PDF Data Sheet Rev. A

FEATURES
PIN CONFIGURATION
AD8420
Maximum supply current: 90 μA
Minimum CMRR: 100 dB
Drives heavy capacitive loads: ~700 pF
Rail-to-rail output
Input voltage range goes below ground
Gain set with 2 external resistors
Can achieve low gain drift at any gain
Very wide power supply range
Single supply: 2.7 V to 36 V
Dual supply: ±2.7 V to ±18 V
Bandwidth (G = 100): 2.5 kHz
Input voltage noise: 55 nV/√Hz
High dc precision
Maximum offset voltage: 125 μV
Maximum offset drift: 1 μV/°C
Maximum differential input voltage: ±1 V
8-lead MSOP package
NC
8
VOUT
+IN 2
+
–
7
FB
–IN
3
–
+
6
REF
–VS
4
5
+VS
1
TOP VIEW
(Not to Scale)
09945-001
Data Sheet
Wide Supply Range, Micropower,
Rail-to-Rail Instrumentation Amplifier
AD8420
Figure 1.
Table 1. Instrumentation Amplifiers by Category1
General
Purpose
AD8221, AD8222
AD8220, AD8224
AD8226, AD8227
AD8228
APPLICATIONS
AD8295, AD8224
Bridge amplifiers
Pressure measurement
Medical instrumentation
Portable data acquisition
Multichannel systems
1
Zero Drift
AD8231
AD8290
AD8293G80,
AD8293G160
AD8553
AD8556
AD8557
Military
Grade
AD620
AD621
AD524
AD526
AD624
Low
Power
AD8420
AD8235
AD8236
AD627
AD8226
AD8227
AD623
AD8223
Digital
Gain
AD8250
AD8251
AD8253
AD8231
See www.analog.com for the latest instrumentation amplifiers.
GENERAL DESCRIPTION
The AD8420 is a low cost, micropower, wide supply range,
instrumentation amplifier with a rail-to-rail output and a novel
architecture that allows for extremely flexible design. It is optimized
to amplify small differential voltages in the presence of large
common-mode signals.
The AD8420 is based on an indirect current feedback architecture
that gives it an excellent input common-mode range. Unlike
conventional instrumentation amplifiers, the AD8420 can easily
amplify signals at or even slightly below ground without requiring
dual supplies. The AD8420 has rail-to-rail output, and the output
voltage swing is completely independent of the input commonmode voltage.
Rev. A
Single-supply operation, micropower current consumption, and
rail-to-rail output swing make the AD8420 ideal for batterypowered applications. Its rail-to-rail output stage maximizes
dynamic range when operating from low supply voltages. Dualsupply operation (±15 V) and low power consumption make
the AD8420 ideal for a wide variety of applications in medical
or industrial instrumentation.
The AD8420 is available in an 8-lead MSOP package. Performance
is specified over the full temperature range of −40°C to +85°C,
and the device is operational from −40°C to +125°C.
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rights of third parties that may result from its use. Specifications subject to change without notice. No
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Tel: 781.329.4700 ©2012–2015 Analog Devices, Inc. All rights reserved.
Technical Support
www.analog.com
AD8420
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Gain Accuracy ............................................................................ 20
Applications ....................................................................................... 1
Input Voltage Range ................................................................... 20
Pin Configuration ............................................................................. 1
Input Protection ......................................................................... 20
General Description ......................................................................... 1
Layout .......................................................................................... 21
Revision History ............................................................................... 2
Driving the Reference Pin ......................................................... 21
Specifications..................................................................................... 3
Input Bias Current Return Path ............................................... 22
Absolute Maximum Ratings............................................................ 7
Radio Frequency Interference (RFI) ........................................ 22
Thermal Resistance ...................................................................... 7
Output Buffering ........................................................................ 23
ESD Caution .................................................................................. 7
Applications Information .............................................................. 24
Pin Configuration and Function Descriptions ............................. 8
AD8420 in Electrocardiography (ECG) .................................. 24
Typical Performance Characteristics ............................................. 9
Classic Bridge Circuit ................................................................ 25
Theory of Operation ...................................................................... 19
4 mA to 20 mA Single-Supply Receiver .................................. 25
Architecture................................................................................. 19
Outline Dimensions ....................................................................... 26
Setting the Gain .......................................................................... 19
Ordering Guide .......................................................................... 26
REVISION HISTORY
1/15—Rev. 0 to Rev. A
Changes to Features Section............................................................ 1
Changes to Specifications Section .................................................. 3
Changes to Quiescent Current Parameter, Table 2 ...................... 4
Changes to Common-Mode Rejection Ratio (CMRR)
Parameter, Table 3 and Quiescent Current Parameter, Table 3......... 5
Changes to ESD Parameter, Table 4 ............................................... 7
Changes to Figure 9, Figure 11, Figure 13, and Figure 14 ......... 10
Changes to Figure 15, Figure 16 Caption, Figure 18 Caption,
and Figure 20 Caption.................................................................... 11
Changes to Figure 38 ...................................................................... 14
Change to Figure 40 Caption ........................................................ 15
Changes to Figure 56 Caption....................................................... 17
Changes to Figure 57 Caption....................................................... 18
Changes to Figure 58 and Architecture Section ......................... 19
Changes to Gain Accuracy Section .............................................. 20
Changes to Driving the Reference Pin Section........................... 21
Changes to Output Buffering Section .......................................... 23
Changes to 4 mA to 20 mA Single-Supply Receiver Section .... 25
3/12—Revision 0: Initial Version
Rev. A | Page 2 of 26
Data Sheet
AD8420
SPECIFICATIONS
+VS = +5 V, −VS = 0 V, VREF = 0.5 V, V+IN = 0.5 V, V−IN = 0.5 V, TA = 25°C, G = 1 to 1000, RL = 20 kΩ, specifications referred to input,
unless otherwise noted. All Table 2 limits are valid from VS = 3 V to VS = ±5 V, unless otherwise specified.
Table 2.
Parameter
COMMON-MODE REJECTION RATIO (CMRR)
CMRR DC to 60 Hz
CMRR at 1 kHz
NOISE
Voltage Noise
Spectral Density
Peak to Peak
Current Noise
Spectral Density
Peak to Peak
VOLTAGE OFFSET
Offset
Average Temperature Coefficient
Offset RTI vs. Supply (PSR)
INPUTS
Input Bias Current1
Average Temperature Coefficient
Input Offset Current
Average Temperature Coefficient
Input Impedance
Differential
Common Mode
Differential Input Operating Voltage
Input Operating Voltage (+IN, −IN, REF, or FB)
DYNAMIC RESPONSE
Small Signal −3 dB Bandwidth
G=1
G = 10
G = 100
G =1000
Settling Time 0.01%
G=1
G = 10
G = 100
Slew Rate
Test Conditions/Comments
VCM = 0 V to 2.7 V
Min
Typ
Max
100
100
Unit
dB
dB
f = 1 kHz, VDIFF ≤ 100 mV
f = 0.1 Hz to 10 Hz, VDIFF ≤ 100 mV
55
1.5
nV/√Hz
μV p-p
f = 1 kHz
f = 0.1 Hz to 10 Hz
80
3
fA/√Hz
pA p-p
VS = 3 V to VS = 5 V
VS = ±5 V
TA = −40°C to +85°C
VS = 2.7 V to 5 V
Valid for REF and FB pair, as well as
+IN and −IN
TA = +25°C
TA = +85°C
TA = −40°C
TA = −40°C to +85°C
TA = +25°C
TA = +85°C
TA = −40°C
TA = −40°C to +85°C
125
150
1
μV
μV
μV/°C
dB
27
24
30
nA
nA
nA
pA/°C
nA
nA
nA
pA/°C
86
20
30
1
1
1
0.5
130||2
1000||2
TA = –40°C to +85°C
TA = +25°C
TA = +85°C
TA = –40°C
VS = ±5 V
−1 V to +1 V output step
−4.5 V to +4.5 V output step
−4.5 V to +4.5 V output step
Rev. A | Page 3 of 26
−1
−VS − 0.15
−VS − 0.05
−VS − 0.2
+1
+VS − 2.2
+VS − 1.8
+VS − 2.7
MΩ||pF
MΩ||pF
V
V
V
V
250
25
2.5
0.25
kHz
kHz
kHz
kHz
3
130
1
1
μs
μs
ms
V/μs
AD8420
Parameter
GAIN2
Gain Range
Gain Error
G=1
G = 10 to 1000
Gain vs. Temperature
OUTPUT
Output Swing
Short-Circuit Current
POWER SUPPLY
Operating Range
Quiescent Current
Data Sheet
Test Conditions/Comments
G = 1 + (R2/R1)
Min
Typ
1
VOUT = 0.1 V to 1.1 V, VREF = 0.1 V
VOUT = 0.2 V to 4.8 V
TA = −40°C to +85°C
VS = 5 V, RL = 10 kΩ to midsupply
VS = ±5 V, RL = 20 kΩ to ground
TA = +25°C
TA = +85°C
TA = −40°C
0.05
−VS + 0.1
−VS + 0.1
−VS + 0.1
Max
Unit
1000
V/V
0.02
0.1
10
%
%
ppm/°C
+VS − 0.15
+VS − 0.2
+VS − 0.15
V
V
V
mA
36
V
90
110
75
μA
μA
μA
+85
+125
°C
°C
10
Single-supply operation3
VS = 5 V
TA = +25°C
TA = +85°C
TA = −40°C
TEMPERATURE RANGE
Specified
Operational4
2.7
55
−40
−40
1
70
The input stage uses PNP transistors; therefore, input bias current always flows out of the device.
For G > 1, errors from External Resistor R1 and External Resistor R2 should be considered in addition to these specifications, including error from FB pin bias current.
Minimum supply voltage indicated for V+IN, V−IN, and VREF = 0 V.
4
See the Typical Performance Characteristics section for operation between 85°C and 125°C.
2
3
Rev. A | Page 4 of 26
Data Sheet
AD8420
+VS = +15 V, −VS = −15 V, VREF = 0 V, TA = 25°C, G = 1 to 1000, RL = 20 kΩ, specifications referred to input, unless otherwise noted.
Table 3.
Parameter
COMMON-MODE REJECTION RATIO (CMRR)
CMRR DC to 60 Hz
CMRR at 1 kHz
NOISE
Voltage Noise
Spectral Density
Peak to Peak
Current Noise
Spectral Density
Peak to Peak
VOLTAGE OFFSET
Offset
Average Temperature Coefficient
Offset RTI vs. Supply (PSR)
INPUTS
Input Bias Current2
Average Temperature Coefficient
Input Offset Current
Average Temperature Coefficient
Input Impedance
Differential
Common Mode
Differential Input Operating Voltage
Input Operating Voltage (+IN, −IN, REF, or FB)
DYNAMIC RESPONSE
Small Signal −3 dB Bandwidth
G=1
G = 10
G = 100
G =1000
Settling Time 0.01%
G=1
G = 10
G = 100
Slew Rate
GAIN3
Gain Range
Gain Error
G=1
G = 10 to 1000
Gain vs. Temperature
Test Conditions/Comments
VCM = −10 V to +10 V
Min
Typ
Max
92
92
Unit
dB
dB
f = 1 kHz, VDIFF ≤ 100 mV
f = 0.1 Hz to 10 Hz, VDIFF ≤ 100 mV
55
1.5
nV/√Hz
μV p-p
f = 1 kHz
f = 0.1 Hz to 10 Hz
80
3
fA/√Hz
pA p-p
VS = ±15 V1
TA = −40°C to +85°C
VS = ±15 V
Valid for REF and FB pair, as well
as +IN and −IN
TA = +25°C
TA = +85°C
TA = −40°C
TA = −40°C to +85°C
TA = +25°C
TA = +85°C
TA = −40°C
TA = −40°C to +85°C
250
1
μV
μV/°C
dB
27
24
30
nA
nA
nA
pA/°C
nA
nA
nA
pA/°C
100
20
30
1
1
1
0.5
130||3
1000||3
TA = −40°C to +85°C
TA = +25°C
TA = +85°C
TA = −40°C
−1
−VS − 0.15
−VS − 0.05
−VS − 0.2
−1 V to +1 V output step
−5 V to +5 V output step
−5 V to +5 V output step
1
+VS − 2.2
+VS − 1.8
+VS − 2.7
MΩ||pF
MΩ||pF
V
V
V
V
250
25
2.5
0.25
kHz
kHz
kHz
kHz
3
130
1
1
μs
μs
ms
V/μs
G = 1 + (R2/R1)
1
VOUT = ±1 V
VOUT = ±10 V
TA = −40°C to +85°C
Rev. A | Page 5 of 26
0.05
1000
V/V
0.02
0.1
10
%
%
ppm/°C
AD8420
Parameter
OUTPUT
Output Swing
RL = 20 kΩ to Ground
Short-Circuit Current
POWER SUPPLY
Operating Range
Quiescent Current
Data Sheet
Test Conditions/Comments
Min
TA = +25°C
TA = +85°C
TA = –40°C
−VS + 0.13
−VS + 0.15
−VS + 0.11
Typ
Max
Unit
+VS − 0.2
+VS − 0.23
+VS − 0.16
V
V
V
mA
±18
V
120
145
110
μA
μA
μA
+85
+125
°C
°C
10
Dual-supply operation4
VS = ±15 V
TA = +25°C
TA = +85°C
TA = −40°C
TEMPERATURE RANGE
Specified
Operational5
±2.7
70
−40
−40
1
85
See the Typical Performance Characteristics section for the offset voltage vs. supply.
The input stage uses PNP transistors; therefore, input bias current always flows out of the device.
3
For G > 1, errors from External Resistor R1 and External Resistor R2 should be considered in addition to these specifications, including error from FB pin bias current.
4
Minimum positive supply voltage indicated for V+IN, V−IN, and VREF = 0 V. With V+IN, V−IN, and VREF = −VS, minimum supply is ±1.35 V.
5
See the Typical Performance Characteristics section for operation between 85°C and 125°C.
2
Rev. A | Page 6 of 26
Data Sheet
AD8420
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 4.
Parameter
Supply Voltage
Output Short-Circuit Current
Maximum Voltage at −IN or +IN
Minimum Voltage at −IN or +IN
Maximum Voltage at REF or FB
Minimum Voltage at REF or FB
Storage Temperature Range
ESD
Human Body Model
Charge Device Model
Machine Model
θJA is specified for a device in free air.
Rating
±18 V
Indefinite
−VS + 40 V
−VS − 0.5 V
+VS + 0.5 V
−VS − 0.5 V
−65°C to +150°C
Table 5.
Package
8-Lead MSOP, 4-Layer JEDEC Board
ESD CAUTION
4 kV
1.25 kV
0.2 kV
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
Rev. A | Page 7 of 26
θJA
135
Unit
°C/W
AD8420
Data Sheet
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
8
VOUT
+IN 2
NC 1
+
–
7
FB
–IN 3
–
+
6
REF
5
+VS
–VS 4
TOP VIEW
(Not to Scale)
09945-002
AD8420
Figure 2. Pin Configuration
Table 6. Pin Function Descriptions
Pin No.
1
Mnemonic
NC
2
3
4
5
6
7
8
+IN
−IN
−VS
+VS
REF
FB
VOUT
Description
This pin is not connected internally. For best CMRR vs. frequency and leakage performance, connect this pin to
negative supply.
Positive Input.
Negative Input.
Negative Supply.
Positive Supply.
Reference Input.
Feedback Input.
Output.
Rev. A | Page 8 of 26
Data Sheet
AD8420
TYPICAL PERFORMANCE CHARACTERISTICS
T = 25°C, +VS = 5 V, RL = 20 kΩ, unless otherwise noted.
700 MEAN: 4.63764
SD: 1.09498
MEAN: –34.8195
SD: 31.3406
600
NUMBER OF HITS
500
400
300
500
400
300
200
200
100
100
0
–150
–100
–50
0
50
100
150
VOS (µV)
0
09945-003
NUMBER OF HITS
600
0
2
4
6
8
10
CMRR, ±15V (µV/V)
Figure 3. Typical Distribution of Input Offset Voltage
09945-008
700
Figure 6. Typical Distribution of CMRR
700 MEAN: 22.706
SD: 0.615728
MEAN: 22.6643
700 SD: 0.6058
600
400
300
500
400
300
200
200
100
100
0
20
21
22
23
24
25
POSITIVE BIAS CURRENT (nA)
0
20
22
23
24
25
gm2 POSITIVE BIAS CURRENT (nA)
Figure 7. Typical Distribution of REF, FB Bias Current
Figure 4. Typical Distribution of Input Bias Current
1200 MEAN: 0.00144205
SD: 0.112088
1000
1000
NUMBER OF HITS
1200 MEAN: 0.000646761
SD: 0.111551
800
600
400
800
600
400
0
–0.9
–0.6
–0.3
0
0.3
0.6
OFFSET CURRENT (nA)
0.9
0
–0.9
–0.6
–0.3
0
0.3
0.6
gm2 OFFSET CURRENT (nA)
Figure 8. Typical Distribution of REF, FB Offset Current
Figure 5. Typical Distribution of Input Offset Current
Rev. A | Page 9 of 26
0.9
09945-007
200
200
09945-005
NUMBER OF HITS
21
09945-006
NUMBER OF HITS
500
09945-004
NUMBER OF HITS
600
AD8420
Data Sheet
10
0.3
1.5
0.2
1.0
0.1
0.5
0
0
–5
0
5
10
15
20
25
30
35
–0.1
40
INPUT VOLTAGE (V)
0
0
–10
–0.4
–15
–20
–15
INPUT COMMON-MODE VOLTAGE (V)
0.2
0
0
–1
–0.2
–2
0
5
10
15
–0.6
25
20
15
INPUT CURRENT (mA)
IIN
–5
Figure 12. Input Overvoltage Performance, G = 100, VS = ±15 V
0.4
1
–10
INPUT VOLTAGE (V)
VOUT
2
OUTPUT VOLTAGE (V)
0.2
–0.2
0.6
VS = ±15V
G=1
5
–5
Figure 9. Input Overvoltage Performance, G = 1
3
0.4
IIN
VOUT
INPUT CURRENT (mA)
0.4
0.6
VS = ±15V
G = 100
09945-312
IIN
2.0
15
OUTPUT VOLTAGE (V)
VOUT
0.5
INPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
VS = +5V
G=1
= 0V
V
2.5 V–IN = 0V
REF
09945-309
3.0
–0.4
10
–1.0V, +12.3V
0.0V, +12.8V
+1.0V, +12.3V
5
0
MAXIMUM VDIFF = ±1V
–5
–10
–15
–1.0V, –14.6V
+1.0V, –14.6V
–10
–5
0
5
10
15
20
–0.6
25
–20
–1.2 –1.0 –0.8 –0.6 –0.4 –0.2
INPUT VOLTAGE (V)
Figure 10. Input Overvoltage Performance, G = 1, VS = ±15 V
6
0.3
3
0.2
VS = 5V
G = 100
V–IN = 0V
VREF = 0V
2
0.1
1
0
–5
INPUT CURRENT (mA)
4
0
0
5
10
15
20
25
30
35
INPUT VOLTAGE (V)
–0.1
40
09945-311
OUTPUT VOLTAGE (V)
0.4
IIN
0.4
0.6
0.8
1.0
1.2
3.0
INPUT COMMON-MODE VOLTAGE (V)
VOUT
0.2
Figure 13. Input Common-Mode Voltage vs. Output Voltage,
G = 1, VS = ±15 V
0.5
5
0
OUTPUT VOLTAGE (V)
Figure 11. Input Overvoltage Performance, G = 100
2.5
+4mV, +2.8V
+1.0V, +2.3V
2.0
1.5
MAXIMUM VDIFF = ±1V
1.0
0.5
+1.0V, +0.4V
+4mV, –0.1V
0
–0.5
–0.2
0
0.2
0.4
0.6
0.8
1.0
1.2
OUTPUT VOLTAGE (V)
Figure 14. Input Common-Mode Voltage vs. Output Voltage,
G = 1, VS = 5 V, VREF = 0 V
Rev. A | Page 10 of 26
09945-314
–15
09945-310
–3
–20
09945-313
0.0V, –15.1V
Data Sheet
3.0
3.5
VREF = 2.5V
RL = 10kΩ TO MIDSUPPLY
+2.5V, +2.8V
2.5
2.0
INPUT COMMON-MODE VOLTAGE (V)
INPUT COMMON-MODE VOLTAGE (V)
3.5
AD8420
+3.03V, +2.46V
+1.5V, +2.3V
1.5
MAXIMUM VFB = +VS – 2.2V
1.0
0.5
+1.5V, +0.4V
+3.03V, +0.16V
0
3.0
2.5
+44mV, +2.8V
+4.8V, +2.78V
+44mV, –0.1V
+4.8V, –80mV
2.0
1.5
1.0
0.5
0
2.0
2.2
2.4
2.6
2.8
3.0
3.2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Figure 15. Input Common-Mode Voltage vs. Output Voltage,
G = 1, VS = 5 V, VREF = 2.5 V
Figure 18. Input Common-Mode Voltage vs. Output Voltage,
G = 100, VS = 5 V, VREF = 0 V
0.6
3.5
+4mV, +0.5V
INPUT COMMON-MODE VOLTAGE (V)
0.5
0.4
0.3
0.2
+0.6V, +0.2V
0.1
0
+4mV, –0.1V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
OUTPUT VOLTAGE (V)
3.0
2.5
+86mV, +2.79V
+2.5V, +2.8V
+4.8V, +2.79V
+86mV, –90mV
+2.5V, –0.1V
+4.8V, –90mV
2.0
3.5
2.0
1.5
1.0
0.5
0
–0.5
–0.5
09945-316
–0.1
–0.2
–0.1
0
0.5
1.0
1.5
2.5
3.0
4.0
4.5
5.0
5.5
OUTPUT VOLTAGE (V)
Figure 16. Input Common-Mode Voltage vs. Output Voltage,
G = 1, VS = 2.7 V, VREF = 0 V
Figure 19. Input Common-Mode Voltage vs. Output Voltage,
G = 100, VS = 5 V, VREF = 2.5 V
20
0.6
15
+14.8V, +12.7V
–14.9V, +12.7V
INPUT COMMON-MODE VOLTAGE (V)
INPUT COMMON-MODE VOLTAGE (V)
0
OUTPUT VOLTAGE (V)
0.0V, +12.8V
10
5
0
–5
–10
–15
–20
–20
0.0V, –15.1V
+14.8V, –15.0V
–14.9V, –15.0V
–15
–10
–5
0
5
10
15
20
OUTPUT VOLTAGE (V)
09945-317
INPUT COMMON-MODE VOLTAGE (V)
OUTPUT VOLTAGE (V)
–0.5
–0.5
Figure 17. Input Common-Mode Voltage vs. Output Voltage,
G = 100, VS = ±15 V
+29mV, +0.5V
+2.53V, +0.49V
+29mV, –0.1V
+2.53V, –90mV
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
OUTPUT VOLTAGE (V)
Figure 20. Input Common-Mode Voltage vs. Output Voltage,
G = 100, VS = 2.7 V, VREF = 0 V
Rev. A | Page 11 of 26
09945-319
1.8
09945-320
1.6
09945-315
–0.5
1.4
09945-318
+2.5V, –0.1V
AD8420
Data Sheet
40
120
–0.2V
POSITIVE PSRR (dB)
30
+2.7V
25
20
15
GAIN = 100
40
GAIN = 10
BANDWIDTH
LIMIT
20
0
0.5
1.0
1.5
2.0
2.5
3.0
0
0.1
120
SPECIFIED
PERFORMANCE RANGE
1k
NEGATIVE PSRR (dB)
0
–100
–200
GAIN = 100
80
60
BANDWIDTH
LIMIT
40
GAIN = 10
GAIN = 1
IBIAS (–IN)
20
–1.0
–0.5
0
0.5
1.0
1.5
2.0
DIFFERENTIAL INPUT VOLTAGE (V)
0
0.1
09945-020
–1.5
1
10
100
1k
10k
100k
FREQUENCY (Hz)
Figure 22. Input Bias Current vs. Differential Input Voltage, VS = ±15
Figure 25. Negative PSRR vs. Frequency, RTI, VS = ±15 V
100
70
80
VS = ±15V
GAIN = 1000
60
50
GAIN = 100
40
GAIN (dB)
GAIN = 1000
60
GAIN = 100
40
GAIN = 10
100
10
GAIN = 1
–20
1k
10k
FREQUENCY (Hz)
100k
09945-500
10
GAIN = 10
20
–10
GAIN = 1
1
30
0
BANDWIDTH
LIMIT
20
0
0.1
100k
GAIN = 1000
100
100
–400
–2.0
10k
VS = ±15V
200
–300
PSRR (dB)
100
Figure 24. Positive PSRR vs. Frequency, RTI, VS = ±15 V
400
IBIAS (+IN)
10
FREQUENCY (Hz)
Figure 21. Input Bias Current vs. Common-Mode Voltage
300
GAIN = 1
1
09945-323
IBIAS (+IN)
IBIAS (–IN)
COMMON-MODE VOLTAGE (V)
INPUT BIAS CURRENT (nA)
GAIN = 1000
60
09945-324
5
–2.0
80
–30
1
10
100
1k
10k
FREQUENCY (Hz)
Figure 26. Gain vs. Frequency
Figure 23. PSRR vs. Frequency on 5 V Supply
Rev. A | Page 12 of 26
100k
1M
09945-023
10
VS = ±15V
100
09945-019
INPUT BIAS CURRENT (nA)
35
Data Sheet
AD8420
70
60
120
VS = 2.7V
GAIN = 1000
100
50
GAIN = 100
80
30
CMRR (dB)
GAIN = 10
20
10
60
GAIN = 1
0
40
–10
20
–20
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
VS = ±15V
VCM = ±10V
0
09945-024
–30
0
0.1
VS = ±15V
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Figure 30. CMRR vs. Differential Input Voltage
120
BANDWIDTH
LIMIT
0.3
DIFFERENTIAL INPUT VOLTAGE (V)
Figure 27. Gain vs. Frequency, 2.7 V Single Supply
140
0.2
09945-329
GAIN (dB)
40
GAIN = 1000
VS = 5V
110
120
100
SUPPLY CURRENT (µA)
100
CMRR (dB)
GAIN = 100
80
60
GAIN = 10
GAIN = 1
40
90
80
70
60
50
40
20
1k
10k
100k
FREQUENCY (Hz)
20
–40
20
50
65
80
95
BIAS CURRENT (nA)
100
GAIN = 100
80
60
GAIN = 10
GAIN = 1
40
150
+IN BIAS CURRENT
15
100
10
50
5
FREQUENCY (Hz)
1k
10k
100k
0
–40
09945-328
100
125
Figure 29. CMRR vs. Frequency, RTI, 1 kΩ Source Imbalance, VS = ±15 V
200
–IN BIAS CURRENT
20
20
10
110
250
25
GAIN = 1000
1
35
30
120
CMRR (dB)
5
Figure 31. Supply Current vs. Temperature, VS = +5 V
VS = ±15V
0
0.1
–10
TEMPERATURE (°C)
Figure 28. CMRR vs. Frequency, RTI, VS = ±15 V
140
–25
0
OFFSET CURRENT
–25
–10
5
20
35
50
65
80
95
110
–50
125
TEMPERATURE (°C)
Figure 32. Input Bias Current and Input Offset Current vs. Temperature
Rev. A | Page 13 of 26
09945-331
100
OFFSET CURRENT (pA)
10
09945-327
1
09945-027
30
0
0.1
AD8420
Data Sheet
200
OFFSET CURRENT (pA)
20
100
+IN BIAS CURRENT
15
50
10
0
OFFSET CURRENT
5
OFFSET VOLTAGE (µV)
150
–IN BIAS CURRENT
–50
–25
–10
5
20
35
50
65
80
95
110
200
100
0
–100
–200
–300
–100
125
–400
–40
TEMPERATURE (°C)
–25
–10
5
1000
5
VIN = ±1V
800 VS = ±15V
4
0
PART B
–200
80
95
110
125
REPRESENTATIVE DATA
NORMALIZED AT 25°C
VS = ±15V
1
PART A: 0.024ppm/°C
0
–1
–400
–2
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
–4
–40
09945-333
–1000
–40
PART B: 0.038ppm/°C
–3
REPRESENTATIVE DATA
NORMALIZED TO 25ºC
20
35
50
65
80
+VS
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
VIN = ±0.1V
800 VS = ±15V
600
400
PART A
200
0
–200
PART B
–600
REPRESENTATIVE DATA
NORMALIZED TO 25ºC
–10
5
20
35
TEMPERATURE (°C)
50
65
80
95
110
125
RL = 20kΩ
–0.1
–0.2
–0.3
–40°C
+25°C
+85°C
+125°C
+0.3
+0.2
+0.1
–VS
09945-334
–25
5
Figure 37. CMRR vs. Temperature, G = 1, VS = ±15 V
1000
–1000
–40
–10
TEMPERATURE (°C)
Figure 34. Gain Error vs. Temperature, G = 1, VIN = ±1 V, VS = ±15 V
–400
–25
09945-032
–600
GAIN ERROR (µV/V)
65
2
200
–800
50
3
PART A
CMRR (µV/V)
GAIN ERROR (µV/V)
600
–800
35
Figure 36. Offset Drift
Figure 33. FB, REF Bias Current and FB, REF Offset Current vs. Temperature
400
20
TEMPERATURE (°C)
2
4
6
8
10
12
14
16
18
SUPPLY VOLTAGE (±VS)
Figure 38. Output Voltage Swing vs. Supply Voltage, RL = 20 kΩ
Figure 35. Gain Error vs. Temperature, G = 1, VIN = ±0.1 V, VS = ±15 V
Rev. A | Page 14 of 26
09945-035
0
–40
NORMALIZED TO 25°C
300
09945-332
BIAS CURRENT (nA)
25
400
09945-031
30
AD8420
+VS
+VS
–0.2
–0.2
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
–0.4
–0.6
–0.8
–40°C
+25°C
+85°C
+125°C
+0.8
+0.6
+0.4
VS = 5V
VREF = 2.5V
+0.2
–0.4
–0.6
–40°C
+25°C
+85°C
+125°C
–0.8
+0.8
+0.6
+0.4
+0.2
10k
100k
1M
LOAD RESISTANCE (Ω)
–VS
0.1
09945-338
–VS
1k
09945-340
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
Data Sheet
1
OUTPUT CURRENT (mA)
Figure 39. Output Voltage Swing vs. Load Resistance, VS = 5 V
Figure 42. Output Voltage Swing vs. Output Current, VS = ±15
+VS
2k
1k
–0.4
–0.6
–0.8
NOISE (nV/ Hz)
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
–0.2
–40°C
+25°C
+85°C
+125°C
+0.8
+0.6
+0.4
GAIN = 1
100
GAIN = 10
VS = 5V
VREF = 2.5V
1
OUTPUT CURRENT (mA)
Figure 40. Output Voltage Swing vs. Output Current, VS = 5 V
20
0.1
GAIN = 100
1
10
100
1k
10k
100k
FREQUENCY (Hz)
Figure 43. Voltage Noise Spectral Density vs. Frequency, RTI
15
5
0
–40°C
+25°C
+85°C
+125°C
–10
–15
1k
10k
100k
1M
LOAD RESISTANCE (Ω)
0.4µV/DIV
1s/DIV
Figure 41. Output Voltage Swing vs. Load Resistance, VS = ±15 V
Figure 44. 0.1 Hz to 10 Hz RTI Voltage Noise, G = 1
Rev. A | Page 15 of 26
09945-043
–5
09945-339
OUTPUT VOLTAGE SWING (V)
10
09945-042
–VS
0.1
09945-501
+0.2
AD8420
Data Sheet
VS = ±5V
1V/DIV
1.78µs TO 0.1%
3.31µs TO 0.01%
100
0.02%/DIV
1
10
100
1k
10k
100k
FREQUENCY (Hz)
09945-348
20µs/DIV
10
09945-149
NOISE (fA/ Hz)
1k
Figure 45. Current Noise Spectral Density vs. Frequency
Figure 48. Large Signal Pulse Response and Settling Time, G = 1
VS = ±5V
4.5V/DIV
67µs TO 0.1%
138µs TO 0.01%
1s/DIV
200µs/DIV
Figure 49. Large Signal Pulse Response and Settling Time, G = 10
Figure 46. 0.1 Hz to 10 Hz Current Noise
30
VS = ±5V
VS = ±15V, G = 15V/V
27
09945-150
1.5pA/DIV
09945-147
0.02%/DIV
21
18
4.5V/DIV
600ms TO 0.1%
1.04ms TO 0.01%
15
12
0.02%/DIV
9
VS = +5V, G = 5V/V
3
20ms/DIV
0
1
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
09945-151
6
09945-148
OUTPUT VOLTAGE (V p-p)
24
Figure 50. Large Signal Pulse Response and Settling Time, G = 100
Figure 47. Large Signal Frequency Response
Rev. A | Page 16 of 26
4µs/DIV
20mV/DIV
Figure 51. Small Signal Pulse Response, G = 1, RL = 20 kΩ, CL = 100 pF
2ms/DIV
09945-054
20mV/DIV
AD8420
09945-051
Data Sheet
Figure 54. Small Signal Pulse Response, G = 1000, RL = 20 kΩ, CL = 100 pF
NO LOAD
220pF
470pF
20µs/DIV
20mV/DIV
09945-055
20mV/DIV
09945-052
780pF
5µs/DIV
Figure 55. Small Signal Response with Various Capacitive Loads,
G = 1, RL = ∞
Figure 52. Small Signal Pulse Response, G = 10, RL = 20 kΩ, CL = 100 pF
90
SUPPLY CURRENT (µA)
85
75
70
65
60
55
50
0
5
10
15
20
25
30
35
SUPPLY VOLTAGE (V)
Figure 53. Small Signal Pulse Response, G = 100, RL = 20 kΩ, CL = 100 pF
Rev. A | Page 17 of 26
Figure 56. Supply Current vs. Total Supply Voltage
40
09945-057
200µs/DIV
09945-053
20mV/DIV
80
AD8420
90
–20
Data Sheet
TESTED WITH DUAL SUPPLIES
CENTERED AT 0V
–60
–80
–100
–120
–140
–160
–180
–200
0
4
8
12
16
20
24
28
SUPPLY VOLTAGE (V)
32
36
09945-502
OFFSET VOLTAGE (µV)
–40
Figure 57. Offset Voltage vs. Total Supply Voltage
Rev. A | Page 18 of 26
Data Sheet
AD8420
THEORY OF OPERATION
AD8420
+VS
–
+
A
VOUT
I3
–VS
+VS
Vb
FB
gm1
–IN
R2
–
ESD
PROTECTION
gm2
I1
ESD
PROTECTION
–VS
+VS
R1
+
I2
REF
–VS
09945-058
+IN
Figure 58. Simplified Schematic
ARCHITECTURE
Table 7. Suggested Resistors for Various Gains, 1% Resistors
The AD8420 is based on an indirect current feedback topology
consisting of three amplifiers: two matched transconductance
amplifiers that convert voltage to current and one integrator
amplifier that converts current to voltage.
R1 (kΩ)
None
49.9
20
10
5
2
1
1
1
1
I3 is integrated to the output, making the output voltage, VOUT,
increase. This voltage continues to increase until the same differential input voltage across the inputs of gm1 is replicated across
the inputs of gm2, generating a current (I2) equal to I1. This reduces
the Difference Current I3 to zero so that the output remains at a
stable voltage. The gain in the configuration shown in Figure 58 is
set by R2 and R1.
In traditional instrumentation amplifiers, the input commonmode voltage can limit the available output swing, typically
depicted in a hexagon plot. Because the AD8420 converts the
input differential signals to current, this limit does not apply. This
is particularly important when amplifying a signal with a commonmode voltage near one of the supply rails.
While the ratio of R2 to R1 sets the gain, the designer determines
the absolute value of the resistors. Larger values reduce power
consumption and output loading; smaller values limit the FB input
bias current and offset current error. For best output swing and
distortion performance, keep (R1 + R2) || RL ≥ 20 kΩ.
A method that allows large value feedback resistors while limiting
FB bias current error is to place a resistor of value R1 || R2 in
series with the REF terminal, as shown in Figure 59. At higher
gains, this resistor can simply be the same value as R1.
To improve robustness and ease of use, the AD8420 includes
differential voltage protection to limit the current into its inputs
to a safe level. This protection scheme allows wide differential
input voltages without damaging the device.
+IN
AD8420
IB–
FB
–IN
R2
R1
+
R1||R2
–
VREF
VOUT = G(V+IN − V−IN) + VREF
REF
IB F
R1
R2
IB R
G=1+
The transfer function of the AD8420 is
VOUT
IB +
SETTING THE GAIN
where G  1 
Gain
1.00
2.00
5.03
10.09
20.06
49.8
101
201
500
1001
09945-059
For the AD8420, assume that all initial voltages and currents are
zero until a positive differential voltage is applied between the
inputs, +IN and −IN. Transconductance Amplifier gm1 converts this
input voltage into a current, I1. Because the voltage across gm2 is
initially zero, I2 is zero and I3 equals I1.
R2 (kΩ)
Short
49.9
80.6
90.9
95.3
97.6
100
200
499
1000
Figure 59. Cancelling Out Error from FB Input Bias Current
R2
.
R1
Rev. A | Page 19 of 26
AD8420
Data Sheet
GAIN ACCURACY
INPUT PROTECTION
Unlike most instrumentation amplifiers, the relative match of
the two gain setting resistors determines the gain accuracy of
the AD8420 rather than a single resistor. For example, if two
resistors have exactly the same absolute error, there is no error
in gain. Conversely, two 1% resistors can cause approximately 2%
maximum gain error at high gains. Temperature coefficient
mismatch of the gain setting resistors increases the gain drift of
the instrumentation amplifier circuit, according to the gain
equation. Because these external resistors do not have to match
any on-chip resistors, resistors with good TC tracking can
achieve excellent gain drift. Even with standard thin film
resistors, the AD8420 can still achieve better gain drift than
most instrumentation amplifiers.
The current into the AD8420 inputs is limited internally. This
ensures that the diodes that limit the differential voltage seen by
the internal amplifier do not draw excessive current when they
turn on. The device can handle large differential input voltages,
regardless of the amount of gain applied, without damage. As a
result, the AD8420 inputs are protected from voltages beyond
the positive rail. If voltages beyond the negative rail are expected,
external protection must be used.
When the differential voltage at the inputs approaches the
differential input limit, the diodes start to conduct, limiting the
voltage seen by the inputs of amplifier gm1. This can look like
increased gain error at large differential inputs. Performance of the
AD8420 is specified for ±1 V differential from −40°C to +85°C.
However, at higher temperatures, the reduced forward voltage of
the diodes limits the differential input to a smaller voltage.
Figure 60 tracks 1% error across the operating temperature
range to show the effect of temperature on the input limit.
VS = ±15V
For applications that require protection beyond the negative rail,
one option is to use an external resistor in series with each input
to limit current during overload conditions. In this case, size the
resistors to limit the current into the AD8420 to 6 mA.
RPROTECT ≥ (Negative Supply − VIN)/6 mA
Although the AD8420 inputs must still be kept within the −VS +
40 V limitation, the I × R drop across the protection resistor
increases the protection on the positive side to approximately
(40 V + Negative Supply) + 300 μA × RPROTECT
An alternate protection method is to place diodes at the AD8420
inputs to limit voltage and resistors in series with the inputs to
limit the current into these diodes. To keep input bias current at
a minimum for normal operation, use low leakage diode clamps,
such as the BAV199. The AD8420 also combines well with TVS
diodes, such as the PTVSxS1UR.
1.8
1.6
1.4
Input Voltages Beyond the Rails
POSITIVE VOLTAGE
1.2
1.0
0.8
RPROTECT
0.6
+
VIN+
–
0.4
0.2
0
–40
AD8420
RPROTECT
–25
–10
5
20
35
50
65
80
95
110
TEMPERATURE (°C)
125
+
VIN–
–
Figure 60. Differential Input Limit vs. Temperature
+VS
RPROTECT
+VS
I
+
VIN+
–
–VS
+VS
AD8420
RPROTECT
–VS
+
VIN–
–
SIMPLE METHOD
INPUT VOLTAGE RANGE
–VS
–VS
ALTERNATE METHOD
Figure 61. Protection for Voltages Beyond the Rails
The allowed input range of the AD8420 is much simpler than
traditional architectures. For the transfer function of the AD8420
to be valid, the input voltage should follow two rules:


+VS
09945-160
NEGATIVE VOLTAGE
09945-503
MAXIMUM INPUT VOLTAGE (1% ERROR)
2.0
Keep all of the AD8420 terminals within the voltage range specified
in the Absolute Maximum Ratings section. All terminals of the
AD8420 are protected against ESD.
Keep the differential input voltage within ±1 V.
Keep the voltage on the +IN, −IN, REF, and FB pins in the
specified input voltage range.
Because the output swing is completely independent of the
input common-mode voltage, there are no hexagonal figures
or complicated formulas to follow, and no limitation for the
output swing the amplifier has for input signals with changing
common mode.
Large Differential Input Voltage
The AD8420 is able to handle large differential input voltage
without damage to the device. Refer to Figure 9, Figure 10,
Figure 11, and Figure 12 for overvoltage performance. The
AD8420 differential voltage is internally limited with diodes to
±1 V. If this limit is exceeded, the diodes start to conduct and
draw current, as shown in Figure 22. This current is limited
internally to a value that is safe for the AD8420, but if the input
current cannot be tolerated in the system, place resistors in
series with each input with the following value:
Rev. A | Page 20 of 26
1V 
1 V

RPROTECT   DIFF

2 
I MAX

Data Sheet
AD8420
LAYOUT
Reference
Common-Mode Rejection Ratio over Frequency
The output voltage of the AD8420 is developed with respect to
the potential on the reference terminal. Take care to tie REF to the
appropriate local ground. The differential voltage at the inputs is
reproduced between the REF and FB pins; therefore, it is important
to set VREF so that the voltage at FB does not exceed the input range.
Poor layout can cause some of the common-mode signal to be
converted to a differential signal before reaching the in-amp. This
conversion can occur when the path to the positive input pin
has a different frequency response than the path to the negative
input pin. For best CMRR vs. frequency performance, the input
source impedance and capacitance of each path should be closely
matched. This includes connecting Pin 1 to −VS, which matches the
parasitic capacitance and the leakage between the inputs and
adjacent pins. Place additional source resistance in the input
path (for example, for input protection) close to the in-amp inputs
to minimize their interaction with the parasitic capacitance from
the printed circuit board (PCB) traces.
DRIVING THE REFERENCE PIN
Traditional instrumentation amplifier architectures require the
reference pin to be driven with a low impedance source. In these
architectures, impedance at the reference pin degrades both CMRR
and gain accuracy. With the AD8420 architecture, resistance at
the reference pin has no effect on CMRR.
+IN
VOUT
Power Supplies
AD8420
Use a stable dc voltage to power the instrumentation amplifier.
Noise on the supply pins can adversely affect performance. For
more information, see the PSRR performance curves in Figure 24
and Figure 25.
+VS
G=1+
R2 + RREF
R1
R1
RREF
R2
09945-062
Place a 0.1 μF capacitor as close as possible to each supply pin.
As shown in Figure 62, a 10 μF tantalum capacitor can be used
farther away from the device. This capacitor, which is intended
to be effective at low frequencies, can usually be shared by other
precision integrated circuits. Keep the traces between these
integrated circuits short to minimize interaction of the trace
parasitic inductance with the shared capacitor.
FB
REF
–IN
VREF
Figure 63. Calculating Gain with Reference Resistance
Resistance at the reference pin does affect the gain of the AD8420,
but if this resistance is constant, the gain setting resistors can be
adjusted to compensate. For example, the AD8420 can be driven
with a voltage divider to level shift the output as shown in
Figure 64.
+IN
0.1µF
10µF
VOUT
AD8420
+IN
FB
VOUT
AD8420
VS
R2
G=1+
–VS
10µF
R2
R2 + R3||R4
R4
R1
Figure 64. Using Resistor Divider to Set Reference Voltage
09945-060
0.1µF
R1
R3
09945-063
R1
–IN
REF
–IN
Figure 62. Supply Decoupling, REF, and Output Referred to Local Ground
Rev. A | Page 21 of 26
AD8420
Data Sheet
INCORRECT
CORRECT
+VS
+VS
VOUT
VOUT
AD8420
AD8420
–VS
–VS
TRANSFORMER
TRANSFORMER
+VS
+VS
VOUT
VOUT
AD8420
AD8420
10MΩ
–VS
–VS
THERMOCOUPLE
THERMOCOUPLE
+VS
+VS
C
C
VOUT
AD8420
VOUT
1
R
fHIGH-PASS = 2πRC
C
AD8420
C
–VS
09945-061
R
–VS
CAPACITIVELY COUPLED
CAPACITIVELY COUPLED
Figure 65. Creating an IBIAS Path
+VS
INPUT BIAS CURRENT RETURN PATH
0.1µF
R
20kΩ
1%
+IN
RADIO FREQUENCY INTERFERENCE (RFI)
–IN
All instrumentation amplifiers can rectify high frequency out-ofband signals. Once rectified, these signals appear as dc offset errors
at the output. High frequency signals can be filtered with a low-pass
RC network placed at the input of the instrumentation amplifier, as
shown in Figure 66. The filter limits the input signal bandwidth
according to the following relationship:
FilterFrequency DIFF 
FilterFrequency CM 
where CD  10 CC.
1
2πR(2C D  C C )
1
2πRCC
10µF
CC
330pF
5%
R
CD
20kΩ 3300pF
1%
VOUT
AD8420
CC
330pF
5%
R1
0.1µF
R2
10µF
–VS
09945-064
The input bias current of the AD8420 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a return current path, create one, as shown in Figure 65.
Figure 66. Suggested RFI Suppression Filter
CD affects the differential signal and CC affects the common-mode
signal. Values of R and CC are chosen to minimize out of band
RFI at the expense of reduced signal bandwidth. Mismatch
between the R × CC at the positive input and the R × CC at the
negative input degrades the CMRR of the AD8420. By using a
value of CD that is at least one magnitude larger than CC, the
effect of the mismatch is reduced and performance is improved.
Rev. A | Page 22 of 26
Data Sheet
AD8420
OUTPUT BUFFERING
The AD8420 is designed to drive loads of 20 kΩ or greater but
can deliver up to 10 mA to heavier loads at lower output voltage
swings (see Figure 42). If more output current is required, buffer
the AD8420 output with a precision op amp. Figure 67 shows
the recommended configuration using the ADA4692-2 with a
single supply. This low power op amp can swing its output from
1 V to 4 V on a single 5 V supply while sourcing or sinking
more than 30 mA of current. When using this configuration,
the load seen by the AD8420 is approximately R1 + R2.
+5V
+VS 0.1µF
AD8420
ADA4692-2
–VS
R1
VOUT
R2
VIN
VREF
–VS
09945-065
VIN
+5V
0.1µF
AD8420
VOUT
0.1µF
CW
R1
R2
R
W
REF
Figure 67. Output Buffering
ADA4692-2
SUGGESTION FOR SECOND
AMPLIFIER: VARIABLE
LEVEL SHIFT WITHOUT
AFFECTING GAIN
Figure 68. Variable Level Shift
Rev. A | Page 23 of 26
CCW
R
09945-066
+VS
0.1µF
Because the ADA4692-2 is a dual op amp, another op amp is now
free for use as an active filter stage or to buffer another AD8420
output on the same PCB. Figure 68 shows another suggestion
for how to use this second op amp. In this circuit, the voltage
from the wiper of a potentiometer is buffered by the ADA4692-2,
allowing a variable level shift of the output. Resistors above and
below the potentiometer reduce the total range of the level shift
but increase the precision. If the potentiometer were connected
directly to the REF pin of the AD8420, gain error would be introduced from the variable resistance. The potentiometer can be tuned
in hardware or software, depending on the type of potentiometer
chosen. For a list of digital potentiometers made by Analog
Devices, Inc., visit www.analog.com/digitalpotentiometers.
AD8420
Data Sheet
APPLICATIONS INFORMATION
A high-pass filter is commonly used in ECG signal conditioning
circuitry to remove electrode offset and motion artifacts. To avoid
degrading the input impedance and CMRR of the system, this
filtering is typically implemented after the instrumentation
amplifier, which limits the gain that can be applied with the
instrumentation amplifier.
With a 3-op-amp instrumentation amplifier, gain is applied in the
first stage. Because of this, the electrode offset is gained and then
must be removed afterward with a high-pass filter. In the AD8420
architecture, the offset can be accounted for in the input stage
200pF
A
B
402kΩ
Figure 69 shows an ECG front end that applies a gain of 100 to
the signal while rejecting dc and high frequencies. This circuit
combines the AD8420 with the AD8657, which is a low power,
low cost, dual, precision CMOS op amp.
THREE-POLE LPF,
BESSEL RESPONSE
FC = 50Hz
INSTRUMENTATION
AMPLIFIER
G = +100
+5V
100kΩ
2000pF
500kΩ
by unbalancing the transconductance amplifier at the REF and
FB pins. In the steady state, the offset at the input is not gained
to the output, and higher frequency signals can be gained and
passed through. Using the AD8420 in this way, the offset tolerance
is nearly the differential input range of the device (±1 V).
AD8420
100kΩ
200pF
FB
REF
–5V
C
110kΩ
200kΩ
0.015μF
0.022μF
200kΩ
+5V
100kΩ
1kΩ
3.3μF
+5V
8200pF
10MΩ
–5V
AD8657-1
–5V
Figure 69. AD8420 in an ECG Front End
Rev. A | Page 24 of 26
AD8657-2
INTEGRATOR PROVIDES
HIGH-PASS POLE AT 0.5Hz
09945-072
AD8420 IN ELECTROCARDIOGRAPHY (ECG)
Data Sheet
AD8420
CLASSIC BRIDGE CIRCUIT
4 mA TO 20 mA SINGLE-SUPPLY RECEIVER
Figure 70 shows the AD8420 configured to amplify the signal from
a classic resistive bridge. This circuit works in dual-supply mode or
single-supply mode. Typically, the same voltage that powers the
instrumentation amplifier excites the bridge. Connecting the
bottom of the bridge to the negative supply of the instrumentation
amplifier sets up an input common-mode voltage that is located
midway between the supply voltages. The voltage on the REF pin
can be varied to suit the application. For example, the REF pin
is tied to the VREF pin of an analog-to-digital converter (ADC)
whose input range is (VREF ± VIN). With an available output swing
on the AD8420 of (−VS + 100 mV) to (+VS − 150 mV), the
maximum programmable gain is simply this output range divided
by the input range.
The 90 μA maximum supply current, input range that goes
below ground, and low drift characteristics make the AD8420 a
very good candidate for use in a 4 mA to 20 mA loop. Figure 71
shows how a signal from a 4 mA to 20 mA transducer can be
interfaced to the AD8420. The signal from a 4 mA to 20 mA
transducer is single-ended, which initially suggests the need for
a simple shunt resistor to ground to convert the current to a voltage.
However, any line resistance in the return path (to the transducer)
adds a current-dependent offset error; therefore, the current must
be sensed differentially.
In this example, a 5 Ω shunt resistor generates a differential voltage
at the inputs of the AD8420 between 20 mV (for 4 mA in) and
100 mV (for 20 mA in) with a very low common-mode value.
With the gain resistors shown, the AD8420 amplifies the 100 mV
input voltage by a factor of 40 to 4.0 V.
+VS
0.1µF
VDIFF
AD8420
VOUT
VREF
09945-069
0.1µF
–VS
Figure 70. Classic Bridge Circuit
5V
0.1µF
–
LINE
IMPEDANCE
+
–
+
POWER
SUPPLY
4mA TO 20mA
5Ω
AD8420
AD627
G = 40
R2 = 97.6kΩ
R1 = 2.49kΩ
Figure 71. 4 mA to 20 mA Receiver Circuit
Rev. A | Page 25 of 26
R1
R2
0.8V TO 4.0V
09945-073
4mA TO 20mA
TRANSDUCER
AD8420
Data Sheet
OUTLINE DIMENSIONS
3.20
3.00
2.80
8
3.20
3.00
2.80
1
5.15
4.90
4.65
5
4
PIN 1
IDENTIFIER
0.65 BSC
0.95
0.85
0.75
15° MAX
1.10 MAX
0.40
0.25
6°
0°
0.23
0.09
COMPLIANT TO JEDEC STANDARDS MO-187-AA
0.80
0.55
0.40
10-07-2009-B
0.15
0.05
COPLANARITY
0.10
Figure 72. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
ORDERING GUIDE
Model1
AD8420ARMZ
AD8420ARMZ-R7
AD8420ARMZ-RL
1
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
8-Lead Mini Small Outline Package [MSOP], Tube
8-Lead Mini Small Outline Package [MSOP], 7-Inch Tape and Reel
8-Lead Mini Small Outline Package [MSOP], 13-Inch Tape and Reel
Z = RoHS Compliant Part.
©2012–2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D09945-0-1/15(A)
Rev. A | Page 26 of 26
Package
Option
RM-8
RM-8
RM-8
Branding
Y3Y
Y3Y
Y3Y