AD8225AR [ADI]
Precision Gain of 5 Instrumentation Amplifier; 5仪表放大器的增益精度
| 型号： | AD8225AR |
| 厂家： | 
ADI |
| 描述： | Precision Gain of 5 Instrumentation Amplifier |
| 文件： | 总16页 (文件大小：276K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Precision Gain of 5
Instrumentation Amplifier
AD8225
FEATURES
FUNCTIONAL BLOCK DIAGRAM
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
AD8225
1
2
3
4
8
7
6
5
NC
+V
NC
–IN
+IN
S
V
OUT
–V
S
REF
NC = NO CONNECT
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/ꢂC Max
Low Input Offset Voltage
150 ꢁV Max
Low Offset Drift
2 ꢁV/ꢂC Max
140
130
120
110
100
90
AD8225
HIGH PERFORMANCE IN AMP
@ GAIN OF 5
80
70
Low Input Bias Current
1.2 nA Max
60
50
40
APPLICATIONS
Patient Monitors
30
1
10
100
1k
10k
100k
Current Transmitters
Multiplexed Systems
4 to 20 mA Converters
Bridge Transducers
Sensor Signal Conditioning
FREQUENCY – Hz
Figure 1. Typical CMRR vs. Frequency
GENERAL DESCRIPTION
CMRR performance of the AD8225 rejects noise from utility
systems, motors, and repair equipment on factory floors, switch-
ing power supplies, and medical equipment.
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.
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.
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
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, norforanyinfringementsofpatentsorotherrightsofthirdpartiesthat
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
Fax: 781/326-8703
www.analog.com
© 2003 Analog Devices, Inc. All rights reserved.
(TA = 25ꢂC, VS = ꢀ15 V, RL = 2 kꢃ, unless otherwise noted.)
AD8225–SPECIFICATIONS
Parameter
Conditions
Min
Typ
Max
Unit
GAIN
Gain
5
V/V
%
ppm
Gain Error
Nonlinearity
vs. Temperature
–0.1
+0.05
2
1
+0.1
10
5
ppm/°C
OFFSET VOLTAGE (RTI)
Offset Voltage
vs. Temperature
50
0.3
100
150
2
µV
µV/°C
vs. Supply (PSRR)
90
dB
INPUT
Input Operating Impedance
Differential
Common Mode
Input Voltage Range
(Common-Mode)
vs. Temperature
10
10
ʈ
ʈ
2
2
GΩ
GΩ
V
ʈ
ʈ
pF
pF
–VS + 1.6
–VS + 2.2
+VS – 1.0
+VS – 1.2
V
Input Bias Current
vs. Temperature
Input Offset Current
vs. Temperature
Common-Mode Rejection Ratio
0.5
3
0.15
1.5
94
1.2
nA
pA/°C
nA
pA/°C
dB
dB
0.5
86
83
80
TA = TMIN to TMAX
f = 10 kHz*
dB
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
RL = 2 kΩ
–VS + 1.4
–VS + 1.5
–VS + 1.0
–VS + 1.2
+VS – 1.4
+VS – 1.6
+VS – 1.1
+VS – 1.0
V
V
V
V
RL = 10 kΩ
Short Circuit Current
18
mA
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
900
75
3.4
kHz
kHz
µs
V
OUT = 20 V p-p
10 V Step
10 V Step
4.8
µs
5
V/µs
NOISE (RTI)
Voltage
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
1.5
45
4
µV p-p
nV/√Hz
pA p-p
fA/√Hz
Current
Spectral Density, 1 kHz
50
REFERENCE INPUT
RIN
IIN
VIN+, VREF = 0
18
60
kΩ
µA
V
Voltage Range
Gain to Output
–VS + 1.4
0.999
+VS – 1.4
1.001
1
POWER SUPPLY
Operating Range
Quiescent Current
1.7
18
1.2
V
mA
1.05
TEMPERATURE RANGE
For Specified Performance
–40
+85
°C
*Pin 1 connected to Pin 4. See Applications section.
Specifications subject to change without notice.
–2–
REV. A
AD8225
(TA = 25ꢂC, VS = ꢀ5 V, RL = 2 kꢃ, unless otherwise noted.)
SPECIFICATIONS
Parameter
Conditions
Min
Typ
Max
Unit
GAIN
Gain
5
V/V
%
ppm
Gain Error
Nonlinearity
vs. Temperature
–0.1
+0.05
2
1
+0.1
10
5
ppm/°C
VOLTAGE OFFSET (RTI)
Offset Voltage
vs. Temperature
125
100
325
2
µV
µV/°C
vs. Supply
90
dB
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
10
10
ʈ
ʈ
2
2
GΩ
GΩ
V
V
nA
pA/°C
nA
pA/°C
dB
dB
ʈ
ʈ
pF
pF
–VS + 1.6
–VS + 2.1
+VS – 1.0
+VS – 1.5
1.2
0.5
3
0.15
1.5
94
0.5
86
83
80
TA = TMIN to TMAX
f = 10 kHz*
dB
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
RL = 2 kΩ
–VS + 0.9
–VS + 1.0
–VS + 0.8
–VS + 0.9
+VS – 1.0
+VS – 1.2
+VS – 1.0
+VS – 1.0
V
V
V
V
RL = 10 kΩ
Short Circuit Current
18
mA
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
900
170
3
kHz
kHz
µs
V
OUT = 7.8 V p-p
7 V Step
7 V Step
4.3
µs
5
V/µs
NOISE (RTI)
Voltage
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
1.5
45
4
µV p-p
nV/√Hz
pA p-p
fA/√Hz
Current
Spectral Density, 1 kHz
50
REFERENCE INPUT
RIN
IIN
18
60
kΩ
µA
V
VINT, VREF = 0
Voltage Range
–VS + 0.9
+VS – 1.0
Gain to Output
0.999
1
1.001
POWER SUPPLY
Operating Range
Quiescent Current
1.7
18
1.2
V
mA
1.05
TEMPERATURE RANGE
For Specified Performance
–40
+85
°C
*Pin 1 connected to Pin 4. See Applications section.
Specifications subject to change without notice.
REV. A
–3–
AD8225
(T = 25ꢂC, V = 5 V, R = 2 kꢃ, unless otherwise noted.)
SPECIFICATIONS
A
S
L
Parameter
Conditions
Min
Typ
Max
Unit
GAIN
Gain
5
V/V
%
ppm
Gain Error
Nonlinearity
vs. Temperature
–0.1
+0.05
2
1
+0.1
10
5
ppm/°C
OFFSET VOLTAGE (RTI)
Offset Voltage
vs. Temperature
150
100
375
2
µV
µV/°C
vs. Supply
90
dB
INPUT
Input Operating Impedance
Differential
Common Mode
Input Voltage Range
(Common-Mode)
vs. Temperature
10
10
ʈ
ʈ
2
2
GΩ
GΩ
V
ʈ
ʈ
pF
pF
1.6
1.7
VS – 1.05
VS – 1.0
V
Input Bias Current
vs. Temperature
Input Offset Current
vs. Temperature
Common-Mode Rejection Ratio
0.5
3
0.15
1.5
94
1.2
nA
pA/°C
nA
pA/°C
dB
dB
0.5
86
83
80
TA = TMIN to TMAX
f = 10 kHz*
dB
OUTPUT
Operating Voltage Range
vs. Temperature
Operating Voltage Range
vs. Temperature
RL = 2 kΩ
0.8
0.9
0.8
0.9
VS – 1.05
VS – 1.2
VS – 1.0
VS – 1.0
V
V
V
V
RL = 10 kΩ
Short Circuit Current
18
mA
DYNAMIC RESPONSE
Small Signal –3 dB Bandwidth
Full Power Bandwidth
Settling Time (0.01%)
Settling Time (0.001%)
Slew Rate
900
420
3.3
5.1
kHz
kHz
µs
VOUT = 3.2 V p-p
2 V Step
2 V Step
µs
5
V/µs
NOISE (RTI)
Voltage
0.1 Hz to 10 Hz
Spectral Density, 1 kHz
0.1 Hz to 10 Hz
1.5
45
4
µV p-p
nV/√Hz
pA p-p
fA/√Hz
Current
Spectral Density, 1 kHz
50
REFERENCE INPUT
RIN
IIN
18
60
kΩ
µA
V
Voltage Range
Gain to Output
0.4
0.999
VS – 0.9
1.001
1
POWER SUPPLY
Operating Range
Quiescent Current
3.4
36
1.2
V
mA
1.05
TEMPERATURE RANGE
For Specified Performance
–40
+85
°C
*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 perma-
nent 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
3
4
5
–IN
+IN
–VS
Inverting Input
Noninverting Input
Negative Supply Voltage
REF
Connect to Desired Output
CMV
6
7
8
VOUT
+VS
NC
Output
Positive Supply Voltage
1.5
1.0
0.5
0
T
= 150 C
J
8-LEAD SOIC PACKAGE
80 90
–50 –40 –30
–10
0
10 20 30 40 50 60 70
–20
AMBIENT TEMPERATURE –
C
Figure 2. Maximum Power Dissipation vs. Temperature
ORDERING GUIDE
Package Description
Model
Temperature Range
Package Options
AD8225AR
–40ºC to +85ºC
–40ºC to +85ºC
–40ºC to +85ºC
8-Lead SOIC
8-Lead SOIC
8-Lead SOIC
Evaluation Board
RN-8
AD8225AR-REEL
AD8225AR-REEL7
AD8225-EVAL
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–
(T = 25ꢂC, R = 2 kꢃ, V = ꢀ15 V, unless otherwise noted.)
AD8225–Typical Performance Characteristics
A
L
S
50
250
LOT SIZE = 3775
45
200
150
100
50
40
35
30
25
20
15
10
5
+BIAS CURRENT
–BIAS CURRENT
0
0
–50
–60
–40
–20
0
20
40
60
80
100
–140–120–100 –80 –60 –40 –20
0
20 40 60 80 100 120
INPUT OFFSETVOLTAGE – ꢁV
TEMPERATURE –
C
TPC 4. Bias Current vs. Temperature
TPC 1. Typical Distribution of Input Offset Voltage,
VS = 15 V
8
6
4
2
0
50
LOT SIZE = 7550
45
40
35
30
25
20
15
10
5
0
0
1
2
3
4
5
–200 –100
0
100 200 300 400 500 600 700 800
INPUT BIAS CURRENT – pA
WARM-UPTIME – Min
TPC 5. Offset Voltage vs. Warm-Up Time
TPC 2. Typical Distribution of Input Bias Current,
VS = 15 V
1000
100
0
50
LOT SIZE = 3775
45
40
35
30
25
20
15
10
5
0
1
10
100
1k
10k
100k
–500 –400 –300 –200 –100
0
100
200
300
400
FREQUENCY – Hz
INPUT OFFSET CURRENT – pA
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
100
0
130
120
110
100
90
80
70
60
50
40
30
1
10
100
1k
10k
1
10
100
1k
10k
100k
FREQUENCY – Hz
FREQUENCY – Hz
TPC 7. Input Current Noise Spectral Density vs.
Frequency
TPC 10. CMR vs. Frequency, RTI
0.10
0.08
0.06
0.04
0.02
0
4
1S
3
100
90
2
1
0
–0.02
–0.04
–0.06
–0.08
–0.10
–1
10
–2
0
–3
–4
–40
–20
0
20
40
60
80
100
0
5
10
TEMPERATURE –
C
TIME – sec
TPC 11. CMRR vs. Temperature
TPC 8. 0.1 Hz to 10 Hz Voltage Noise, RTI
15
8
V
= 15V
1S
S
10
5
6
100
90
4
V
= 5V
2
0
S
0
–2
–5
–10
–15
10
–4
0
–6
–8
0
OUTPUTVOLTAGE –V
10
15
–15
–10
–5
5
0
5
10
TIME – sec
TPC 12. CMV Range vs. VOUT, Dual Supplies
TPC 9. 0.1 Hz to 10 Hz Current Noise
REV. A
–7–
AD8225
40
30
5
20
4
3
2
1
V
= 5V
S
10
0
–10
–20
–30
–40
–50
–60
0
0
100
1k
10k
100k
1M
10M
1
2
3
4
5
FREQUENCY – Hz
OUTPUTVOLTAGE –V
TPC 13. CMV vs. VOUT, Single Supply
TPC 16. Large Signal Frequency Response,
OUT = 4 V p-p
V
140
120
100
80
+V –0.0
S
–0.5
–1.0
–1.5
+V
S
–2.0
2.0
0
5
10
15
20
60
–V
S
1.5
1.0
0.5
40
20
–V +0.0
S
0
0
5
10
15
20
0.1
1
10
100
1k
10k
100k
1M
SUPPLYVOLTAGE – ꢀV
FREQUENCY – Hz
TPC 14. PSRR vs. Frequency, RTI
TPC 17. Input Common Mode Voltage Range vs.
Supply Voltage
40
30
+V
0
S
–0.5
–1.0
–1.5
R
= 10kꢃ
= 2kꢃ
20
L
10
R
L
0
–2.0
2.0
–10
–20
–30
–40
–50
0
2
4
6
8
10
12
14
16
18
20
1.5
1.0
0.5
R
= 2kꢃ
= 10kꢃ
6
L
R
L
–60
100
–V
0
S
1k
10k
100k
1M
10M
0
2
4
8
10
12
14
16
18
20
FREQUENCY – Hz
SUPPLYVOLTAGE – ꢀV
TPC 18. Output Voltage Swing vs. Supply Voltage
and Load Resistance
TPC 15. Small Signal Frequency Response,
OUT = 200 mV p-p
V
–8–
REV. A
AD8225
30
25
20
15
10
5
10
9
8
7
6
5
4
3
2
1
0
0.001%
0.01%
0
10
1k
100
LOAD RESISTANCE –ꢃ
100k
0
5
10
15
20
STEP SIZE –V
TPC 19. Output Voltage Swing vs. Load Resistance
TPC 22. Settling Time vs. Step Size
4
HORIZ
(4ꢁs/DIV)
100mV
2V
CH 1 = 5V/DIV
3
2
100
90
100
90
OUTPUT (5V/DIV)
1
0
TEST
CIRCUIT
OUTPUT
(0.001%/DIV)
CH 2 = 10mV/DIV
–1
10
0
–2
10
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
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
INPUT
100
90
1
+85 C
+25 C
–40 C
OUTPUT
10
0
2
CH 1 = 10mV, CH 2 = 20mV, H = 2ꢁs
0
2
4
6
8
10
12
14
16
18
20
SUPPLYVOLTAGE –
V
TPC 21. Small Signal Pulse Response, CL = 100 pF
TPC 24. ISUPPLY vs. VSUPPLY and Temperature
REV. A
–9–
AD8225
Test Circuits
4k⍀
G = 101
AD829
G = 100
AD797
100⍀
20k⍀
G = 100
LPF
G = 5
SCOPE
AD8225
G = 5
2k⍀
AD8225
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
+V
S
APPLICATIONS
Precision V-to-I Converter
+V
–V
+V
–V
S
S
V
B
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.
+IN
Q1
–IN
Q2
A1
A2
R1
+V
–V
R2
C2
C1
S
S
S
3kꢃ
V
REF
15kꢃ
Figure 5 illustrates how the AD8225 may be used as the trans-
mitter and receiver in a current loop system. The full-scale
output is 5 mA.
+V
S
A3
15kꢃ
S
3kꢃ
OUT
R
FULL SCALE
3
2
SH
I
OUT
e
CURRENT = 5mA
–V
IN
20ꢃ
S
3
2
AD8225
200mV
pk FS
6
1kꢃ
V
SH
6
8ꢃ
Figure 3. Simplified Schematic
AD8225
5
e
OUT
200mV
pk FS
5
47pF
THEORY OF OPERATION
The AD8225 is a monolithic, three op amp instrumentation
9kꢃ
GND OR
REFV
amplifier. Laser wafer trimming and proprietary circuit tech-
niques 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.
OP27
V
0.5 e
IN
SH
I
=
=
OUT
R
R
SH
SH
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).
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.
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.
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.
eIN = 398mV p-p, eOUT = 398mV p-p,
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 com-
mon, the stray capacitance at Pins 2 and 3 is balanced.
I
= 10.3mA p-p
OUT
1
2
eIN
eOUT
AC
GROUND
AD8225
8
NC
I
OUT
–IN
+IN
7
6
5
+V
V
S
3
OUT
REF
AC
GROUND
CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA,
H = 200ꢁs
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
–11–
AD8225
Driving a High Resolution ADC
100 pF of capacitance at its output, a 75 Ω series resistor is
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.
required at each in amp output to prevent oscillation.
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 differen-
tial 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
75ꢃ
6
+IN
AD8225
2
2.7nF
5
AD7675
100kSPS
3
2
75ꢃ
6
–IN
AD8225
2.7nF
5
ALTERNATE
CONNECTION
FOR SE SOURCE
4.99kꢃ
4.99kꢃ
1.25V
OP177
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 gener-
ated 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 = 3TO 10
A1
DIGITAL DATA
GAIN AND ADC
TO SYSTEM
A3
TOTAL G = 1000
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.
RA-LA 1
LA-LL 2
Gain developed in the input stage results in a trade-off in common-
mode 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.
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
OP77
G = 5
G = 200
19.6kꢃ 301ꢃ
100ꢃ
S1A
S1B
0.2V, 2V
AD8225
S2A
DA
OP77
G = 5
S2B
G = 200
1
AD8225
ADG409
S3A
S3B
S4A
S4B
DB
19.6kꢃ 301ꢃ
100ꢃ
REF
4
AD8225
OP77
G = 5
G = 200
Figure 11. Connection to an ADG409 Analog MUX
19.6kꢃ 301ꢃ
100ꢃ
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 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 10 shows test waveforms observed from the circuit of
Figure 9.
SMALL SIGNAL
(200mV/DIV)
INPUT
SIGNAL
TRAN-
SITION
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
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.
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
+V
S
C1
0.1ꢁF
C4
R3
R2
0.1ꢁF
100kꢃ*
100ꢃ
+IN
GND
–IN
W3
W4
7
3
6
R4
100ꢃ
A1
4
OUTPUT
EXT_REF
2
5
1
R8
R5
C3
0.1ꢁF
100kꢃ*
W12
C2
0.1ꢁF
W13
W11
W14
AUX
–V
S
USER-SUPPLIED
+V
AUX
C12
10ꢁF, 25V
C10
0.1ꢁF
CS1
J500
240ꢁA
W7
+V
+V
S
GND
C7
0.1ꢁF
OFFSET
ADJ
R9
5.9kꢃ, 1%
W6
3
2
R1
10kꢃ
7
4
–V
–V
S
AUX
6
C11
10ꢁF, 25V
A1
R10
5.9kꢃ, 1%
C9
0.1ꢁF
C8
0.1ꢁF
CS2
J500
240ꢁA
–V
AUX
NOTES
AD707JN
–V
AUX
REMOVE W3ANDW4 FORAC COUPLING
*INSTALL FOR AC COUPLING
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)
8
1
5
4
6.20 (0.2440)
5.80 (0.2284)
4.00 (0.1574)
3.80 (0.1497)
1.27 (0.0500)
BSC
0.50 (0.0196)
0.25 (0.0099)
ꢄ45ꢂ
1.75 (0.0688)
1.35 (0.0532)
0.25 (0.0098)
0.10 (0.0040)
8ꢂ
0ꢂ
0.51 (0.0201)
0.33 (0.0130)
1.27 (0.0500)
0.41 (0.0160)
COPLANARITY
0.10
0.25 (0.0098)
0.19 (0.0075)
SEATING
PLANE
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.
Updated ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Change to TPC 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Change to TPC 20 caption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Edit to Precision V-to-I Converter section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
OUTLINE DIMENSIONS updated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
–16–
REV. A
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