AD AD8225 Precision gain of 5 instrumentation amplifier Datasheet

Precision Gain of 5
Instrumentation Amplifier
AD8225
APPLICATIONS
Patient Monitors
Current Transmitters
Multiplexed Systems
4 to 20 mA Converters
Bridge Transducers
Sensor Signal Conditioning
FUNCTIONAL BLOCK DIAGRAM
NC 1
AD8225
8
NC
–IN 2
7
+VS
+IN 3
6
VOUT
–VS 4
5
REF
NC = NO CONNECT
140
130
120
110
CMRR – dB
FEATURES
No External Components Required
Highly Stable, Factory Trimmed Gain of 5
Low Power, 1.2 mA Max Supply Current
Wide Power Supply Range (1.7 V to 18 V)
Single- and Dual-Supply Operation
Excellent Dynamic Performance
High CMRR
86 dB Min @ DC
80 dB Min to 10 kHz
Wide Bandwidth
900 kHz
4 V to 36 V Single Supply
High Slew Rate
5 V/s Min
Outstanding DC Precision
Low Gain Drift
5 ppm/C Max
Low Input Offset Voltage
150 V Max
Low Offset Drift
2 V/C Max
Low Input Bias Current
1.2 nA Max
100
AD8225
90
HIGH PERFORMANCE IN AMP
@ GAIN OF 5
80
70
60
50
40
30
1
10
100
1k
FREQUENCY – Hz
10k
100k
Figure 1. Typical CMRR vs. Frequency
GENERAL DESCRIPTION
The AD8225 is an instrumentation amplifier with a fixed gain
of 5, which sets new standards of performance. The superior
CMRR of the AD8225 enables rejection of high frequency
common-mode voltage (80 dB Min @ 10 kHz). As a result,
higher ambient levels of noise from utility lines, industrial
equipment, and other radiating sources are rejected. Extended
CMV range enables the AD8225 to extract low level differential
signals in the presence of high common-mode dc voltage levels
even at low supply voltages.
Ambient electrical noise from utility lines is present at 60 Hz
and harmonic frequencies. Power systems operating at 400 Hz
create high noise environments in aircraft instrument clusters.
Good CMRR performance over frequency is necessary if power
system generated noise is to be rejected. The dc to 10 kHz
CMRR performance of the AD8225 rejects noise from utility
systems, motors, and repair equipment on factory floors, switching power supplies, and medical equipment.
Low input bias currents combined with a high slew rate of 5 V/µs
make the AD8225 ideally suited for multiplexed applications.
The AD8225 provides excellent dc precision, with maximum
input offset voltage of 150 µV and drift of 2 µV/°C. Gain drift is
5 ppm/°C or less.
Operating on either single or dual supplies, the fixed gain of 5
and wide input common-mode voltage range make the AD8225
well suited for patient monitoring applications.
The AD8225 is packaged in an 8-lead SOIC package and is
specified over the standard industrial temperature range, –40°C
to +85°C.
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. 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 companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© 2003 Analog Devices, Inc. All rights reserved.
AD8225* Product Page Quick Links
Last Content Update: 11/01/2016
Comparable Parts
Tools and Simulations
View a parametric search of comparable parts
• In-Amp Error Calculator
• AD8225 SPICE Macro-Model
Evaluation Kits
• AD8225 Evaluation Board
Documentation
Application Notes
• AN-1401: Instrumentation Amplifier Common-Mode
Range: The Diamond Plot
• AN-244: A User's Guide to I.C. Instrumentation Amplifiers
• AN-245: Instrumentation Amplifiers Solve Unusual Design
Problems
• AN-282: Fundamentals of Sampled Data Systems
• AN-589: Ways to Optimize the Performance of a
Difference Amplifier
• AN-669: Effectively Applying the AD628 Precision Gain
Block
• AN-671: Reducing RFI Rectification Errors in In-Amp
Circuits
Data Sheet
• AD8225: Precision Gain of 5 Instrumentation Amplifer
Data Sheet
Technical Books
• A Designer's Guide to Instrumentation Amplifiers, 3rd
Edition, 2006
User Guides
• UG-261: Evaluation Boards for the AD62x, AD822x and
AD842x Series
Reference Materials
Technical Articles
• Auto-Zero Amplifiers
• Current Measurement in Solenoids for Automotive Control
Systems
• High-performance Adder Uses Instrumentation Amplifiers
• Input Filter Prevents Instrumentation-amp RF-Rectification
Errors
• The AD8221 - Setting a New Industry Standard for
Instrumentation Amplifiers
Design Resources
•
•
•
•
AD8225 Material Declaration
PCN-PDN Information
Quality And Reliability
Symbols and Footprints
Discussions
View all AD8225 EngineerZone Discussions
Sample and Buy
Visit the product page to see pricing options
Technical Support
Submit a technical question or find your regional support
number
* This page was dynamically generated by Analog Devices, Inc. and inserted into this data sheet. Note: Dynamic changes to
the content on this page does not constitute a change to the revision number of the product data sheet. This content may be
frequently modified.
AD8225–SPECIFICATIONS (T = 25C, V = 15 V, R = 2 k, unless otherwise noted.)
A
Parameter
S
Conditions
Min
GAIN
Gain
Gain Error
Nonlinearity
vs. Temperature
–0.1
OFFSET VOLTAGE (RTI)
Offset Voltage
vs. Temperature
vs. Supply (PSRR)
90
INPUT
Input Operating Impedance
Differential
Common Mode
Input Voltage Range
(Common-Mode)
vs. Temperature
Input Bias Current
vs. Temperature
Input Offset Current
vs. Temperature
Common-Mode Rejection Ratio
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
NOISE (RTI)
Voltage
Current
REFERENCE INPUT
RIN
IIN
Voltage Range
Gain to Output
Typ
Max
Unit
5
+0.05
2
1
+0.1
10
5
V/V
%
± ppm
± ppm/°C
150
2
±µV
±µV/°C
dB
50
0.3
100
10储2
10储2
–VS + 1.6
+VS – 1.0
–VS + 2.2
86
83
80
TA = TMIN to TMAX
f = 10 kHz*
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
Short Circuit Current
L
RL = 2 kΩ
0.5
3
0.15
1.5
94
–VS + 1.4
–VS + 1.5
–VS + 1.0
–VS + 1.2
RL = 10 kΩ
+VS – 1.2
1.2
0.5
+VS – 1.4
+VS – 1.6
+VS – 1.1
+VS – 1.0
18
GΩ储pF
GΩ储pF
V
V
nA
pA/°C
nA
pA/°C
dB
dB
dB
V
V
V
V
mA
900
75
3.4
4.8
kHz
kHz
µs
µs
V/µs
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
1.5
45
4
50
µV p-p
nV/√Hz
pA p-p
fA/√Hz
VIN+, VREF = 0
18
60
kΩ
µA
V
VOUT = 20 V p-p
10 V Step
10 V Step
5
–VS + 1.4
0.999
POWER SUPPLY
Operating Range
Quiescent Current
1
+VS – 1.4
1.001
1.05
18
1.2
±V
mA
+85
°C
1.7
TEMPERATURE RANGE
For Specified Performance
–40
*Pin 1 connected to Pin 4. See Applications section.
Specifications subject to change without notice.
–2–
REV. A
AD8225
SPECIFICATIONS (T = 25C, V = 5 V, R = 2 k, unless otherwise noted.)
A
Parameter
S
L
Conditions
Min
GAIN
Gain
Gain Error
Nonlinearity
vs. Temperature
–0.1
VOLTAGE OFFSET (RTI)
Offset Voltage
vs. Temperature
vs. Supply
90
NOISE (RTI)
Voltage
Current
REFERENCE INPUT
RIN
IIN
Voltage Range
Gain to Output
5
+0.05
2
1
+0.1
10
5
V/V
%
± ppm
± ppm/°C
325
2
100
–VS + 1.6
–VS + 2.1
86
83
80
RL = 2 kΩ
0.5
3
0.15
1.5
94
–VS + 0.9
–VS + 1.0
–VS + 0.8
–VS + 0.9
RL = 10 kΩ
+VS – 1.0
+VS – 1.5
1.2
0.5
+VS – 1.0
+VS – 1.2
+VS – 1.0
+VS – 1.0
18
±µV
±µV/°C
dB
GΩ储pF
GΩ储pF
V
V
nA
pA/°C
nA
pA/°C
dB
dB
dB
V
V
V
V
mA
900
170
3
4.3
kHz
kHz
µs
µs
V/µs
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
1.5
45
4
50
µV p-p
nV/√Hz
pA p-p
fA/√Hz
VINT, VREF = 0
18
60
kΩ
µA
V
VOUT = 7.8 V p-p
7 V Step
7 V Step
5
–VS + 0.9
0.999
POWER SUPPLY
Operating Range
Quiescent Current
1
+VS – 1.0
1.001
1.05
18
1.2
±V
mA
+85
°C
1.7
TEMPERATURE RANGE
For Specified Performance
–40
*Pin 1 connected to Pin 4. See Applications section.
Specifications subject to change without notice.
REV. A
Unit
10储2
10储2
TA = TMIN to TMAX
f = 10 kHz*
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
Max
125
INPUT
Input Operating Impedance
Differential
Common-Mode
Input Operating Voltage Range
vs. Temperature
Input Bias Current
vs. Temperature
Input Offset Current
vs. Temperature
Common-Mode Rejection Ratio
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
Short Circuit Current
Typ
–3–
AD8225
SPECIFICATIONS (T = 25C, V = 5 V, R = 2 k, unless otherwise noted.)
A
Parameter
S
L
Conditions
Min
GAIN
Gain
Gain Error
Nonlinearity
vs. Temperature
–0.1
OFFSET VOLTAGE (RTI)
Offset Voltage
vs. Temperature
vs. Supply
90
NOISE (RTI)
Voltage
Current
Unit
5
+0.05
2
1
+0.1
10
5
V/V
%
± ppm
± ppm/°C
375
2
100
10储2
10储2
VS – 1.05
1.6
1.7
86
83
80
TA = TMIN to TMAX
f = 10 kHz*
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
Max
150
INPUT
Input Operating Impedance
Differential
Common Mode
Input Voltage Range
(Common-Mode)
vs. Temperature
Input Bias Current
vs. Temperature
Input Offset Current
vs. Temperature
Common-Mode Rejection Ratio
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
Short Circuit Current
Typ
RL = 2 kΩ
0.5
3
0.15
1.5
94
0.8
0.9
0.8
0.9
RL = 10 kΩ
VS – 1.0
1.2
0.5
VS – 1.05
VS – 1.2
VS – 1.0
VS – 1.0
18
VOUT = 3.2 V p-p
2 V Step
2 V Step
REFERENCE INPUT
RIN
IIN
Voltage Range
Gain to Output
0.4
0.999
POWER SUPPLY
Operating Range
Quiescent Current
–40
V
nA
pA/°C
nA
pA/°C
dB
dB
dB
V
V
V
V
mA
kHz
kHz
µs
µs
V/µs
1.5
45
4
50
µV p-p
nV/√Hz
pA p-p
fA/√Hz
18
60
kΩ
µA
V
1
VS – 0.9
1.001
1.05
36
1.2
V
mA
+85
°C
3.4
TEMPERATURE RANGE
For Specified Performance
GΩ储pF
GΩ储pF
V
900
420
3.3
5.1
5
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
±µV
±µV/°C
dB
*Pin 1 connected to Pin 4. See Applications section.
Specifications subject to change without notice.
–4–
REV. A
AD8225
ABSOLUTE MAXIMUM RATINGS*
PIN FUNCTION DESCRIPTIONS
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . 650 mW
Input Voltage (Common-Mode) . . . . . . . . . . . . . . . . . . . . ± VS
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ± 25 V
Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Storage Temperature . . . . . . . . . . . . . . . . . . –65ºC to +125ºC
Operating Temperature Range . . . . . . . . . . . –40ºC to +85ºC
Lead Temperature Range (10 sec Soldering) . . . . . . . . . 300ºC
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and 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.
Pin Number
Mnemonic
Function
1
NC
May be Connected to Pin 4 to
Balance CIN
2
–IN
Inverting Input
3
+IN
Noninverting Input
4
–VS
Negative Supply Voltage
5
REF
Connect to Desired Output
CMV
6
VOUT
Output
7
+VS
Positive Supply Voltage
8
NC
1.5
POWER DISSIPATION – W
TJ = 150 C
8-LEAD SOIC PACKAGE
1.0
0.5
0
–50 –40 –30 –20 –10 0 10 20 30 40 50 60
AMBIENT TEMPERATURE – C
70
80
90
Figure 2. Maximum Power Dissipation vs. Temperature
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Options
AD8225AR
AD8225AR-REEL
AD8225AR-REEL7
AD8225-EVAL
–40ºC to +85ºC
–40ºC to +85ºC
–40ºC to +85ºC
8-Lead SOIC
8-Lead SOIC
8-Lead SOIC
Evaluation Board
RN-8
13" REEL
7" REEL
RN-8
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although the
AD8225 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended
to avoid performance degradation or loss of functionality.
REV. A
–5–
AD8225–Typical Performance Characteristics (T = 25C, R = 2 k, V
A
50
L
S
= 15 V, unless otherwise noted.)
250
LOT SIZE = 3775
45
200
40
BIAS CURRENT – pA
% OF UNITS
35
30
25
20
15
150
+BIAS CURRENT
100
–BIAS CURRENT
50
10
0
5
0
–140 –120 –100 –80 –60 –40 –20 0 20 40 60
INPUT OFFSET VOLTAGE – V
–50
–60
80 100 120
–40
–20
0
20
40
TEMPERATURE – C
60
80
100
TPC 4. Bias Current vs. Temperature
TPC 1. Typical Distribution of Input Offset Voltage,
VS = ± 15 V
8
50
LOT SIZE = 7550
CHANGE IN OFFSET VOLTAGE – V
45
40
% OF UNITS
35
30
25
20
15
10
6
4
2
5
0
–200 –100
0
0
100 200 300 400 500
INPUT BIAS CURRENT – pA
600
700
800
0
1
2
3
WARM-UP TIME – Min
4
5
TPC 5. Offset Voltage vs. Warm-Up Time
TPC 2. Typical Distribution of Input Bias Current,
VS = ± 15 V
1000
50
LOT SIZE = 3775
VOLTAGE NOISE DENSITY – nV/ Hz
45
40
% OF UNITS
35
30
100
25
20
15
10
5
0
–500
0
–400
–300 –200 –100
0
100
200
INPUT OFFSET CURRENT – pA
300
400
1
10
100
1k
FREQUENCY – Hz
10k
100k
TPC 6. Voltage Noise Spectral Density vs. Frequency (RTI)
TPC 3. Typical Distribution of Input Offset Current,
VS = ± 15 V
–6–
REV. A
AD8225
1000
130
VOLTAGE NOISE DENSITY – nV/ Hz
120
110
CMR – dB
100
100
90
80
70
60
50
40
0
1
10
100
FREQUENCY – Hz
30
10k
1k
1
TPC 7. Input Current Noise Spectral Density vs.
Frequency
10
100
1k
FREQUENCY – Hz
100k
10k
TPC 10. CMR vs. Frequency, RTI
0.10
4
0.08
1S
3
0.06
100
90
CMRR DRIFT – ppm/ C
NOISE – RTI – V
2
1
0
–1
–2
10
0
0.04
0.02
0
–0.02
–0.04
–0.06
–3
–0.08
–4
0
5
TIME – sec
–0.10
–40
10
–20
0
80
100
10
15
20
40
60
TEMPERATURE – C
TPC 11. CMRR vs. Temperature
TPC 8. 0.1 Hz to 10 Hz Voltage Noise, RTI
15
8
1S
10
COMMON-MODE VOLTAGE – V
CURRENT NOISE – 2pA/div
6
100
4
90
2
0
–2
–4
10
0
5
VS = 5V
0
–5
–10
–6
–8
0
5
TIME – sec
–15
–15
10
–10
0
–5
5
OUTPUT VOLTAGE – V
TPC 12. CMV Range vs. VOUT, Dual Supplies
TPC 9. 0.1 Hz to 10 Hz Current Noise
REV. A
VS = 15V
–7–
AD8225
40
5
30
20
COMMON-MODE VOLTAGE – V
4
VS = 5V
10
0
GAIN – dB
3
–10
–20
2
–30
–40
1
–50
0
0
1
2
3
OUTPUT VOLTAGE – V
4
–60
100
5
TPC 13. CMV vs. VOUT, Single Supply
10M
+VS –0.0
INPUT VOLTAGE 5 – V
(REFERRED TO SUPPLY VOLTAGES)
120
100
PSRR – dB
1M
TPC 16. Large Signal Frequency Response,
VOUT = 4 V p-p
140
80
+VS
60
–VS
40
20
0
0.1
10k
100k
FREQUENCY – Hz
1k
–0.5
–1.0
–1.5
–2.0
0
5
0
5
15
20
10
15
SUPPLY VOLTAGE – V
20
10
2.0
1.5
1.0
0.5
–VS +0.0
1
10
100
1k
FREQUENCY – Hz
10k
1M
100k
TPC 14. PSRR vs. Frequency, RTI
TPC 17. Input Common Mode Voltage Range vs.
Supply Voltage
40
+VS 0
OUTPUT VOLTAGE SWING – V
(REFERENCED TO SUPPLY VOLTAGES)
30
20
GAIN – dB
10
0
–10
–20
–30
–40
–50
–60
100
1k
10k
100k
FREQUENCY – Hz
1M
–0.5
RL = 2k
–1.5
–2.0
0
2
4
6
8
10
12
14
16
18
20
8
10
12
14
SUPPLY VOLTAGE – V
16
18
20
2.0
1.5
RL = 2k
1.0
RL = 10k
0.5
–VS 0
10M
RL = 10k
–1.0
0
2
4
6
TPC 18. Output Voltage Swing vs. Supply Voltage
and Load Resistance
TPC 15. Small Signal Frequency Response,
VOUT = 200 mV p-p
–8–
REV. A
AD8225
30
10
8
7
SETTLING TIME – s
OUTPUT VOLTAGE SWING – V p-p
9
25
20
15
10
6
5
0.001%
4
3
0.01%
2
5
1
0
10
1k
100
LOAD RESISTANCE – 0
100k
0
TPC 19. Output Voltage Swing vs. Load Resistance
5
10
STEP SIZE – V
15
20
TPC 22. Settling Time vs. Step Size
4
CH 1 = 5V/DIV
100
100mV
HORIZ
(4s/DIV)
100
OUTPUT (5V/DIV)
NONLINEARITY – ppm
90
CH 2 = 10mV/DIV
2V
3
TEST
CIRCUIT
OUTPUT
(0.001%/DIV)
10
2
90
1
0
–1
–2
10
0
0
–3
–4
–10
0
10
OUTPUT VOLTAGE – V
TPC 20. Large Signal Pulse Response and Settling
Time to 0.001%
TPC 23. Gain Nonlinearity
1.5
1.4
INPUT
1.3
SUPPLY CURRENT – mA
100
90
1
OUTPUT
10
1.2
1.1
+85 C
1.0
+25 C
0.9
0.8
–40 C
0.7
0
2
0.6
CH 1 = 10mV, CH 2 = 20mV, H = 2s
0.5
TPC 21. Small Signal Pulse Response, CL = 100 pF
REV. A
0
2
4
6
8
10
12
14
SUPPLY VOLTAGE – V
16
18
TPC 24. ISUPPLY vs. VSUPPLY and Temperature
–9–
20
AD8225
Test Circuits
4k⍀
G = 100
AD797
G=5
AD8225
100⍀
G = 100
LPF
G = 101
SCOPE
G=5
AD829
20k⍀
AD8225
2k⍀
20⍀
2k⍀
Test Circuit 2. Settling Time to 0.01%
Test Circuit 1. 1 Hz to 10 Hz Voltage Noise Test
–10–
REV. A
AD8225
+VS
APPLICATIONS
Precision V-to-I Converter
+VS
+VS
VB
–IN
A1
–VS
+IN
Q1
Q2
A2
C2
R2
–VS
R1
C1
+VS
3k
VREF
15k
+VS
A3
3k
–VS
15k
OUT
When small analog voltages are transmitted across significant
distances, errors may develop due to ambient electrical noise,
stray capacitance, or series impedance effects. If the desired
voltage is converted to a current, however, the effects of ambient
noise are mitigated. All that is required is a voltage to current
conversion at the source, and an I-to-V conversion at the other
end to reverse the process.
Figure 5 illustrates how the AD8225 may be used as the transmitter and receiver in a current loop system. The full-scale
output is 5 mA.
3
–VS
Figure 3. Simplified Schematic
eIN
200mV
pk FS
IOUT
AD8225
2
Referring to Figure 3, the input buffers consist of super-beta
NPN transistors Q1 and Q2, and op amps A1 and A2. The
transistors are compensated so that the bias currents are
extremely low, typically 100 pA or less. As a result, current noise
is also low, at 50 fA/√Hz. The unity gain input buffers drive a
gain-of-five difference amplifier. Because the 3 kΩ and 15 kΩ
resistors are ratio matched, gain stability is better than 5 ppm/°C
over the rated temperature range.
The AD8225 also has five times the gain bandwidth of a typical
in amp. This wider GBW results from compensation at a fixed
gain of 5, which can be one fifth of that required if the amplifier
were compensated for unity gain.
2
6
5
V
0.5 eIN
IOUT = SH =
RSH
RSH
Figure 5. Precision Voltage-to-Current Converter
As noted in Figure 5, an additional op amp and four resistors are
required to complete the converter. The precision gain of 5 in the
AD8225s, used in the transmit and receive sections, preserves
the integrity of the desired signal, while the high frequency
common-mode performance at the receiver rejects noise on the
transmission line. The reference of the receiver may be connected
to local ground or the reference pin of an A/D converter (ADC).
Figure 6 shows bench measurements of the input and output
voltages, and output current of the circuit of Figure 5. The
transmission media is 10 feet of insulated hook-up wire for the
current drive and return lines.
eIN = 398mV p-p, eOUT = 398mV p-p,
IOUT = 10.3mA p-p
eIN
eOUT
2
AD8225
8
NC
7
+VS
6
VOUT
5
REF
eOUT
200mV
pk FS
GND OR
REF V
OP27
1
IOUT
3
AC
GROUND
CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA,
H = 200s
Figure 6. V-to-I Converter Waveforms (CH1: VIN,
CH2: VOUT, CH3: IOUT)
PIN 1 HAS NO INTERNAL CONNECTION
Figure 4. Pinout for Symmetrical Input Stray Capacitance
REV. A
AD8225
8
9k
High frequency performance is also enhanced by the innovative
pinout of the AD8225. Since Pins 1 and 8 are uncommitted,
Pin 1 may be connected to Pin 4. Since Pin 4 is also ac common, the stray capacitance at Pins 2 and 3 is balanced.
+IN
3
VSH
47pF
The AD8225 is a monolithic, three op amp instrumentation
amplifier. Laser wafer trimming and proprietary circuit techniques enable the AD8225 to boast the lowest output offset
voltage and drift of any currently available in amp (150 µV
RTI), as well as a higher common-mode voltage range.
–IN
FULL SCALE
CURRENT = 5mA
5
THEORY OF OPERATION
AC
GROUND
6
1k
RSH
20
–11–
AD8225
100 pF of capacitance at its output, a 75 Ω series resistor is
required at each in amp output to prevent oscillation.
Driving a High Resolution ADC
Most high precision ADCs feature differential analog inputs.
Differential inputs offer an inherent 6 dB improvement in S/N
ratio and resultant bit resolution. These advantages are easy to
realize using a pair of AD8225s.
Using the Reference Input
Note in the example in Figure 7 that Pin 5, the reference input, is
driven by a voltage source. This is because the reference pin is
internally connected to a 15 kΩ resistor, which is carefully trimmed
to optimize common-mode rejection. Any additional resistance
connected to this node will unbalance the bridge network formed
by the two 3 kΩ and two 15 kΩ resistors, resulting in an error
voltage generated by common-mode voltages at the input pins.
AD8225s can be configured to drive an ADC with differential
inputs by using either single-ended or differential inputs to the
AD8225s. Figure 7 shows the circuit connections for a differential input. A single-ended input may be configured by connecting
the negative input terminal to ground.
AD8225 Used as an EKG Front End
5V
The topology of the instrumentation amplifier has made it the
circuit configuration of choice for designers of EKG and other
low level biomedical amplifiers. CMRR and common-mode
voltage advantages of the instrumentation amplifier are tailor
made to meet the challenges of detecting minuscule cardiac
generated voltage levels in the presence of overwhelming levels
of noise and dc offset voltage. The subtracter circuit of the in
amp will extract and amplify low level signals that are virtually
obscured by the presence of high common-mode dc and ac
potentials.
3
6
AD8225
75
+IN
2
5
AD7675
100kSPS
3
AD8225
6
75
2
ALTERNATE
CONNECTION
FOR SE SOURCE
16 BITS
2.7nF
–IN
2.7nF
5
4.99k
1.25V
OP177
4.99k
A typical circuit block diagram of an EKG amplifier is shown in
Figure 8. Using discrete op amps in the in amp and gain stages,
the signal chain usually includes several filters, high voltage
protection, lead-select circuitry, patient lead buffering, and an
ADC. Designers who roll their own instrumentation amplifiers
must provide precision custom trimmed resistor networks and
well matched op amps.
2.5V
AD780
RERERENCE
Figure 7. Driver for Differential ADC
The AD8225 instrumentation amplifier not only replaces all the
components shown in the highlighted block in Figure 8, but also
provides a solution to many of the difficult design problems
encountered in EKG front ends. Among these are patient generated errors from ac noise sources and errors generated by unequal
electrode potentials. Alone, these error voltages can exceed the
desired QRS complex by orders of magnitude.
The AD7675 ADC illustrated in Figure 7 is a SAR type converter.
When the input is sampled, the internal sample-and-hold capacitor
is charged to the input voltage level. Since the output of the
AD8225 cannot track the instantaneous current surge, a voltage
glitch develops. To source the momentary current surge, a
capacitor is connected from the A/D input terminal to ground.
Since the AD8225 cannot tolerate greater than approximately
PATIENT
ISOLATION
BARRIER
INSTRUMENTATION AMPLIFIER
G = 3 TO 10
A1
A3
GAIN AND ADC
TOTAL G = 1000
DIGITAL DATA
TO SYSTEM
MAINFRAME
LEAD
SELECT,
HV
PROTECTION,
FILTERING
A2
Figure 8. Block Diagram, EKG Monitor Front End Using Discrete Components
–12–
REV. A
AD8225
In the classical three op amp in amp topology shown in Figure 8,
gain is developed differentially between the two input amplifiers
A1 and A2, sacrificing CMV (common-mode voltage) range.
The gain of the in amp is typically 10 or less, and an additional
gain stage increases the overall gain to approximately 1000.
Gain developed in the input stage results in a trade-off in commonmode voltage range, constraining the ability of the amplifier to
tolerate high dc electrode errors. Although the AD8225 is also
a three amplifier design, its gain of 5 is developed at the output
amplifier, improving the CMV range at the input. Using ± 5 V
supplies, the CMV range of the AD8225 is from –3.4 V to
+4 V, compared to –3.1 V to +3.8 V, a 7% improvement in
input headroom over conventional in amps with the same gain.
G = 5 OP77
G = 200
19.6k
LA-LL 2
RA-LL 3
CH 1 = 2V, CH 2 = 2V, CH 3 = 2V, H = 200ms
Figure 10. EKG Waveform Using Circuit of Figure 9
Benefits of Fast Slew Rates
At 5 V/µs, the slew rate of the AD8225 is as fast as many op amp
circuits. This is an advantage in systems applications using multiple
sensors. For example, an analog multiplexer (see Figure 11) may
be used to select pairs of leads connected to several sensors. If
the AD8225 drives an ADC, the acquisition time is constrained
by the ability of the in amp to settle to a stable level after a new
set of leads is selected. Fast slew rates contribute greatly to
this function, especially if the difference in input levels is large.
AD8225
100
RA-LA 1
301
S1A
S1B
0.2V, 2V
AD8225
S2A
G = 5 OP77
G = 200
S3A
100
19.6k
DA
S2B
301
1
ADG409
AD8225
DB
S3B
REF
S4A
4
S4B
AD8225
G = 5 OP77
G = 200
Figure 11. Connection to an ADG409 Analog MUX
100
19.6k
301
Figure 9. EKG Monitor Front End
Figure 9 illustrates how an AD8225 may be used in an EKG
front end. In a low cost system, the AD8225 can be connected to
the patient. If buffers are required, the AD8225 can replace the
expensive precision resistor network and op amp.
Figure 12 illustrates the response of an AD8225 connected to
an ADG409 analog multiplexer in the circuit shown in Figure 11
at two signal levels. Two of the four MUX inputs are connected
to test dc levels. The remaining two are at ground potential so
that the output slews as the inputs A0 and A1 are addressed. As
can be seen, the output response settles well within 4 µs of the
applied level.
Figure 10 shows test waveforms observed from the circuit of
Figure 9.
INPUT
SIGNAL
TRANSITION
SMALL SIGNAL
(200mV/DIV)
LARGE SIGNAL
(2V/DIV)
CH 1 = 200mV, CH 2 = 2V, H = 500ns
Figure 12. Slew Responses After MUX Selection
REV. A
–13–
AD8225
Evaluation Board
Figure 13 is a schematic of an evaluation board available for the
AD8225. The board is shipped with an AD8225 already installed
and tested. The user need only connect power and an input to
conduct measurements. The supply may be configured for dual
or single supplies, and the input may be dc- or ac-coupled. A
circuit is provided on the board so that the user can zero the
output offset. If desired, a reference may be applied from an
external voltage source.
+VS
C1
0.1F
+IN
GND
R2
100
R4
100
W3
3
W4
C3
0.1F
–IN
C4
0.1F
R3
100k*
2
R5
100k*
7
6
A1
4
1
5
OUTPUT
R8
W12
C2
0.1F
W13
EXT_REF
W11
W14
–VS
USER-SUPPLIED
+VAUX
C12
10F, 25V
C10
0.1F
W7
+VAUX
+VS
GND
W6
–VS
–VAUX
C11
10F, 25V
6
3
7
A1
OFFSET
ADJ
CS1
J500
240A
R1
10k
2
4
C9
0.1F
–VAUX
NOTES
REMOVE W3 AND W4 FOR AC COUPLING
*INSTALL FOR AC COUPLING
AD707JN
C7
R9
0.1F
5.9k, 1%
R10
5.9k, 1%
C8
0.1F
CS2
J500
240A
–VAUX
Figure 13. Evaluation Board Schematic
–14–
REV. A
AD8225
OUTLINE DIMENSIONS
8-Lead Standard Small Outline Package (SOIC)
(RN-8)
Dimensions shown in millimeters and (inches)
5.00 (0.1968)
4.80 (0.1890)
4.00 (0.1574)
3.80 (0.1497)
8
5
1
4
1.27 (0.0500)
BSC
0.25 (0.0098)
0.10 (0.0040)
COPLANARITY
SEATING
0.10
PLANE
6.20 (0.2440)
5.80 (0.2284)
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
0.33 (0.0130)
0.50 (0.0196)
45
0.25 (0.0099)
8
0.25 (0.0098) 0 1.27 (0.0500)
0.41 (0.0160)
0.19 (0.0075)
COMPLIANT TO JEDEC STANDARDS MS-012AA
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
REV. A
–15–
AD8225
Revision History
Location
Page
2/03—Data Sheet changed from REV. 0 to REV. A.
Change to TPC 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Change to TPC 20 caption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Edit to Precision V-to-I Converter section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
PRINTED IN U.S.A.
OUTLINE DIMENSIONS updated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
C02771–0–2/03(A)
Updated ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
–16–
REV. A