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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 GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM
NC 1 -IN 2 +IN 3 -VS 4
AD8225
8 7 6 5
NC +VS VOUT REF
NC = NO CONNECT
140 130 120 110
CMRR - dB
100 AD8225 90 80 70 60 50 40 30 1 10 100 1k FREQUENCY - Hz 10k 100k HIGH PERFORMANCE IN AMP @ GAIN OF 5
Figure 1. Typical CMRR vs. Frequency
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
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.
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/s make the AD8225 ideally suited for multiplexed applications. The AD8225 provides excellent dc precision, with maximum input offset voltage of 150 V and drift of 2 V/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, -40C to +85C.
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 (c) 2003 Analog Devices, Inc. All rights reserved.
AD8225-SPECIFICATIONS (T = 25 C, V =
A S
15 V, RL = 2 k , unless otherwise noted.)
Min Typ 5 +0.05 2 1 50 0.3 100 Max Unit V/V % ppm ppm/C V V/C dB
Parameter GAIN Gain Gain Error Nonlinearity vs. Temperature OFFSET VOLTAGE (RTI) Offset Voltage vs. Temperature vs. Supply (PSRR) 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
Conditions
-0.1
+0.1 10 5 150 2
90
10 2 10 2 -VS + 1.6 -VS + 2.2 0.5 3 0.15 1.5 94 +VS - 1.0 +VS - 1.2 1.2 0.5
G pF G pF V V nA pA/C nA pA/C dB dB dB V V V V mA kHz kHz s s V/s V p-p nV/Hz pA p-p fA/Hz k A V
TA = TMIN to TMAX f = 10 kHz* OUTPUT Operating Voltage Range vs. Temperature Operating Voltage Range vs. Temperature Short Circuit Current 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 POWER SUPPLY Operating Range Quiescent Current TEMPERATURE RANGE For Specified Performance
*Pin 1 connected to Pin 4. See Applications section. Specifications subject to change without notice.
86 83 80 -VS + 1.4 -VS + 1.5 -VS + 1.0 -VS + 1.2
RL = 2 k RL = 10 k
+VS - 1.4 +VS - 1.6 +VS - 1.1 +VS - 1.0 18 900 75 3.4 4.8
VOUT = 20 V p-p 10 V Step 10 V Step 5 0.1 Hz to 10 Hz Spectral Density, 1 kHz 0.1 Hz to 10 Hz Spectral Density, 1 kHz VIN+, VREF = 0 -VS + 1.4 0.999 1.7
1.5 45 4 50 18 60 1 +VS - 1.4 1.001 18 1.2 +85
1.05 -40
V mA C
-2-
REV. A
AD8225
SPECIFICATIONS (T = 25 C, V =
A S
5 V, RL = 2 k , unless otherwise noted.)
Min Typ 5 +0.05 2 1 125 90 100 Max Unit V/V % ppm ppm/C V V/C dB
Parameter GAIN Gain Gain Error Nonlinearity vs. Temperature VOLTAGE OFFSET (RTI) Offset Voltage vs. Temperature vs. Supply 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
Conditions
-0.1
+0.1 10 5 325 2
10 2 10 2 -VS + 1.6 -VS + 2.1 0.5 3 0.15 1.5 94 +VS - 1.0 +VS - 1.5 1.2 0.5
TA = TMIN to TMAX f = 10 kHz* OUTPUT Operating Voltage Range vs. Temperature Operating Voltage Range vs. Temperature Short Circuit Current 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 POWER SUPPLY Operating Range Quiescent Current TEMPERATURE RANGE For Specified Performance
*Pin 1 connected to Pin 4. See Applications section. Specifications subject to change without notice.
86 83 80 -VS + 0.9 -VS + 1.0 -VS + 0.8 -VS + 0.9
G pF G pF V V nA pA/C nA pA/C dB dB dB V V V V mA kHz kHz s s V/s V p-p nV/Hz pA p-p fA/Hz k A V
RL = 2 k RL = 10 k
+VS - 1.0 +VS - 1.2 +VS - 1.0 +VS - 1.0 18 900 170 3 4.3
VOUT = 7.8 V p-p 7 V Step 7 V Step 5 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 18 60 -VS + 0.9 0.999 1.7 1.05 -40 1 +VS - 1.0 1.001 18 1.2 +85
VINT, VREF = 0
V mA C
REV. A
-3-
AD8225
SPECIFICATIONS (T = 25 C, V = 5 V, R = 2 k
A S L
, unless otherwise noted.)
Min Typ 5 +0.05 2 1 150 90 100 Max Unit V/V % ppm ppm/C V V/C dB
Parameter GAIN Gain Gain Error Nonlinearity vs. Temperature OFFSET VOLTAGE (RTI) Offset Voltage vs. Temperature vs. Supply 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
Conditions
-0.1
+0.1 10 5 375 2
10 2 10 2 1.6 1.7 0.5 3 0.15 1.5 94 VS - 1.05 VS - 1.0 1.2 0.5
G pF G pF V V nA pA/C nA pA/C dB dB dB V V V V mA kHz kHz s s V/s V p-p nV/Hz pA p-p fA/Hz k A V
TA = TMIN to TMAX f = 10 kHz* OUTPUT Operating Voltage Range vs. Temperature Operating Voltage Range vs. Temperature Short Circuit Current 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 POWER SUPPLY Operating Range Quiescent Current TEMPERATURE RANGE For Specified Performance
*Pin 1 connected to Pin 4. See Applications section. Specifications subject to change without notice.
86 83 80 0.8 0.9 0.8 0.9
RL = 2 k RL = 10 k
VS - 1.05 VS - 1.2 VS - 1.0 VS - 1.0 18 900 420 3.3 5.1
VOUT = 3.2 V p-p 2 V Step 2 V Step 5 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 18 60 0.4 0.999 3.4 1.05 -40 1 VS - 0.9 1.001 36 1.2 +85
V mA C
-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 . . . . . . . . . . . . . . . . . . -65C to +125C Operating Temperature Range . . . . . . . . . . . -40C to +85C Lead Temperature Range (10 sec Soldering) . . . . . . . . . 300C
*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 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
POWER DISSIPATION - W
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 AD8225AR AD8225AR-REEL AD8225AR-REEL7 AD8225-EVAL
Temperature Range -40C to +85C -40C to +85C -40C to +85C
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-
AD8225-Typical Performance Characteristics (T = 25 C, R = 2 k
A L
50 LOT SIZE = 3775 45 40 BIAS CURRENT - pA 35
% OF UNITS
, VS =
15 V, unless otherwise noted.)
250
200
150 +BIAS CURRENT 100 -BIAS CURRENT 50
30 25 20 15 10 5 0 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 INPUT OFFSET VOLTAGE - V 80 100 120
0
-50 -60
-40
-20
0 20 40 TEMPERATURE - C
60
80
100
TPC 1. Typical Distribution of Input Offset Voltage, VS = 15 V
TPC 4. Bias Current vs. Temperature
50 LOT SIZE = 7550 45 40 35
% OF UNITS
8
CHANGE IN OFFSET VOLTAGE - V
6
30 25 20 15 10 5 0 -200 -100
4
2
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 2. Typical Distribution of Input Bias Current, VS = 15 V
TPC 5. Offset Voltage vs. Warm-Up Time
50 LOT SIZE = 3775 45 40 35
% OF UNITS
1000
30 25 20 15 10 5 0 -500
VOLTAGE NOISE DENSITY - nV/ Hz
100
0 -400 -300 -200 -100 0 100 200 INPUT OFFSET CURRENT - pA 300 400
1
10
100 1k FREQUENCY - Hz
10k
100k
TPC 3. Typical Distribution of Input Offset Current, VS = 15 V
TPC 6. Voltage Noise Spectral Density vs. Frequency (RTI)
-6-
REV. A
AD8225
1000
130 120
VOLTAGE NOISE DENSITY - nV/ Hz
110 100
CMR - dB
1 10 100 FREQUENCY - Hz 1k 10k
90 80 70 60 50 40
100
0
30
1
10
100 1k FREQUENCY - Hz
10k
100k
TPC 7. Input Current Noise Spectral Density vs. Frequency
TPC 10. CMR vs. Frequency, RTI
0.10 4 1S 3
100
0.08 0.06 CMRR DRIFT - ppm/ C 0.04 0.02 0 -0.02 -0.04 -0.06 -0.08
2
90
V NOISE - RTI -
1 0 -1 -2 -3 -4 0 5 TIME - sec 10
10 0
-0.10 -40
-20
0
20 40 60 TEMPERATURE - C
80
100
TPC 8. 0.1 Hz to 10 Hz Voltage Noise, RTI
TPC 11. CMRR vs. Temperature
15
8 1S 6
10
COMMON-MODE VOLTAGE - V
VS = 15V
CURRENT NOISE - 2pA/div
100
4 2 0 -2 -4 -6 -8
90
5 VS = 5V 0
-5
10 0
-10
0
5 TIME - sec
10
-15 -15
-10
0 -5 5 OUTPUT VOLTAGE - V
10
15
TPC 9. 0.1 Hz to 10 Hz Current Noise
TPC 12. CMV Range vs. VOUT, Dual Supplies
REV. A
-7-
AD8225
5 40 30
COMMON-MODE VOLTAGE - V
4 VS = 5V
20 10 GAIN - dB
3
0
-10 -20 -30
2
1
-40 -50
0
0
1
2 3 OUTPUT VOLTAGE - V
4
5
-60 100
1k
10k 100k FREQUENCY - Hz
1M
10M
TPC 13. CMV vs. VOUT, Single Supply
TPC 16. Large Signal Frequency Response, VOUT = 4 V p-p
140
+VS -0.0
INPUT VOLTAGE 5 - V (REFERRED TO SUPPLY VOLTAGES)
120
-0.5 -1.0 -1.5 -2.0 2.0 1.5 1.0 0.5 0 5 10 15 20
100
PSRR - dB
80 60
+VS -VS
40 20 0 0.1
-VS +0.0 1 10 100 1k FREQUENCY - Hz 10k 100k 1M
0
5
10 SUPPLY VOLTAGE -
15 V
20
TPC 14. PSRR vs. Frequency, RTI
TPC 17. Input Common Mode Voltage Range vs. Supply Voltage
40
+VS 0
30
OUTPUT VOLTAGE SWING - V (REFERENCED TO SUPPLY VOLTAGES)
-0.5 -1.0 -1.5 -2.0 2.0 1.5 0 2 4
20 10 GAIN - dB 0
RL = 10k RL = 2k
-10 -20 -30 -40 -50 -60 100 1k 10k 100k FREQUENCY - Hz 1M 10M
6
8
10
12
14
16
18
20
RL = 2k 1.0 0.5 -VS 0 0 RL = 10k
2
4
6
8 10 12 14 SUPPLY VOLTAGE - V
16
18
20
TPC 15. Small Signal Frequency Response, VOUT = 200 mV p-p
TPC 18. Output Voltage Swing vs. Supply Voltage and Load Resistance
-8-
REV. A
AD8225
30 10 9
OUTPUT VOLTAGE SWING - V p-p
25 SETTLING TIME - s
8 7 6 5 4 3 2 1 0.01% 0.001%
20
15
10
5
0 10
1k 100 LOAD RESISTANCE -
100k
0
0
5
10 STEP SIZE - V
15
20
TPC 19. Output Voltage Swing vs. Load Resistance
TPC 22. Settling Time vs. Step Size
4
CH 1 = 5V/DIV
100 90
HORIZ (4 s/DIV)
100mV 3
100
2V
OUTPUT (5V/DIV)
NONLINEARITY - ppm
2 1 0 -1 -2 -3 -4
90
CH 2 = 10mV/DIV
10 0
TEST CIRCUIT OUTPUT (0.001%/DIV)
10 0
-10
0 OUTPUT VOLTAGE - V
10
TPC 20. Large Signal Pulse Response and Settling Time to 0.001%
TPC 23. Gain Nonlinearity
1.5
INPUT
100 90
1.4 1.3 SUPPLY CURRENT - mA 1.2 1.1 1.0 0.9 0.8 0.7 0.6 +25 C -40 C +85 C
1
OUTPUT
10 0
2 CH 1 = 10mV, CH 2 = 20mV, H = 2 s
0.5
0
2
4
6
8 10 12 14 SUPPLY VOLTAGE - V
16
18
20
TPC 21. Small Signal Pulse Response, CL = 100 pF
TPC 24. ISUPPLY vs. VSUPPLY and Temperature
REV. A
-9-
AD8225 Test Circuits
G = 100
4k G = 101 100
AD797
G=5
G = 100 LPF SCOPE
G=5
AD829
AD8225
20k
2k 20
AD8225
2k
Test Circuit 1. 1 Hz to 10 Hz Voltage Noise Test
Test Circuit 2. Settling Time to 0.01%
-10-
REV. A
AD8225
+VS +VS VB -IN Q2 A1 -VS R2 C2 A2 C1 R1 3k 15k A3 3k 15k OUT +VS -VS VREF -VS +VS Q1 +IN +VS
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 transmitter and receiver in a current loop system. The full-scale output is 5 mA.
3 eIN 200mV pk FS IOUT RSH 20 VSH FULL SCALE CURRENT = 5mA 3 8 2
-VS
AD8225
2 5
Figure 3. Simplified Schematic
THEORY OF OPERATION
6 1k
AD8225
5 GND OR REF V
6 eOUT 200mV pk FS
47pF 9k
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 V 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 k and 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 common, the stray capacitance at Pins 2 and 3 is balanced.
OP27
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
1
AC GROUND
eIN
eOUT
2
AD8225
8 7 6 5
NC +VS VOUT REF
CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA, H = 200 s 3 IOUT
-IN +IN
AC GROUND PIN 1 HAS NO INTERNAL CONNECTION
Figure 4. Pinout for Symmetrical Input Stray Capacitance
Figure 6. V-to-I Converter Waveforms (CH1: VIN, CH2: VOUT, CH3: IOUT)
REV. A
-11-
AD8225
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. 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.
5V 3 75 +IN 2.7nF 5 16 BITS
100 pF of capacitance at its output, a 75 series resistor is 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 k 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 k and two 15 k resistors, resulting in an error voltage generated by common-mode voltages at the input pins.
AD8225 Used as an EKG Front End
AD8225
2
6
AD7675
100kSPS
3
AD8225
2 ALTERNATE CONNECTION FOR SE SOURCE 5 1.25V
6
75 2.7nF 4.99k
-IN
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. 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. 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.
OP177
4.99k 2.5V
AD780
RERERENCE
Figure 7. Driver for Differential ADC
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
INSTRUMENTATION AMPLIFIER G = 3 TO 10 A1 A3 LEAD SELECT, HV PROTECTION, FILTERING A2 GAIN AND ADC TOTAL G = 1000
PATIENT ISOLATION BARRIER DIGITAL DATA TO SYSTEM MAINFRAME
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.
RA-LA 1
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
AD8225
G = 5 OP77 G = 200 19.6k 301
100
At 5 V/s, 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.
S1A S1B
AD8225
G = 5 OP77 G = 200 19.6k 301
0.2V, 2V
S2A S2B S3A
DA
ADG409
DB
1
AD8225
REF 4
100
S3B S4A 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 10 shows test waveforms observed from the circuit of Figure 9.
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 s of the applied level.
SMALL SIGNAL (200mV/DIV)
INPUT SIGNAL TRANSITION
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 0.1 F R2 100 R4 100 R3 100k * W3 W4 C3 0.1 F 3 2 R5 100k * 7 A1 4 1 6 5 R8 W12 C2 0.1 F W13 W11 W14 -VS C12 10 F, 25V W7 +VS GND W6 -VS C11 10 F, 25V -VAUX 6 7 A1 4 C9 0.1 F -VAUX NOTES REMOVE W3 AND W4 FOR AC COUPLING *INSTALL FOR AC COUPLING CS2 J500 240 A -VAUX 3 2 +VAUX C10 0.1 F CS1 J500 240 A C7 R9 0.1 F 5.9k , 1% R10 5.9k , 1% C8 0.1 F USER-SUPPLIED +VAUX EXT_REF OUTPUT C4 0.1 F
+IN GND -IN
OFFSET ADJ
R1 10k
AD707JN
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 5 4
4.00 (0.1574) 3.80 (0.1497)
1
6.20 (0.2440) 5.80 (0.2284)
1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY SEATING 0.10 PLANE
1.75 (0.0688) 1.35 (0.0532) 8 0.25 (0.0098) 0 0.19 (0.0075)
0.50 (0.0196) 45 0.25 (0.0099)
0.51 (0.0201) 0.33 (0.0130)
1.27 (0.0500) 0.41 (0.0160)
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 2/03--Data Sheet changed from REV. 0 to REV. A. Page
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
C02771-0-2/03(A) PRINTED IN U.S.A.
-16-
REV. A
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