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AD8225 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD8225
Beschreibung Precision Gain of 5 Instrumentation Amplifier
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 17 Seiten
AD8225 Datasheet, Funktion
Precision Gain of 5
Instrumentation Amplifier
AD8225
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
APPLICATIONS
Patient Monitors
Current Transmitters
Multiplexed Systems
4 to 20 mA Converters
Bridge Transducers
Sensor Signal Conditioning
FUNCTIONAL BLOCK DIAGRAM
NC 1
–IN 2
+IN 3
–VS 4
AD8225
8 NC
7 +VS
6 VOUT
5 REF
NC = NO CONNECT
140
130
120
110
100
90
80
HIGH PERFORMANCE IN AMP
@ GAIN OF 5
70
AD8225
60
50
40
30
1 10 100 1k 10k 100k
FREQUENCY – Hz
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
REV. A
CMRR performance of the AD8225 rejects noise from utility
systems, motors, and repair equipment on factory floors, switch-
ing 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.
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 Datasheet, Funktion
AD8225
ABSOLUTE MAXIMUM RATINGS*
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 FUNCTION DESCRIPTIONS
Pin Number
1
2
3
4
5
6
7
8
Mnemonic
NC
IN
+IN
VS
REF
VOUT
+VS
NC
Function
May be Connected to Pin 4 to
Balance CIN
Inverting Input
Noninverting Input
Negative Supply Voltage
Connect to Desired Output
CMV
Output
Positive Supply Voltage
1.5
TJ = 150 C
8-LEAD SOIC PACKAGE
1.0
0.5
Model
AD8225AR
AD8225AR-REEL
AD8225AR-REEL7
AD8225-EVAL
0
–50 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90
AMBIENT TEMPERATURE – C
Figure 2. Maximum Power Dissipation vs. Temperature
Temperature Range
40ºC to +85ºC
40ºC to +85ºC
40ºC to +85ºC
ORDERING GUIDE
Package Description
8-Lead SOIC
8-Lead SOIC
8-Lead SOIC
Evaluation Board
Package Options
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–

6 Page









AD8225 pdf, datenblatt
AD8225
+VS
+VS +VS
VB
–IN Q2
Q1 +IN
–VS R2
A1
C2
A2
C1
R1 –VS +VS
3k
A3
15k
3k
15k
+VS –VS
VREF
OUT
–VS
Figure 3. Simplified Schematic
THEORY OF OPERATION
The AD8225 is a monolithic, three op amp instrumentation
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.
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 kand 15 k
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.
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.
AC
GROUND
–IN
+IN
AC
GROUND
AD8225
8 NC
7 +VS
6 VOUT
5 REF
PIN 1 HAS NO INTERNAL CONNECTION
Figure 4. Pinout for Symmetrical Input Stray Capacitance
APPLICATIONS
Precision V-to-I Converter
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 trans-
mitter and receiver in a current loop system. The full-scale
output is 5 mA.
eIN
200mV
pk FS
3 IOUT
AD8225 6
2 5 1k
RSH
20
VSH
47pF
9k
OP27
FULL SCALE
CURRENT = 5mA
8
3
AD8225
25
6
GND OR
REF V
IOUT
=
VSH
RSH
=
0.5 eIN
RSH
eOUT
200mV
pk FS
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
1 eIN eOUT
2
IOUT
3
CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA,
H = 200s
Figure 6. V-to-I Converter Waveforms (CH1: VIN,
CH2: VOUT, CH3: IOUT)
REV. A
–11–

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