD8218 rejects high common-mode voltages (up to 80 V) and provides a ground-referenced, buffered output. Figure 27 shows a simplified schematic of the AD8218.
5V ILOAD ICHARGE VS GND CF
The AD8218 is configured as a difference amplifier. The transfer function is OUT = ((R4/R1) x (V1 - V2)) + VREF Resistors R4 and R1 are matched to within 0.01% and have values of 1.5 M and 75 k, respectively, meaning an inputto-output total gain of 20 V/V for the AD8218. The difference between V1 and V2 is the voltage across the shunt resistor, or VIN. Therefore, the input-to-output transfer function of the AD8218 is OUT (V) = (20 x VIN) + VREF The AD8218 accurately amplifies the input differential signal, rejecting high voltage common modes ranging from 4 V to 80 V. The main amplifier uses a novel zero-drift architecture, providing the end user with breakthrough temperature stability. The offset drift is typically less than 100 nV/C. This performance leads to optimal accuracy and dynamic range.
09592-027
AD8218
-IN V2 LOAD V1 4V TO 80V SHUNT +IN R2 ENB REF VREF LDO R3 R1
R4
OUT
GND
OUTPUT CLAMPING
After the input common-mode voltage in the application is above 5.2 V, the internal LDO output of the AD8218 also reaches its maximum value of 5.2 V, which is the maximum output range of the AD8218. Because in typical applications the output interfaces with a converter, clamping the AD8218 output voltage to 5.2 V ensures that the ADC input is not damaged due to excessive overvoltage.
Figure 27. Simplified Schematic
Rev. A | Page 10 of 16
AD8218 APPLICATION NOTES
SUPPLY (VS) CONNECTIONS
The AD8218 includes an internal LDO, which allows the user to leave the VS pin floating, powering the AD8218 directly from the voltage present at Pin 1 (+IN), provided this voltage is in the 4 V to 80 V range. A typical connection for the part in this configuration is shown in Figure 28.
ILOAD 4V TO 80V VS SHUNT +IN -IN REF 2.5V ICHARGE
ENABLE PIN (ENB) OPERATION
The AD8218 includes an internal reference that can be enabled by connecting Pin 3 (ENB) to ground. This mode of operation is shown in Figure 30.
ILOAD 4V TO 80V VS SHUNT +IN -IN REF
BATTERY
LOAD
BATTERY
LOAD
AD8218
ENB GND
09592-030
OUT
AD8218
ENB GND OUT
09592-028
Figure 30. Enabling the Internal 80 mV Reference
Figure 28. Operation with No VS Connection
The AD8218 can also be powered from a separate low impedance supply at Pin 2 (VS); however, this voltage can only be in the 4 V to 5.5 V range. In cases where the high voltage bus is susceptible to noise, transients, or high voltage fluctuations and a 5 V supply is available, the AD8218 can be used in the mode depicted in Figure 29.
ILOAD 4V TO 80V VS CF 5V SHUNT +IN -IN REF 2.5V ICHARGE
In this configuration, the internal 80 mV reference is activated, and the output of the AD8218 is 80 mV when the differential input voltage is 0 V and the voltage at Pin 7 (REF) is also 0 V. This internal reference is useful in unidirectional current measurements where the current being monitored has a very wide range. Setting the output starting point to 80 mV means that when the load current through the shunt resistor is 0 A, the output is 80 mV. This ensures that the output errors due to initial offset and the output saturation range of the amplifier are overcome. In this mode, the transfer function of the AD8218 becomes OUT (V) = OUT (V) = (20 x VIN) + 0.08 V If Pin 3 is connected to ground, and therefore the internal reference is enabled, 80 mV must always be added to the transfer function of the AD8218.
BATTERY
LOAD
AD8218
ENB GND OUT
Figure 29. 5 V Supply Operation
09592-029
Rev. A | Page 11 of 16
AD8218 APPLICATIONS INFORMATION
UNIDIRECTIONAL HIGH-SIDE CURRENT SENSING
In the unidirectional high-side current sensing configuration, the shunt resistor is referenced to the battery (see Figure 31). High voltage is present at the inputs of the current sense amplifier. When the shunt is battery referenced, the AD8218 produces a linear ground-referenced analog output. The supply pin, VS, of the AD8218 can either be connected to a 5 V supply or left floating (see the Supply (VS) Connections section).
ILOAD VS
ILOAD
VS
AD8218
-IN V2 LOAD V1 BATTERY (4V TO 80V) ENB SHUNT +IN R2 LDO R3 R1
R4
OUT
REF 2.5V
GND
09592-033
AD8218
-IN V2 LOAD V1 BATTERY (4V TO 80V) ENB SHUNT +IN R2 LDO R3 R1
R4
Figure 33. Bidirectional Operation Using a 2.5 V Reference Input
OUT
The output transfer function curve for bidirectional operation using a 2.5 V reference input is shown in Figure 34.
5.0 4.5
09592-031
REF
GND
4.0
OUTPUT VOLTAGE (V)
3.5 3.0 2.5 2.0 1.5 1.0 0.5
-0.10 -0.05 0 0.05 0.10 0.15
09592-034
Figure 31. Unidirectional Operation with ENB Connected to GND
The output transfer function curve for unidirectional operation with ENB connected to GND is shown in Figure 32.
320 280 240 200 160 120 80 40
09592-032
OUTPUT VOLTAGE (mV)
0 -0.15
INPUT VOLTAGE (V)
Figure 34. Transfer Function When Using a 2.5 V Reference Input
MOTOR CONTROL CURRENT SENSING
The AD8218 is a practical, accurate solution for high-side current sensing in motor control applications. In cases where the shunt resistor is referenced to a battery and the current flowing is bidirectional (as shown in Figure 35), the AD8218 monitors the current with no additional supply pin necessary.
BATTERY IMOTOR
0
0 1 2 3 4 5 6 7 8 9 10
INPUT VOLTAGE (mV)
Figure 32. Output Transfer Function with ENB Connected to GND
BIDIRECTIONAL HIGH-SIDE CURRENT SENSING
Inputting a voltage at Pin 7 (REF) offsets the output of the AD8218 and allows for bidirectional current sensing. The transfer function from the REF pin to the output is 1 V/V. For example, a 2.5 V REF input offsets the output of the AD8218 to 2.5 V. See Figure 33 for typical connections. The user must ensure that the voltage applied at Pin 7 (REF) is from a low impedance source.
+IN VS
-IN REF VREF
MOTOR
AD8218
ENB GND OUT
Figure 35. High-Side Current Sensing in Motor Control
Rev. A | Page 12 of 16
09592-035
AD8218 OUTLINE DIMENSIONS
3.20 3.00 2.80
8
5
3.20 3.00 2.80 PIN 1 IDENTIFIER
1
5.15 4.90 4.65
4
0.65 BSC 0.95 0.85 0.75 0.15 0.05 COPLANARITY 0.10 0.40 0.25 15 MAX 1.10 MAX 0.80 0.55 0.40
10-07-2009-B
6 0
0.23 0.09
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 36. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
ORDERING GUIDE
Model1 AD8218BRMZ AD8218BRMZ-RL
1
Temperature Range -40C to +125C -40C to +125C
Package Description 8-Lead Mini Small Outline Package [MSOP] 8-Lead Mini Small Outline Package [MSOP]
Package Option RM-8 RM-8
Branding Y3K Y3K
Z = RoHS Compliant Part.
Rev. A | Page 13 of 16
AD8218 NOTES
Rev. A | Page 14 of 16
AD8218 NOTES
Rev. A | Page 15 of 16
AD8218 NOTES
(c)2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D09592-0-2/11(A)
Rev. A | Page 16 of 16

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