AD AD8235 Low noise, low gain drift, g = 2000 Datasheet

FEATURES
Fixed gain of 2000
Access to internal nodes provides flexibility
Low noise: 1.5 nV/√Hz input voltage noise
High accuracy dc performance
Gain drift: 5 ppm/°C
Offset drift: 0.3 μV/°C
Gain accuracy: 0.05%
CMRR: 140 dB min
Excellent ac specifications
Bandwidth: 3.5 MHz
Slew rate: 40 V/μs
Power supply range: ±4 V to ±18 V
8-lead SOIC package
ESD protection: 5000 V (HBM)
Temperature range for specified performance:
−40°C to +85°C
Operational up to 125°C
APPLICATIONS
Sensor interface
Medical instrumentation
Patient monitoring
FUNCTIONAL BLOCK DIAGRAM
–IN
3kΩ
The AD8428 is an ultralow noise instrumentation amplifier
designed to accurately measure tiny, high speed signals. It
delivers industry-leading gain accuracy, noise, and bandwidth.
All gain setting resistors for the AD8428 are internal to the part
and are precisely matched. Care is taken in both the chip pinout
and layout. This results in excellent gain drift and quick settling
to the final gain value after the part is powered on.
The high CMRR of the AD8428 prevents unwanted signals
from corrupting the signal of interest. The pinout of the AD8428
is designed to avoid parasitic capacitance mismatches that can
degrade CMRR at high frequencies.
6kΩ
6kΩ
120kΩ
OUT
30.15Ω
3kΩ
6kΩ
6kΩ
120kΩ
REF
+IN
AD8428
–VS
+FIL
Figure 1.
Table 1. Instrumentation Amplifiers by Category1
GeneralPurpose
AD8220
AD8221
AD8222
AD8224
AD8228
AD8295
1
GENERAL DESCRIPTION
–FIL
+VS
09731-001
Data Sheet
Low Noise, Low Gain Drift, G = 2000
Instrumentation Amplifier
AD8428
Zero
Drift
AD8231
AD8290
AD8293
AD8553
AD8556
AD8557
Military
Grade
AD620
AD621
AD524
AD526
AD624
Low
Power
AD627
AD623
AD8235
AD8236
AD8426
AD8226
AD8227
AD8420
Low
Noise
AD8428
AD8429
See www.analog.com for the latest instrumentation amplifiers.
The AD8428 is one of the fastest instrumentation amplifiers
available. The circuit architecture is designed for high bandwidth
at high gain. The AD8428 uses a current feedback topology for
the initial preamplifier gain stage of 200, followed by a difference
amplifier stage of 10. This architecture results in a 3.5 MHz
bandwidth at a gain of 2000 for an equivalent gain bandwidth
product of 7 GHz.
The AD8428 pinout allows access to internal nodes between the
first and second stages. This feature can be useful for modifying
the frequency response between the two amplification stages,
thereby preventing unwanted signals from contaminating the
output results.
The performance of the AD8428 is specified over the industrial
temperature range of −40°C to +85°C. It is available in an 8-lead
plastic SOIC 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 ©2011–2012 Analog Devices, Inc. All rights reserved.
AD8428
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Filter Terminals........................................................................... 13
Applications....................................................................................... 1
Reference Terminal .................................................................... 13
Functional Block Diagram .............................................................. 1
Input Voltage Range................................................................... 14
General Description ......................................................................... 1
Layout .......................................................................................... 14
Revision History ............................................................................... 2
Input Bias Current Return Path ............................................... 15
Specifications..................................................................................... 3
Input Protection ......................................................................... 15
Absolute Maximum Ratings............................................................ 5
Radio Frequency Interference (RFI)........................................ 16
Thermal Resistance ...................................................................... 5
Calculating the Noise of the Input Stage................................. 16
ESD Caution.................................................................................. 5
Applications Information .............................................................. 18
Pin Configuration and Function Descriptions............................. 6
Effect of Passive Network Across the Filter Terminals.......... 18
Typical Performance Characteristics ............................................. 7
Circuits Using the Filter Terminals.......................................... 18
Theory of Operation ...................................................................... 13
Outline Dimensions ....................................................................... 20
Architecture................................................................................. 13
Ordering Guide .......................................................................... 20
REVISION HISTORY
4/12—Rev. 0 to Rev. A
Changes to Features Section and Table 1 ...................................... 1
Added B Grade Column to Table 2 ................................................ 3
Changes to Figure 3, Figure 4, Figure 5, Figure 6, Figure 7,
and Figure 8....................................................................................... 7
Changes to Filter Terminals Section ............................................ 13
Added Applications Information Section ................................... 18
Changes to Ordering Guide .......................................................... 20
10/11—Revision 0: Initial Version
Rev. A | Page 2 of 20
Data Sheet
AD8428
SPECIFICATIONS
VS = ±15 V, VREF = 0 V, TA = 25°C, G = 2000, RL = 10 kΩ, unless otherwise noted.
Table 2.
Parameter
COMMON-MODE REJECTION RATIO
(CMRR)
DC to 60 Hz
At 50 kHz
NOISE (RTI)
Voltage Noise
Current Noise
VOLTAGE OFFSET
Input Offset, VOSI
Average TC
Offset RTI vs. Supply (PSRR)
INPUT CURRENT
Input Bias Current
Over Temperature
Input Offset Current
Over Temperature
DYNAMIC RESPONSE
−3 dB Small Signal Bandwidth
Settling Time to 0.01%
Settling Time to 0.001%
Slew Rate
GAIN
First Stage Gain
Subtractor Stage Gain
Total Gain Error
Total Gain Nonlinearity
Gain Drift
INPUT
Impedance (Pin to Ground) 1
Input Operating Voltage Range
Over Temperature
OUTPUT
Output Voltage Swing
Over Temperature
Output Voltage Swing
Over Temperature
Short-Circuit Current
REFERENCE INPUT
Input Impedance, RIN
Input Current, IIN
Voltage Range
Reference Gain to Output
Reference Gain Error
Test Conditions/
Comments
RTI, VCM = ±10 V
Min
A Grade
Typ
Max
130
110
VIN+, VIN− = 0 V
f = 1 kHz
f = 0.1 Hz to 10 Hz
f = 1 kHz
f = 0.1 Hz to 10 Hz
Min
B Grade
Typ
Max
140
120
1.3
40
1.5
150
1.5
50
dB
dB
1.3
40
1.5
150
100
1
TA = −40°C to +85°C
120
Unit
1.5
50
nV/√Hz
nV p-p
pA/√Hz
pA p-p
25
0.3
μV
μV/°C
dB
50
130
TA = −40°C to +85°C
250
200
TA = −40°C to +85°C
20
20
nA
pA/°C
nA
pA/°C
10 V step
10 V step
3.5
0.75
1.4
50
3.5
0.75
1.4
50
MHz
μs
μs
V/μs
200
10
0.05
5
5
V/V
V/V
%
ppm
ppm/°C
GΩ||pF
V
V
250
50
40
10
40
200
10
VOUT = −10 V to +10 V
VOUT = −10 V to +10 V
0.2
5
10
1||2
1||2
VS = ±4 V to ±18 V
TA = −40°C to +85°C
−VS + 2.5
−VS + 2.5
+VS − 2.5
+VS − 2.5
−VS + 2.5
−VS + 2.5
+VS − 2.5
+VS − 2.5
RL = 2 kΩ
TA = −40°C
TA = +85°C
RL = 10 kΩ
TA = −40°C
TA = +85°C
−VS + 1.7
−VS + 2.0
−VS + 1.6
−VS + 1.7
−VS + 1.8
−VS + 1.4
+VS − 1.2
+VS − 1.3
+VS − 1.1
+VS − 1.0
+VS − 1.2
+VS − 0.9
−VS + 1.7
−VS + 2.0
−VS + 1.6
−VS + 1.7
−VS + 1.8
−VS + 1.4
+VS − 1.2
+VS − 1.3
+VS − 1.1
+VS − 1.0
+VS − 1.2
+VS − 0.9
VIN+, VIN− = 0 V
30
30
132
6.5
132
6.5
−VS
+VS
1
0.01
Rev. A | Page 3 of 20
−VS
+VS
1
0.01
V
V
V
V
V
V
mA
kΩ
μA
V
V/V
%
AD8428
Parameter
FILTER TERMINALS
Input Impedance, RIN 2
Voltage Range
POWER SUPPLY
Operating Range
Quiescent Current
Over Temperature
1
2
Data Sheet
Test Conditions/
Comments
Min
A Grade
Typ
Max
Min
+VS
−VS
±18
6.8
8
±4
6
−VS
±4
6.5
TA = −40°C to +85°C
Max
Unit
+VS
kΩ
V
±18
6.8
8
V
mA
mA
6
The differential and common-mode input impedances can be calculated from the pin impedance: ZDIFF = 2(ZPIN); ZCM = ZPIN/2.
To calculate the actual impedance, see Figure 1.
Rev. A | Page 4 of 20
B Grade
Typ
6.5
Data Sheet
AD8428
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 3.
Parameter
Supply Voltage
Output Short-Circuit Current Duration
Maximum Voltage at −IN, +IN1
Maximum Voltage at −FIL, +FIL
Differential Input Voltage1
Maximum Voltage at REF
Storage Temperature Range
Specified Temperature Range
Maximum Junction Temperature
ESD
Human Body Model
Charged Device Model
Machine Model
1
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Rating
±18 V
Indefinite
±VS
±VS
±1 V
±VS
−65°C to +150°C
−40°C to +85°C
140°C
Table 4. Thermal Resistance
Package
8-Lead SOIC_N
ESD CAUTION
5000 V
1250 V
400 V
For voltages beyond these limits, use input protection resistors. See the
Input Protection section for more information.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Rev. A | Page 5 of 20
θJA
121
Unit
°C/W
AD8428
Data Sheet
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
8
+VS
–FIL 2
7
OUT
+FIL 3
6
REF
+IN 4
5
–VS
TOP VIEW
(Not to Scale)
09731-002
AD8428
–IN 1
Figure 2. Pin Configuration
Table 5. Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
Mnemonic
−IN
−FIL
+FIL
+IN
−VS
REF
OUT
+VS
Description
Negative Input Terminal.
Negative Filter Terminal.
Positive Filter Terminal.
Positive Input Terminal.
Negative Power Supply Terminal.
Reference Voltage Terminal. Drive this terminal with a low impedance voltage source to level-shift the output.
Output Terminal.
Positive Power Supply Terminal.
Rev. A | Page 6 of 20
Data Sheet
AD8428
TYPICAL PERFORMANCE CHARACTERISTICS
VS = ±15 V, VREF = 0 V, TA = 25°C, G = 2000, RL = 10 kΩ, unless otherwise noted.
1600
N = 5170
MEAN = 2.12
SD = 7.332
1200
1000
1000
HITS
1200
800
600
400
400
200
200
–40
–20
0
20
40
60
VOSI (µV)
N = 5171
MEAN = –10.8
SD = 6.67496
800
600
N = 5171
MEAN = –10.2
SD = 6.52901
0
–60
40
60
1200
1000
1000
HITS
1200
800
800
600
600
400
400
200
200
–40
–20
0
20
40
N = 5171
MEAN = –0.53
SD = 1.41655
1400
60
VOSI (µV)
0
–8
–6
–4
–2
0
2
4
6
8
IOS (nA)
09731-007
N = 5169
MEAN = –2.57
SD = 7.31066
09731-004
HITS
20
1600
1400
Figure 7. Typical Distribution of Input Offset Current
Figure 4. Typical Distribution of Input Offset Voltage, VS = ±15 V
1600
1600
N = 5166
MEAN = 0.398
SD = 0.42707
1400
1200
1000
1000
HITS
1200
800
800
600
600
400
400
200
200
–2
–1
0
1
2
VOSI DRIFT (µV/°C)
N = 3487
MEAN = –53.9
SD = 86.7774
1400
3
09731-005
HITS
0
Figure 6. Typical Distribution of Input Bias Current
1600
0
–3
–20
IBIAS (nA)
Figure 3. Typical Distribution of Input Offset Voltage, VS = ±5 V
0
–60
–40
0
–600
–400
–200
0
200
400
GAIN ERROR (µV/V)
Figure 8. Typical Distribution of Gain Error, Gain = 2000,
VS = ±15 V, RL = 10 kΩ
Figure 5. Typical Distribution of Input Offset Voltage Drift
Rev. A | Page 7 of 20
600
09731-008
0
–60
POSITIVE INPUT IBIAS
NEGATIVE INPUT IBIAS
1400
09731-003
HITS
1400
09731-006
1600
AD8428
Data Sheet
10
72
66
VS = ±15V
60
54
VS = ±12V
48
5
42
GAIN (dB)
INPUT COMMON-MODE VOLTAGE (V)
15
VS = ±5V
0
36
30
24
18
–5
12
6
–10
0
0
5
10
15
OUTPUT VOLTAGE (V)
–12
100
1k
14
150
12
140
CMRR (dB)
160
10
8
4
100
0
2
4
6
8
GAIN = 2000
90
VCM = +12V
–4 –2
10
12
14
COMMON-MODE VOLTAGE (V)
80
1
10
100
1k
10k
100k
1M
100k
1M
FREQUENCY (Hz)
Figure 13. CMRR vs. Frequency
Figure 10. Input Bias Current vs. Common-Mode Voltage,
VS = ±15 V
120
140
GAIN = 2000
110
120
100
90
+PSRR
100
80
CMRR (dB)
–PSRR
80
60
70
60
50
40
40
30
20
20
10
0
0.1
1
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
09731-011
PSRR (dB)
100M
120
110
2
10M
130
6
09731-010
INPUT BIAS CURRENT (nA)
170
VCM = –11.8V
–6
1M
Figure 12. Gain vs. Frequency
16
0
–14 –12 –10 –8
100k
FREQUENCY (Hz)
Figure 9. Input Common-Mode Voltage vs. Output Voltage,
VS = ±5 V, VS = ±12 V, VS = ±15 V
18
10k
09731-015
–5
09731-016
–10
09731-009
–15
–15
09731-014
–6
Figure 11. PSRR vs. Frequency
0
1
10
100
1k
10k
FREQUENCY (Hz)
Figure 14. CMRR vs. Frequency, 1 kΩ Source Imbalance
Rev. A | Page 8 of 20
Data Sheet
AD8428
70
60
4
NORMALIZED AT 25°C
CMRR (nV/V)
40
2
1
30
20
10
0
0
–10
–1
–20
0
10
20
30
40
50
60
70
80
90
100 110 120
WARM-UP TIME (Seconds)
–30
–40
1.2
9.0
0.8
8.5
0.4
IBIAS–
0
0
–5
–0.4
–10
–0.8
–15
–1.2
–20
–1.6
–25
SUPPLY CURRENT (mA)
5
IBIAS+
5
20
35
50
65
80
95
110
125
Figure 18. CMRR vs. Temperature, Normalized at 25°C
INPUT OFFSET CURRENT (nA)
IOS
10
–10
TEMPERATURE (°C)
Figure 15. Change in Input Offset Voltage (VOSI) vs. Warm-Up Time
15
–25
09731-020
–2
INPUT BIAS CURRENT (nA)
REPRESENTATIVE DATA
50
3
09731-017
CHANGE IN INPUT OFFSET VOLTAGE (µV)
5
8.0
7.5
7.0
6.5
6.0
5.5
–2.0
–10
5
20
35
50
65
80
95
110
–2.4
125
5.0
–40
TEMPERATURE (°C)
200
40
SHORT-CIRCUIT CURRENT (mA)
50
0
–50
–100
–150
20
35
50
65
80
95
110
125
110
125
ISHORT+
30
20
10
0
–10
–20
ISHORT–
–30
–40
REPRESENTATIVE DATA
–200
5
20
35
–40 –25 –10
NORMALIZED AT 25°C
50
65
80
95
110
125
TEMPERATURE (°C)
09731-019
GAIN ERROR (µV/V)
50
5
Figure 19. Supply Current vs. Temperature
250
100
–10
TEMPERATURE (°C)
Figure 16. Input Bias Current and Input Offset Current vs. Temperature,
Normalized at 25°C
150
–25
–50
–40
–25
–10
5
20
35
50
65
80
95
TEMPERATURE (°C)
Figure 20. Short-Circuit Current vs. Temperature
Figure 17. Gain Error vs. Temperature, Normalized at 25°C
Rev. A | Page 9 of 20
09731-022
–25
09731-018
–30
–40
09731-021
NORMALIZED AT 25°C
AD8428
Data Sheet
+VS
90
–0.4
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
100
–SR
70
60
50
+SR
40
30
20
10
9
+125°C
–0.8
–1.2
+2.0
+1.6
+1.2
+0.8
5
20
35
50
65
80
95
110
125
–VS
6
8
10
11
12
13
14
15
16
17
Figure 24. Output Voltage Swing vs. Supply Voltage, RL = 10 kΩ
+VS
90
–0.4
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
100
80
70
–SR
50
40
5
SUPPLY VOLTAGE (±VS)
Figure 21. Slew Rate vs. Temperature, VS = ±15 V
60
4
09731-026
–10
09731-023
–25
TEMPERATURE (°C)
+SR
30
20
10
–40°C
+25°C
7
9
+85°C
+125°C
–0.8
–1.2
+2.0
+1.6
+1.2
+0.8
+0.4
–25
–10
5
20
35
50
65
80
95
110
125
TEMPERATURE (°C)
–VS
09731-024
0
–40
5
6
8
10
11
12
13
14
15
16
17
SUPPLY VOLTAGE (±VS)
Figure 22. Slew Rate vs. Temperature, VS = ±5 V
+VS
4
09731-027
SLEW RATE (V/µs)
7
+85°C
+0.4
0
–40
Figure 25. Output Voltage Swing vs. Supply Voltage, RL = 2 kΩ
15
–40°C
+25°C
+85°C
+125°C
–1.0
–1.5
–40°C
+25°C
+85°C
+125°C
10
OUTPUT VOLTAGE SWING (V)
–0.5
–2.0
–2.5
+2.5
+2.0
+1.5
+1.0
5
0
–5
–10
+0.5
–VS
4
6
8
10
12
14
16
SUPPLY VOLTAGE (±VS)
18
09731-025
INPUT VOLTAGE (V)
REFERRED TO SUPPLY VOLTAGES
+25°C
–15
100
1k
10k
100k
LOAD RESISTANCE (Ω)
Figure 26. Output Voltage Swing vs. Load Resistance, VS = ±15 V
Figure 23. Input Voltage Limit vs. Supply Voltage
Rev. A | Page 10 of 20
09731-028
SLEW RATE (V/µs)
80
–40°C
Data Sheet
AD8428
+VS
+25°C
+85°C
+125°C
–1.0
–1.5
+1.5
+1.0
+0.5
0.1
1
10
OUTPUT CURRENT (mA)
Figure 30. RTI Voltage Noise, 0.1 Hz to 10 Hz
Figure 27. Output Voltage Swing vs. Output Current, VS = ±15 V
20
1s/DIV
09731-029
20nV/DIV
–VS
0.01
09731-032
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES
–40°C
–0.5
16
15
GAIN = 2000
15
14
CURRENT NOISE (pA/√Hz)
GAIN NONLINEARITY (ppm)
13
10
5
0
–5
–10
12
11
10
9
8
7
6
5
4
3
–15
–6
–4
–2
0
2
4
6
8
10
OUTPUT VOLTAGE (V)
10
100
1k
10k
100k
FREQUENCY (Hz)
GAIN = 2000
10
50pA/DIV
0.1
0.1
1
10
100
1k
10k
100k
FREQUENCY (Hz)
1s/DIV
Figure 29. RTI Voltage Noise Spectral Density vs. Frequency
Figure 32. Current Noise, 0.1 Hz to 10 Hz
Rev. A | Page 11 of 20
09731-034
1
09731-031
VOLTAGE NOISE (nV/√Hz)
1
Figure 31. Current Noise Spectral Density vs. Frequency
Figure 28. Gain Nonlinearity, RL = 10 kΩ
100
1
09731-033
–8
09731-030
2
–20
–10
AD8428
Data Sheet
5V/DIV
752ns TO 0.01%
1408ns TO 0.001%
NO LOAD
0.002%/DIV
09731-035
1µs/DIV
CL = 770pF
50mV/DIV
1µs/DIV
09731-037
CL = 500pF
TIME (µs)
Figure 33. Large Signal Pulse Response and Settling Time,
10 V Step, VS = ±15 V
Figure 35. Small Signal Pulse Response with Various Capacitive Loads,
No Resistive Load
1800
GAIN = 2000
1600
SETTLING TIME (ns)
1400
1200
SETTLED TO 0.001%
1000
800
600
SETTLED TO 0.01%
400
200
0
2
4
6
8
10
12
14
16
STEP SIZE (V)
Figure 36. Settling Time vs. Step Size
Figure 34. Small Signal Pulse Response, RL = 10 kΩ, CL = 100 pF
Rev. A | Page 12 of 20
18
20
09731-038
1µs/DIV
09731-036
20mV/DIV
Data Sheet
AD8428
THEORY OF OPERATION
I
VB
A1
A2
C1
–FIL
2
IB
COMPENSATION
C2
NODE 1
+VS
–VS
R3
6kΩ
R4
6kΩ
120kΩ
R7
+VS
7
+VS
R1
3kΩ
1
+VS
R2
3kΩ
Q1
–IN
4
Q2
+IN
R5
6kΩ
R6
6kΩ
+VS
–VS
–RG
RG
+RG
+VS
–VS
–VS
120kΩ
R8
3
+FIL
30.15Ω
–VS
OUT
A3
NODE 2
6
REF
09731-042
I
IB
COMPENSATION
–VS
Figure 37. Simplified Schematic
ARCHITECTURE
REFERENCE TERMINAL
The AD8428 is based on the classic 3-op-amp topology. This
topology has two stages: a gain stage (preamplifier) to provide
differential amplification by a factor of 200, followed by a difference amplifier (subtractor) stage to remove the common-mode
voltage and provide additional amplification by a factor of 10.
Figure 37 shows a simplified schematic of the AD8428.
The output voltage of the AD8428 is developed with respect to
the potential on the reference terminal. This is useful when the
output signal must be offset to a precise midsupply level. For
example, a voltage source can be tied to the REF pin to levelshift the output so that the AD8428 can drive a single-supply
ADC. The REF pin is protected with ESD diodes and should
not exceed either +VS or −VS.
The first stage works as follows. To keep its two inputs matched,
Amplifier A1 must keep the collector of Q1 at a constant voltage.
It does this by forcing −RG to be a constant diode drop from −IN.
Similarly, A2 forces +RG to be a constant diode drop from +IN.
Therefore, a replica of the differential input voltage is placed across
the gain setting resistor, RG. The current that flows across this
resistor must also flow through the R1 and R2 resistors, creating
a gained differential signal between the A2 and A1 outputs.
The second stage is a G = 10 difference amplifier, composed of
Amplifier A3 and Resistors R3 through R8. This stage removes
the common-mode signal from the amplified differential signal.
The transfer function of the AD8428 is
For best performance, the source impedance to the REF
terminal should be kept well below 1 Ω. As shown in Figure 37,
the reference terminal, REF, is at one end of a 120 kΩ resistor.
Additional impedance at the REF terminal adds to this 120 kΩ
resistor and results in amplification of the signal connected to
the positive input. The amplification from the additional RREF
can be calculated as follows:
2 × (120 kΩ + RREF)/(240 kΩ + RREF)
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades the CMRR of
the amplifier.
INCORRECT
VOUT = 2000 × (VIN+ − VIN−) + VREF
CORRECT
FILTER TERMINALS
AD8428
REF
AD8428
REF
V
V
+
OP1177
–
Figure 38. Driving the Reference Pin
Rev. A | Page 13 of 20
09731-043
The −FIL and +FIL terminals allow access between R3 and R4,
and between R5 and R6, respectively. Adding a filter between
these two terminals modifies the gain that is applied to the signal
before it reaches the second amplifier stage (see the Applications
Information section).
AD8428
Data Sheet
INPUT VOLTAGE RANGE
Power Supplies and Grounding
The 3-op-amp architecture of the AD8428 applies gain in the
first stage before removing the common-mode voltage in the
difference amplifier stage. Internal nodes between the first and
second stages (Node 1 and Node 2 in Figure 37) experience a
combination of an amplified differential signal, a common-mode
signal, and a diode drop. This combined signal can be limited by
the voltage supplies even when the individual input and output
signals are not limited. Figure 9 shows the allowable input
common-mode voltage ranges for various output voltages and
supply voltages.
Use a stable dc voltage to power the instrumentation amplifier.
Noise on the supply pins can adversely affect performance. See
the PSRR performance curves in Figure 11 for more information.
LAYOUT
To ensure optimum performance of the AD8428 at the PCB
level, care must be taken in the design of the board layout. The
pins of the AD8428 are especially arranged to simplify board
layout and to help minimize parasitic imbalance between the
inputs.
Place a 0.1 μF capacitor as close as possible to each supply pin.
Because the length of the bypass capacitor leads is critical at
high frequency, surface-mount capacitors are recommended.
Parasitic inductance in the bypass ground trace works against
the low impedance created by the bypass capacitor.
As shown in Figure 40, a 10 μF capacitor can be used farther
away from the device. For larger value capacitors, which are
intended to be effective at lower frequencies, the current return
path distance is less critical. In most cases, the 10 μF capacitor
can be shared by other precision integrated circuits.
+VS
0.1µF
10µF
AD8428
+VS
–FIL 2
7
OUT
+FIL 3
6
REF
+IN 4
5
–VS
TOP VIEW
(Not to Scale)
+IN
OUT
AD8428
LOAD
REF
–IN
Figure 39. Pinout Diagram
0.1µF
Common-Mode Rejection Ratio over Frequency
–VS
Poor layout can cause some of the common-mode signals to
be converted to differential signals before reaching the in-amp.
Such conversions occur when one input path has a frequency
response that is different from the other.
To maintain high CMRR over frequency, the input source
impedance and capacitance of each path should be closely
matched. Additional source resistance in the input paths (for
example, for input protection) should be placed close to the
in-amp inputs to minimize the interaction of the inputs with
parasitic capacitance from the PCB traces.
Parasitic capacitance at the filter pins can also affect CMRR over
frequency. If the board design has a component at the filter pins,
the component should be chosen so that the parasitic capacitance
is as small as possible.
10µF
09731-045
8
09731-044
–IN 1
Figure 40. Supply Decoupling, REF, and Output Referred to Local Ground
A ground plane layer is helpful to reduce undesired parasitic
inductances and to minimize voltage drops with changes in
current. The area of the current path is directly proportional
to the magnitude of parasitic inductances and, therefore, the
impedance of the path at high frequency. Large changes in
currents in an inductive decoupling path or ground return
create unwanted effects due to the coupling of such changes
into the amplifier inputs.
Because load currents flow from the supplies, the load should
be connected at the same physical location as the bypass capacitor grounds.
Reference Pin
The output voltage of the AD8428 is developed with respect to
the potential on the reference terminal. Ensure that REF is tied
to the appropriate local ground.
Rev. A | Page 14 of 20
Data Sheet
AD8428
INPUT BIAS CURRENT RETURN PATH
Input Voltages Beyond the Rails
The input bias current of the AD8428 must have a return path
to ground. When the source, such as a thermocouple, cannot
provide a current return path, one should be created, as shown
in Figure 41.
If voltages beyond the rails are expected, use an external resistor
in series with each input to limit current during overload conditions. The limiting resistor at each input can be computed using
the following equation:
CORRECT
+VS
RPROTECT ≥
+VS
AD8428
REF
–VS
–VS
TRANSFORMER
TRANSFORMER
+VS
+
VIN+
–
I
AD8428
+
REF
+VS
RPROTECT
+
VIN+
–
–VS
VIN–
–
+VS
33Ω
I
–VS
AD8428
+VS
RPROTECT
RPROTECT
AD8428
REF
+VS
RPROTECT
+VS
AD8428
I MAX
Noise sensitive applications may require a lower protection
resistance. Low leakage diode clamps, such as the BAV199, can
be used at the inputs to shunt current away from the AD8428
inputs and, therefore, allow smaller protection resistor values.
To ensure that current flows primarily through the external
protection diodes, place a small value resistor, such as a 33 Ω
resistor, between the diodes and the AD8428.
AD8428
REF
VIN − VSUPPLY
33Ω
+
VIN–
–
–VS
–VS
10MΩ
–VS
SIMPLE METHOD
–VS
THERMOCOUPLE
LOW NOISE METHOD
09731-047
INCORRECT
Figure 42. Protection for Voltages Beyond the Rails
THERMOCOUPLE
Large Differential Input Voltage at High Gain
+VS
C
If large differential voltages at high gain are expected, use
an external resistor in series with each input to limit current
during overload conditions. The limiting resistor at each input
can be computed using the following equation:
C
C
R
1
fHIGH-PASS = 2πRC
AD8428
REF
AD8428
C
REF
R PROTECT ≥
–VS
CAPACITIVELY COUPLED
09731-046
R
–VS
CAPACITIVELY COUPLED
Figure 41. Creating an Input Bias Current Return Path
INPUT PROTECTION
Do not allow the inputs of the AD8428 to exceed the ratings
stated in the Absolute Maximum Ratings section. If these ratings
cannot be adhered to, add protection circuitry in front of the
AD8428 to limit the maximum current into the inputs (see the
IMAX section).
⎞
1 ⎛⎜ VDIFF − 1 V
×
− RG ⎟
⎟
2 ⎜
I MAX
⎝
⎠
Noise sensitive applications may require a lower protection
resistance. Low leakage diode clamps, such as the BAV199,
can be used across the AD8428 inputs to shunt current away
from the inputs and, therefore, allow smaller protection resistor
values.
RPROTECT
+ I
VDIFF
–
RPROTECT
IMAX
The maximum current into the AD8428 inputs, IMAX, depends
on time and temperature. At room temperature, the device can
withstand a current of 10 mA for at least one day. This time is
cumulative over the life of the device.
Rev. A | Page 15 of 20
AD8428
09731-048
+VS
Figure 43. Protection for Large Differential Voltages
AD8428
Data Sheet
RADIO FREQUENCY INTERFERENCE (RFI)
For best results, place the RFI filter network as close to the
amplifier as possible. Layout is critical to ensure that RF signals
are not picked up on the traces after the filter. If RF interference
is too strong to be filtered, shielding is recommended.
Because of its high gain and low noise properties, the AD8428
is a highly sensitive amplifier. Therefore, RF rectification can be
a problem if the AD8428 is used in applications that have strong
RF signal sources present. The problem is intensified if long leads
or PCB traces are required to connect the amplifier to the signal
source. The disturbance can appear as a dc offset voltage or as a
train of pulses.
CALCULATING THE NOISE OF THE INPUT STAGE
High frequency signals can be filtered with a low-pass filter
network at the input of the instrumentation amplifier, as shown
in Figure 44.
The total noise of the amplifier front end depends on much
more than the specifications in this data sheet. The three main
contributors to noise are as follows:
+VS
0.1µF
CC
1nF
R
CD
10nF
OUT
AD8428
R
REF
–IN
33Ω
CC
1nF
0.1µF
Source Resistance Noise
*CHIP FERRITE BEAD.
09731-049
10µF
–VS
Figure 44. RFI Suppression
The filter limits both the differential and common-mode bandwidth, as shown in the following equations:
FilterFrequency DIFF =
FilterFrequency CM =
Source resistance
Voltage noise of the instrumentation amplifier
Current noise of the instrumentation amplifier
In the following calculations, noise is referred to the input (RTI);
that is, all sources of noise are calculated as if the source appeared
at the amplifier input. To calculate the noise referred to the amplifier output (RTO), multiply the RTI noise by the gain of the
instrumentation amplifier.
+IN
33Ω
L*
•
•
•
10µF
1
2 πR(2C D + CC )
Any sensor connected to the AD8428 has some output resistance.
There may also be resistance placed in series with the inputs for
protection from either overvoltage or radio frequency interference.
This combined resistance is labeled R1 and R2 in Figure 45. Any
resistor, no matter how well made, has an intrinsic level of noise.
This noise is proportional to the square root of the resistor value.
At room temperature, the value is approximately equal to
4 nV/√Hz × √(resistor value in kΩ).
SENSOR
1
2πRCC
R1
where CD ≥ 10 CC.
CD affects the differential signal, and CC affects the commonmode signal. Choose values of R and CC that minimize RFI. A
mismatch between R × CC at the positive input and R × CC at
the negative input degrades the CMRR of the AD8428. By using
a value of CD one order of magnitude larger than CC, the effect
of the mismatch is reduced, and performance is improved.
R2
AD8428
09731-050
L*
Note that the resistors used for the RFI filter can be the same
as those used for input protection (see the Input Protection
section).
Figure 45. Source Resistance from Sensor and Protection Resistors
For example, assuming that the combined sensor and protection resistance is 4 kΩ on the positive input and 1 kΩ on the
negative input, the total noise from the input resistance is
Resistors add noise; therefore, the choice of resistor and capacitor values depends on the desired trade-off between noise, input
impedance at high frequencies, and RFI immunity. To achieve
low noise and sufficient RFI filtering, the use of inductive ferrite
beads is recommended (see Figure 44). Using inductive ferrite
beads allows the value of the resistors to be reduced, which helps
to minimize the noise at the input.
Rev. A | Page 16 of 20
(4 × 4 ) + (4 × 1 )
2
2
= 64 + 16 = 8.9 nV/ Hz
Data Sheet
AD8428
Voltage Noise of the Instrumentation Amplifier
Total Noise Density Calculation
Unlike other instrumentation amplifiers in which an external
resistor is used to set the gain, the voltage noise specification
of the AD8428 already includes the input noise, output noise,
and the RG resistor noise.
To determine the total noise of the in-amp, referred to input,
combine the source resistance noise, voltage noise, and current
noise contribution by the sum of squares method.
Current Noise of the Instrumentation Amplifier
The contribution of current noise to the input stage in nV/√Hz
is calculated by multiplying the source resistance in kΩ by the
specified current noise of the instrumentation amplifier in
pA/√Hz.
For example, if the R1 source resistance in Figure 45 is 4 kΩ
and the R2 source resistance is 1 kΩ, the total noise, referred
to input, is
For example, if the R1 source resistance in Figure 45 is 4 kΩ
and the R2 source resistance is 1 kΩ, the total effect from the
current noise is calculated as follows:
(4 × 1.5) + (1 × 1.5)
2
2
=
36 + 2.25 = 6.2 nV/ Hz
Rev. A | Page 17 of 20
8.9 2 + 1.52 + 6.2 2 = 11.0 nV/ Hz
AD8428
Data Sheet
APPLICATIONS INFORMATION
The classic 3-op-amp topology used for instrumentation
amplifiers typically places all the gain in the first stage and
subtracts the common-mode signals only in the second stage.
When operated at high gain, any amplifier is sensitive to large
interfering signals that can saturate it, thus making it impossible
to recover the signal of interest.
The AD8428 splits the total gain of 2000 into two stages: 200 in the
preamplification stage and 10 in the subtractor stage. Reducing the
gain of the first stage helps to increase the common-mode range
vs. differential signal range by avoiding saturation of the preamps.
Because this resistor appears inside the feedback of the subtractor
stage, it modifies the gain of the subtractor as well. The total gain
formula is a simplified version of the transfer function equation
(Equation 1).
G=
RG =
6000 × G
2000 − G
The AD8428 defaults to G = 2000 when no gain resistor is used.
When setting the amplifier to a different gain, the absolute gain
accuracy is only 10%. In addition, the temperature mismatch of
the external gain resistor increases the gain drift of the instrumentation amplifier. Gain error and gain drift are at a guaranteed
minimum when a gain resistor is not used. For applications that
require accuracy at different gains, low noise, and wide bandwidth,
the AD8429 should be considered.
10
5
SINGLE STAGE GAIN, G = 2000
AD8428
–5
–10
Low-Pass Filter
–15
–15
To help limit undesired differential signals, a first-order, low-pass
filter can be implemented by adding a capacitor across the filter
terminals of the AD8428, as shown in Figure 47.
–5
0
5
10
15
OUTPUT VOLTAGE (V)
Figure 46. AD8428 vs. Single Stage Gain Topology, G = 2000
+IN
In addition, filtering between stages can help to attenuate signals
before they reach the second amplification stage. This filtering
helps to prevent saturation of the second stage amplifier as long
as the signals are located in frequencies other than the signal of
interest.
EFFECT OF PASSIVE NETWORK ACROSS THE
FILTER TERMINALS
The AD8428 filter terminals allow access between the two
amplification stages. Adding a passive network between the two
terminals can shape the transfer function over the frequency of
the amplifier. The general expression for the transfer function is
represented by Equation 1.
G(s) =
2000 × Z(s)
Z(s) + 6000
(1)
where Z(s) is the frequency dependent impedance of the
network across the filter terminals.
CIRCUITS USING THE FILTER TERMINALS
Setting the Amplifier to Different Gains
In its simplest form, the transfer function equation (Equation 1)
implies that the AD8428 can be configured for gains lower than
2000. This can be achieved by attaching a resistor across the filter
pins. Unlike the gain configuration of traditional instrumentation
amplifiers, this resistor attenuates the signal that was previously
amplified by the initial gain of 200.
+
+FIL
CF
AD8428
–FIL
–IN
–
OUT
09731-146
–10
09731-246
INPUT COMMON-MODE VOLTAGE (V)
(2)
RG + 6000
The RG unit is in ohms. The resistor value required to obtain the
desired gain can be calculated using the following formula:
15
0
2000 × RG
Figure 47. Differential Low-Pass Filter
This single-pole filter limits the signal bandwidth, as shown in
the following equation:
fC =
1
2π(6 kΩ)C F
The 6 kΩ factor comes from the internal resistor values. The
tolerance of these resistors is 10%; therefore, using capacitors
with a tolerance better than 5% does not provide a significant
improvement on the absolute tolerance of the cutoff frequency.
Limiting the bandwidth of the amplifier also helps to minimize
the amount of out-of-band noise present at the output.
Note that filtering common-mode signals by adding a capacitor
on each filter terminal to ground degrades the performance of
the amplifier. This practice is generally discouraged because it
degrades CMRR performance. In addition, filtering commonmode signals has little effect on preventing the saturation of the
internal nodes. On the contrary, the load added to the preamplifiers
causes them to saturate with even smaller common-mode signals.
Rev. A | Page 18 of 20
Data Sheet
AD8428
Notch Filter
In cases where the frequency of the interfering signal is well
known, a notch filter can be implemented to help minimize the
impact of the known signal on the measurement. The filter can
be realized by adding a series LC network between the filter
pins, as shown in Figure 48.
72
+
+FIL
–3dB
Figure 48. Notch Filter Example
The inductor and capacitor form a resonant circuit that rejects
frequencies near the notch. The center frequency can be
calculated using the following equation:
fN =
1
6000
54
48
42
36
0.01f N
0.1fN
fN
10fN
100f N
FREQUENCY (Hz)
LF
CF
Figure 49. Notch Filter Attenuation with Q = 0.1 and Q = 1
Around the Center Frequency
The accuracy of the center frequency, fN, depends only on the
tolerance of the capacitor and inductor values, not on the value
of the internal resistors. However, the Q of the circuit depends on
both the tolerance of the external components and the absolute
tolerance of the internal resistors, which is typically 10%.
The Q factor is a filter parameter that indicates how narrow the
notch filter is. It is defined as follows:
Q =
Q = 0.1
–20dB
1
2π L F C F
The Q factor of the filter is given by the following equation:
Q=
60
09731-249
–
MAGNITUDE (dB)
–FIL
CF
Q=1
66
OUT
09731-147
AD8428
fN
fB − fA
The maximum attenuation that can be achieved with a notch
filter is at its center frequency, fN. This maximum attenuation
(or depth of the notch) depends on the equivalent series
resistance of the inductor and capacitor at the center frequency.
Choosing components with high quality factors improves the
rejection at the filter’s center frequency. For information about
calculating the maximum allowed series resistance at the frequency
of interest to obtain the desired attenuation, see the Setting the
Amplifier to Different Gains section.
Extracting the Common-Mode Voltage of the Input
where fA and fB are the frequencies at which there is −3 dB
attenuation on each side of the notch.
This equation indicates that the higher the Q, the narrower the
notch—that is, high values of Q increase the selectivity of the
notch. In other words, although high values of Q reduce the
effect of the notch on the amplitude and phase in neighboring
frequencies, the ability to reject the undesired frequency may
also be reduced due to mismatch between it and the actual
center frequency. This mismatch can be caused by frequency
variations on the affecting source and the tolerance of the filter
inductor and capacitor values.
In contrast, low values of Q work better to ensure that the
interfering frequency is attenuated, but these low values also
affect the signal of interest if it is located close to the center
frequency of the notch.
For example, if the goal is to attenuate the interfering signal by
20 dB, a large Q value reduces the frequency range where the
notch is effective, as shown in Figure 49.
The common-mode signal present at the input terminals can be
extracted by inserting two resistors between the filter terminals
and tapping from the center, as shown in Figure 50. The commonmode voltage, VCM, is the average of the voltages present at the
two inputs minus a 0.6 V drop.
+
+IN
R
R
VCM
–IN
+FIL
AD8428
–FIL
–
OUT
09731-148
LF
In contrast, a small Q value increases the range for the same
attenuation, which relaxes the tolerance requirements between
the inductor and capacitor and the frequency uncertainty of the
undesired signal. However, the lower Q value has a significant
effect on signal bandwidth one decade before the notch
frequency.
Figure 50. Extracting the Common-Mode Voltage
Use resistor values that are high enough to minimize the impact
on gain accuracy. For example, resistor values of 2 MΩ introduce
an additional gain error of less than 0.2%. For information about
the impact of these resistors on the gain of the amplifier, see the
Effect of Passive Network Across the Filter Terminals section.
Rev. A | Page 19 of 20
AD8428
Data Sheet
OUTLINE DIMENSIONS
5.00 (0.1968)
4.80 (0.1890)
8
1
5
4
1.27 (0.0500)
BSC
0.25 (0.0098)
0.10 (0.0040)
COPLANARITY
0.10
SEATING
PLANE
6.20 (0.2441)
5.80 (0.2284)
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
0.31 (0.0122)
0.50 (0.0196)
0.25 (0.0099)
45°
8°
0°
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
COMPLIANT TO JEDEC STANDARDS MS-012-AA
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.
012407-A
4.00 (0.1574)
3.80 (0.1497)
Figure 51. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body
(R-8)
Dimensions shown in millimeters and (inches)
ORDERING GUIDE
Model 1
AD8428ARZ
AD8428ARZ-RL
AD8428BRZ
AD8428BRZ-RL
1
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
8-Lead SOIC_N
8-Lead SOIC_N, 13” Tape and Reel
8-Lead SOIC_N
8-Lead SOIC_N, 13” Tape and Reel
Z = RoHS Compliant Part.
©2011–2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D09731-0-4/12(A)
Rev. A | Page 20 of 20
Package Option
R-8
R-8
R-8
R-8