a single supply, rail-to-rail, low cost instrumentation...
TRANSCRIPT
REV. A
Information furnished by Analog Devices is believed to be accurate andreliable. However, no responsibility is assumed by Analog Devices for itsuse, nor for any infringements of patents or other rights of third partieswhich may result from its use. No license is granted by implication orotherwise under any patent or patent rights of Analog Devices.
aAD623
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 World Wide Web Site: http://www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 1997
Single Supply, Rail-to-Rail, Low CostInstrumentation Amplifier
CONNECTION DIAGRAM
8-Lead Plastic Mini-DIP (N),SOIC (R) and microSOIC (RM) Packages
8
7
6
5
3
4
2RG
2IN
1IN
2VS
1RG
1VS
OUTPUT
REFAD623
1
2
120
110
100
90
80
70
60
50
40
301 10 100 1k 10k 100k
FREQUENCY – Hz
CM
R –
dB
x1000
x100
x10
x1
Figure 1. CMR vs. Frequency, +5 VS, 0 VS
FEATURES
Easy to Use
Higher Performance than Discrete Design
Single and Dual Supply Operation
Rail-to-Rail Output Swing
Input Voltage Range Extends 150 mV Below Ground
(Single Supply)
Low Power, 575 mA Max Supply Current
Gain Set with One External Resistor
Gain Range 1 (No Resistor) to 1,000
HIGH ACCURACY DC PERFORMANCE
0.1% Gain Accuracy (G = 1)
0.35% Gain Accuracy (G > 1)
25 ppm Gain Drift (G = 1)
200 mV Max Input Offset Voltage (AD623A)
2 mV/8C Max Input Offset Drift (AD623A)
100 mV Max Input Offset Voltage (AD623B)
1 mV/8C Max Input Offset Drift (AD623B)
25 nA Max Input Bias Current
NOISE
35 nV/√Hz RTI Noise @ 1 kHz (G = 1)
EXCELLENT AC SPECIFICATIONS
90 dB Min CMRR (G = 10); 84 dB Min CMRR (G = 5)
(@ 60 Hz, 1K Source Imbalance)
800 kHz Bandwidth (G = 1)
20 ms Settling Time to 0.01% (G = 10)
APPLICATIONS
Low Power Medical Instrumentation
Transducer Interface
Thermocouple Amplifier
Industrial Process Controls
Difference Amplifier
Low Power Data Acquisition
PRODUCT DESCRIPTIONThe AD623 is an integrated single supply instrumentation am-plifier that delivers rail-to-rail output swing on a single supply(+3 V to +12 V supplies). The AD623 offers superior user flex-ibility by allowing single gain set resistor programming, andconforming to the 8-lead industry standard pinout configura-tion. With no external resistor, the AD623 is configured forunity gain (G = 1) and with an external resistor, the AD623 canbe programmed for gains up to 1,000.
The AD623 holds errors to a minimum by providing superiorAC CMRR that increases with increasing gain. Line noise, aswell as line harmonics, will be rejected since the CMRR remainsconstant up to 200 Hz. The AD623 has a wide input common-
mode range and can amplify signals that have a common-modevoltage 150 mV below ground. Although the design of the AD623has been optimized to operate from a single supply, the AD623still provides superior performance when operated from a dualvoltage supply (±2.5 V to ±6.0 V).
Low power consumption (1.5 mW at 3 V), wide supply voltagerange, and rail-to-rail output swing make the AD623 ideal forbattery powered applications. The rail-to-rail output stage maxi-mizes the dynamic range when operating from low supply volt-ages. The AD623 replaces discrete instrumentation amplifierdesigns and offers superior linearity, temperature stability andreliability in a minimum of space. Until the AD623, this level ofinstrumentation amplifier performance has not been achieved.
–2– REV. A
AD623–SPECIFICATIONSSINGLE SUPPLYModel AD623A AD623ARM AD623BSpecification Conditions Min Typ Max Min Typ Max Min Typ Max Units
GAIN G = 1 + (100 k/RG)Gain Range 1 1000 1 1000 1 1000Gain Error1 G1 VOUT =
0.05 V to 3.5 VG > 1 VOUT =0.05 V to 4.5 V
G = 1 0.03 0.10 0.03 0.10 0.03 0.10 %G = 10 0.10 0.35 0.10 0.35 0.10 0.35 %G = 100 0.10 0.35 0.10 0.35 0.10 0.35 %G = 1000 0.10 0.35 0.10 0.35 0.10 0.35 %
Nonlinearity, G1 VOUT =0.05 V to 3.5 VG > 1 VOUT =0.05 V to 4.5 V
G = 1–1000 50 50 50 ppmGain vs. Temperature
G = 1 5 5 5 ppm/°CG > 1 50 50 50 ppm/°C
VOLTAGE OFFSET Total RTI Error = VOSI + VOSO/G
Input Offset, VOSI 25 200 200 500 25 100 µVOver Temperature 350 650 160 µVAverage TC 0.1 2 0.1 2 0.1 1 µV/°C
Output Offset, VOSO 200 1000 500 2000 200 500 µVOver Temperature 1500 2600 1100 µVAverage TC 2.5 10 2.5 10 2.5 10 µV/°C
Offset Referred to the Input vs. Supply (PSR)
G = 1 80 100 80 100 80 100 dBG = 10 100 120 100 120 100 120 dBG = 100 120 140 120 140 120 140 dBG = 1000 120 140 120 140 120 140 dB
INPUT CURRENTInput Bias Current 17 25 17 25 17 25 nA
Over Temperature 27.5 27.5 27.5 nAAverage TC 25 25 25 pA/°C
Input Offset Current 0.25 2 0.25 2 0.25 2 nAOver Temperature 2.5 2.5 2.5 nAAverage TC 5 5 5 pA/°C
INPUTInput Impedance
Differential 2i2 2i2 2i2 GΩipFCommon-Mode 2i2 2i2 2i2 GΩipF
Input Voltage Range2 VS = +3 V to +12 V (–VS) – 0.15 (+VS) – 1.5 (–VS) – 0.15 (+VS) – 1.5 (–VS) – 0.15 (+VS) – 1.5 VCommon-Mode Rejection at
60 Hz with 1 kΩ SourceImbalance
G = 1 VCM = 0 V to 3 V 70 80 70 80 77 86 dBG = 10 VCM = 0 V to 3 V 90 100 90 100 94 100 dBG = 100 VCM = 0 V to 3 V 105 110 105 110 105 110 dBG = 1000 VCM = 0 V to 3 V 105 110 105 110 105 110 dB
OUTPUTOutput Swing RL = 10 kΩ +0.01 (+VS) – 0.5 +0.01 (+VS) – 0.5 +0.01 (+VS) – 0.5 V
RL = 100 kΩ +0.01 (+VS) – 0.15 +0.01 (+VS) – 0.15 +0.01 (+VS) – 0.15 V
DYNAMIC RESPONSESmall Signal –3 dBBandwidthG = 1 800 800 800 kHzG = 10 100 100 100 kHzG = 100 10 10 10 kHzG = 1000 2 2 2 kHz
Slew Rate 0.3 0.3 0.3 V/µsSettling Time to 0.01% VS = +5 V
G = 1 Step Size: 3.5 V 30 30 30 µsG = 10 Step Size: 4 V,
VCM = 1.8 V 20 20 20 µs
(typical @ +258C Single Supply, VS = +5 V, and RL = 10 kV, unless otherwise noted)
–3–REV. A
AD623
DUAL SUPPLIESModel AD623A AD623ARM AD623BSpecification Conditions Min Typ Max Min Typ Max Min Typ Max Units
GAIN G = 1 + (100 k/RG)Gain Range 1 1000 1 1000 1 1000Gain Error1 G1 VOUT =
–4.8 V to 3.5 VG > 1 VOUT =0.05 V to 4.5 V
G = 1 0.03 0.10 0.03 0.10 0.03 0.10 %G = 10 0.10 0.35 0.10 0.35 0.10 0.35 %G = 100 0.10 0.35 0.10 0.35 0.10 0.35 %G = 1000 0.10 0.35 0.10 0.35 0.10 0.35 %
Nonlinearity, G1 VOUT =–4.8 V to 3.5 V G > 1 VOUT =–4.8 V to 4.5 V
G = 1–1000 50 50 50 ppmGain vs. Temperature
G = 1 5 5 5 ppm/°CG > 1 50 50 50 ppm/°C
VOLTAGE OFFSET Total RTI Error = VOSI + VOSO/G
Input Offset, VOSI 25 200 200 500 25 100 µVOver Temperature 350 650 160 µVAverage TC 0.1 2 0.1 2 0.1 1 µV/°C
Output Offset, VOSO 200 1000 500 2000 200 500 µVOver Temperature 1500 2600 1100 µVAverage TC 2.5 10 2.5 10 2.5 10 µV/°C
Offset Referred to the Input vs. Supply (PSR)
G = 1 80 100 80 100 80 100 dBG = 10 100 120 100 120 100 120 dBG = 100 120 140 120 140 120 140 dBG = 1000 120 140 120 140 120 140 dB
INPUT CURRENTInput Bias Current 17 25 17 25 17 25 nA
Over Temperature 27.5 27.5 27.5 nAAverage TC 25 25 25 pA/°C
Input Offset Current 0.25 2 0.25 2 0.25 2 nAOver Temperature 2.5 2.5 2.5 nAAverage TC 5 5 5 pA/°C
INPUTInput Impedance
Differential 2i2 2i2 2i2 GΩipFCommon-Mode 2i2 2i2 2i2 GΩipF
Input Voltage Range2 VS = +2.5 V to ±6 V (–VS) – 0.15 (+VS) – 1.5 (–VS) –0.15 (+VS) – 1.5 (–VS) – 0.15 (+VS) – 1.5 VCommon-Mode Rejection at
60 Hz with 1 kΩ SourceImbalance
G = 1 VCM = +3.5 V to –5.15 V 70 80 70 80 77 86 dBG = 10 VCM = +3.5 V to –5.15 V 90 100 90 100 94 100 dBG = 100 VCM = +3.5 V to –5.15 V 105 110 105 110 105 110 dBG = 1000 VCM = +3.5 V to –5.15 V 105 110 105 110 105 110 dB
OUTPUTOutput Swing RL = 10 kΩ, VS = ±5 V (–VS) +0. 2 (+VS) – 0.5 (–VS) + 0.2 (+VS) – 0.5 (–VS) + 0.2 (+VS) – 0.5 V
RL = 100 kΩ (–VS) + 0.05 (+VS) – 0.15 (–VS) + 0.05 (+VS) – 0.15 (–VS) + 0.05 (+VS) – 0.15 V
DYNAMIC RESPONSESmall Signal –3 dBBandwidthG = 1 800 800 800 kHzG = 10 100 100 100 kHzG = 100 10 10 10 kHzG = 1000 2 2 2 kHz
Slew Rate 0.3 0.3 0.3 V/µsSettling Time to 0.01% VS = ±5 V, 5 V Step
G = 1 30 30 30 µsG = 10 20 20 20 µs
(typical @ +258C Dual Supply, VS = 65 V, and RL = 10 kV, unless otherwise noted)
–4– REV. A
AD623–SPECIFICATIONS
ABSOLUTE MAXIMUM RATINGS1
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±6 VInternal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 650 mWDifferential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ±6 VOutput Short Circuit Duration . . . . . . . . . . . . . . . . IndefiniteStorage Temperature Range
(N, R, RM) . . . . . . . . . . . . . . . . . . . . . . . –65°C to +125°COperating Temperature Range
AD623 (A) . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°CLead Temperature Range
(Soldering 10 seconds) . . . . . . . . . . . . . . . . . . . . . . +300°C
NOTES1Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of thedevice at these or any other conditions above those indicated in the operationalsection of this specification is not implied. Exposure to absolute maximum ratingconditions for extended periods may affect device reliability.
2Specification is for device in free air:8-Lead Plastic Package: θJA = 95°C/Watt8-Lead SOIC Package: θJA = 160°C/Watt8-Lead microSOIC Package: θJA = 200°C/Watt.
ORDERING GUIDE
Model Package Description Package Option
AD623AN 8-Lead Plastic DIP N-8AD623AR 8-Lead SOIC R-8AD623ARM 8-Lead microSOIC RM-8AD623AR-REEL 8 L SOIC 13" ReelAD623AR-REEL7 8 L SOIC 7" Reel
ESD SUSCEPTIBILITYESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 volts, whichreadily accumulate on the human body and on test equipment, can discharge without detection.Although the AD623 features proprietary ESD protection circuitry, permanent damage may stilloccur on these devices if they are subjected to high energy electrostatic discharges. Therefore, properESD precautions are recommended to avoid any performance degradation or loss of functionality.
BOTH DUAL AND SINGLE SUPPLIESModel AD623A AD623ARM AD623BSpecification Conditions Min Typ Max Min Typ Max Min Typ Max Units
NOISEVoltage Noise, 1 kHz Total RTI Noise =
eni
2
+ eno /G
2
Input, Voltage Noise, eni 35 35 35 nV/√HzOutput, Voltage Noise, eno 50 50 50 nV/√Hz
RTI, 0.1 Hz to 10 HzG = 1 3.0 3.0 3.0 µV p-pG = 1000 1.5 1.5 1.5 µV p-p
Current Noise f = 1 kHz 100 100 100 fA/√Hz0.1 Hz to 10 Hz 1.5 1.5 1.5 pA p-p
REFERENCE INPUTRIN 100 ± 20% 100 ± 20% 100 ± 20% kΩIIN VIN+, VREF = 0 +50 +60 +50 +60 +50 +60 µAVoltage Range –VS +VS –VS +VS –VS +VS VGain to Output 1 ± 0.0002 1 ± 0.0002 1 ± 0.0002 V
POWER SUPPLYOperating Range Dual Supply ± 2.5 ± 6 ± 2.5 ± 6 ± 2.5 ± 6 V
Single Supply +3 +12 +3 +12 +3 +12 VQuiescent Current 375 575 375 575 375 575 µA
Over Temperature 625 625 625 µA
TEMPERATURE RANGEFor Specified Performance –40 to +85 –40 to +85 –40 to +85 °C
NOTES1Does not include effects of external resistor RG.2One input grounded. G = 1.
Specifications subject to change without notice.
WARNING!
ESD SENSITIVE DEVICE
AD623
–5–REV. A
Typical Characteristics (@ +258C VS = 65 V, RL = 10 kΩ unless otherwise noted)—
INPUT OFFSET VOLTAGE – mV–100 140–60 –20–40 20 120
280
0
UN
ITS
240
200
160
80
40
120
260
220
180
140
60
20
100
1008060400–80
300
Figure 2. Typical Distribution of Input Offset Voltage;Package Option N-8, R-8
OUTPUT OFFSET VOLTAGE 2 mV
180
0–800 –600
UN
ITS
–400 –200 0 200 400 600 800
300
240
60
120
360
420
480
Figure 3. Typical Distribution of Output Offset Voltage;Package Option N-8, R-8
INPUT OFFSET VOLTAGE – mV
12
6
0–80 –60
UN
ITS
–40 –20 0 20 40 60 80 100
10
8
4
2
14
16
18
20
22
Figure 4. Typical Distribution of Input Offset Voltage,VS = +5, Single Supply, VREF = –0.125 V; Package OptionN-8, R-8
OUTPUT OFFSET VOLTAGE – mV
12
6
0–500 0–400
UN
ITS
–300 –200 –100 100 200 300 400
10
8
4
2
14
16
18
20
22
–600 500
Figure 5. Typical Distribution of Output Offset Voltage,VS = +5, Single Supply, VREF = –0.125 V; Package OptionN-8, R-8
INPUT OFFSET CURRENT – nA
150
0–0.245 –0.21
UN
ITS
–0.24 –0.235 –0.23 –0.225 –0.22 –0.215
90
60
30
120
180
210
Figure 6. Typical Distribution for Input Offset Current;Package Option N-8, R-8
INPUT OFFSET CURRENT – nA
20
0–0.025 0.01–0.02
UN
ITS
–0.015 –0.01 –0.005 0 0.005
18
8
6
4
2
14
10
16
12
Figure 7. Typical Distribution for Input Offset Current,VS = +5, Single Supply, VREF = –0.125 V; Package OptionN-8, R-8
AD623
–6– REV. A
CMRR 2 dB
1600
600
075 80
UN
ITS
85 90 95 100 105 110 115 120 125 130
1400
800
400
200
1200
1000
Figure 8. Typical Distribution for CMRR (G = 1)
GAIN = 1000
1k
100
10
NO
ISE
– n
V/!
Hz,
RT
I
1 10 100 1k 10k 100kFREQUENCY – Hz
GAIN = 100
GAIN = 10
GAIN = 1
-
Figure 9. Voltage Noise Spectral Density vs. Frequency
CMV – Volts
21
–5 4–4
I BIA
S –
NA
–2 0 2
18
17
16
15
20
19
Figure 10. IBIAS vs. CMR, VS = ±5 V
TEMPERATURE – 8C
30
15
0–60 –40
INP
UT
BIA
S C
UR
RE
NT
– N
A
–20 0 20 40 60 80 100 120 140
25
20
10
5
Figure 11. Input Bias Current vs. Temp
FREQUENCY – Hz
1k
100
101 1k10
CU
RR
EN
T N
OIS
E –
fA/!
Hz
100
Figure 12. Current Noise Spectral Density vs. Frequency
CMV – Volts
19.5
19.0
–3 –2 –1 0 1
18.5
18.0
17.5
17.0
16.5
I BIA
S –
NA
Figure 13. IBIAS vs. CMV, VS = ±2.5 V
AD623
–7–REV. A
Figure 14. 0.1 Hz to 10 Hz Current Noise (0.71 pA/Div)
Figure 15. 0.1 Hz to 10 Hz RTI Voltage Noise (1 DIV = 1 µV p-p)
120
110
100
90
80
70
60
50
40
301 10 100 1k 10k 100k
FREQUENCY – Hz
CM
R –
dB
x1000
x100
x10
x1
Figure 16. CMR vs. Frequency, +5, 0 VS, VREF = 2.5 V
120
110
100
90
80
70
60
50
40
301 10 100 1k 10k 100k
FREQUENCY – Hz
CM
R –
dB
x1000
x10
x1
x100
Figure 17. CMR vs. Frequency, ±5 VS
100 1k 10k 100k 1M
FREQUENCY – Hz
70
60
50
40
30
20
10
0
–10
–20
–30
GA
IN –
dB
Figure 18. Gain vs. Frequency (VS = +5 V, 0 V), VREF = 2.5 V
COMMON MODE INPUT – Volts
OU
TP
UT
– V
olts
5
–5–6 –5
4
3
0
2
VS = 65
VS = 62.5
–4 –3 –2 –1 0 1 2 3 4 5
–4
–3
–2
–1
1
Figure 19. Maximum Output Voltage vs. Common Mode,G = 1, RL = 100 kΩ
RTO
RTI
AD623
–8– REV. A
COMMON MODE INPUT – Volts
OU
TP
UT
– V
olts
5
–5–6 –5
4
3
0
2
VS = 65VS = 62.5
–4 –3 –2 –1 0 1 2 3 4 5
–4
–3
–2
–1
1
Figure 20. Maximum Output Voltage vs. Common Mode,G ≥ 10, RL = 100 kΩ
COMMON MODE INPUT – Volts
5
4
0–1 50
OU
TP
UT
– V
olts
1 2 3 4
3
2
1
Figure 21. Maximum Output Voltage vs. Common Mode,G = 1, VS = +5 V, RL = 100 kΩ
COMMON MODE INPUT – Volts
5
4
0–1 50
OU
TP
UT
– V
olts
1 2 3 4
3
2
1
Figure 22. Maximum Output Voltage vs. Common Mode,G ≥ 10, VS = +5 V, RL = 100 kΩ
120
100
40
20
01 10 100 1k 10k 100k
FREQUENCY – Hz
PS
RR
– d
B
60
140
80
G = 1000
G = 100
G = 10
G = 1
Figure 23. Positive PSRR vs. Frequency, ±5 VS
120
100
40
20
01 10 100 1k 10k 100k
FREQUENCY – Hz
PS
RR
– d
B
60
140
80
G = 1000
G = 100
G = 10
G = 1
Figure 24. Positive PSRR vs. Frequency, +5 VS, 0 VS
120
100
40
20
01 10 100 1k 10k 100k
FREQUENCY – Hz
PS
RR
– d
B
60
140
80
G = 1000
G = 100
G = 10
G = 1
Figure 25. Negative PSRR vs. Frequency, ±5 VS
AD623
–9–REV. A
8
00 40
V p
–p
6
4
2VS = 62.5
VS = 65
20 8060 100
FREQUENCY – kHz
10
Figure 26. Large Signal Response, G ≤ 10
GAIN – V/V
1000
100
11 10
SE
TT
LIN
G T
IME
– m
s
100
10
1000
Figure 27. Settling Time to 0.01% vs. Gain, for a 5 V Stepat Output, CL = 100 pF, VS = ±5 V
Figure 28. Large Signal Pulse Response and SettlingTime, G = –1 (0.250 mV = 0.01%), CL = 100 pF
Figure 29. Large Signal Pulse Response and Settling Time,G = –10 (0.250 mV = 0.01%), CL = 100 pF
Figure 30. Large Signal Pulse Response and Settling Time, G = 100, CL = 100 pF
Figure 31. Large Signal Pulse Response and SettlingTime, G = –1000 (5 mV = 0.01%), CL = 100 pF
AD623
–10– REV. A
Figure 35. Small Signal Pulse Response, G = 1000,RL = 10 kΩ, CL = 100 pF
Figure 36. Gain Nonlinearity, G = –1 (50 ppm/Div)
Figure 37. Gain Nonlinearity, G = –10 (6 ppm/Div)
Figure 32. Small Signal Pulse Response, G = 1, RL = 10 kΩ,CL = 100 pF
Figure 33. Small Signal Pulse Response, G = 10,RL = 10 kΩ, CL = 100 pF
Figure 34. Small Signal Pulse Response G = 100,RL = 10 kΩ, CL = 100 pF
AD623
–11–REV. A
Figure 38. Gain Nonlinearity (G = –100, 15 ppm/Div)
V–0 0.5
SW
ING
– V
olts
1
(V–) +0.5
(V+) –1.5
V+
(V+) –1.5
(V+) –0.5
1.5
OUTPUT CURRENT – mA
2
Figure 39. Output Voltage Swing vs. Output Current
THEORY OF OPERATIONThe AD623 is an instrumentation amplifier based on a modifiedclassic three op amp approach, to assure single or dual supplyoperation even at common-mode voltages at the negative supplyrail. Low voltage offsets, input and output, as well as absolutegain accuracy, and one external resistor to set the gain, make theAD623 one of the most versatile instrumentation amplifiers inits class.
The input signal is applied to PNP transistors acting as voltagebuffers and providing a common-mode signal to the inputamplifiers (Figure 40). An absolute value 50 kΩ resistor in eachof the amplifiers’ feedback assures gain programmability.
The differential output is
VO = 1+
100 kΩRG
VC
The differential voltage is then converted to a single-endedvoltage using the output amplifier, which also rejects any common-mode signal at the output of the input amplifiers.
Since all the amplifiers can swing to either supply rails, as wellas have their common-mode range extended to below the nega-tive supply rail, the range over which the AD623 can operate isfurther enhanced (Figures 19 and 20).
The output voltage at Pin 6 is measured with respect to thepotential at Pin 5. The impedance of the reference pin is 100 kΩ,so in applications requiring V/I conversion, a small resistorbetween Pins 5 and 6 is all that is needed.
50kV 50kV 50kV
7POS SUPPLY
INVERTING2
14
50kV 50kV 50kV8
4NEG SUPPLY
NON-INVERTING
3
7
GAINOUT
6
REF5
+
–
+
–
+
–
Figure 40. Simplified Schematic
The bandwidth of the AD623 is reduced as the gain is increased,since all the amplifiers are of voltage feedback type. At unitygain, it is the output amplifier that limits the bandwidth. There-fore even at higher gains the AD623 bandwidth does not roll offas quickly.
APPLICATIONSBasic ConnectionFigure 41 shows the basic connection circuit for the AD623.The +VS and –VS terminals are connected to the power supply.The supply can be either bipolar (VS = ±2.5 V to ±6 V) orsingle supply (–VS = 0 V, +VS = 3.0 V to 12 V). Power suppliesshould be capacitively decoupled close to the devices powerpins. For best results, use surface mount 0.1 µF ceramic chipcapacitors and 10 µF electrolytic tantalum capacitors.
The input voltage, which can be either single-ended (tie either–IN or +IN to ground) or differential is amplified by the pro-grammed gain. The output signal appears as the voltage differencebetween the Output pin and the externally applied voltage onthe REF input. For a ground referenced output, REF should begrounded.
GAIN SELECTIONThe AD623’s gain is resistor programmed by RG, or more pre-cisely, by whatever impedance appears between Pins 1 and 8.The AD623 is designed to offer accurate gains using 0.1%–1%tolerance resistors. Table I shows required values of RG forvarious gains. Note that for G = 1, the RG terminals are uncon-nected (RG = `). For any arbitrary gain, RG can be calculatedby using the formula
RG = 100 kΩ/(G – 1)
REFERENCE TERMINALThe reference terminal potential defines the zero output voltageand is especially useful when the load does not share a preciseground with the rest of the system. It provides a direct means ofinjecting a precise offset to the output. The reference terminal isalso useful when bipolar signals are being amplified as it can beused to provide a virtual ground voltage. The voltage on thereference terminal can be varied from –VS to +VS.
AD623
–12– REV. A
Table I. Required Values of Gain Resistors
Desired 1% Std Table Calculated GainGain Value of RG, V Using 1% Resistors
2 100 k 25 24.9 k 5.0210 11 k 10.0920 5.23 k 20.1233 3.09 k 33.3640 2.55 k 40.2150 2.05 k 49.7865 1.58 k 64.29100 1.02 k 99.04200 499 201.4500 200 5011000 100 1001
INPUT AND OUTPUT OFFSET VOLTAGEThe low errors of the AD623 are attributed to two sources,input and output errors. The output error is divided by theprogrammed gain when referred to the input. In practice, theinput errors dominate at high gains and the output errors domi-nate at low gains. The total VOS for a given gain is calculated as:
Total Error RTI = Input Error + (Output Error/G)
Total Error RTO = (Input Error × G) + Output Error
RTI offset errors and noise voltages for different gains are shownbelow in Table II.
Table II. RTI Error Sources
Max MaxTotal Input Total Input Total InputOffset Error Offset Drift Referred Noise
Gain mV mV mV/8C mV/8C (nV/√Hz)
AD623A AD623B AD623A AD623B AD623A & AD623B
1 1200 600 12 11 622 700 350 7 6 455 400 200 4 3 3810 300 150 3 2 3520 250 125 2.5 1.5 3550 220 110 2.2 1.2 35100 210 105 2.1 1.1 351000 200 100 2 1 35
INPUT PROTECTIONInternal supply referenced clamping diodes allow the input,reference, output and gain terminals of the AD623 to safelywithstand overvoltages of 0.3 V above or below the supplies.This is true for all gains, and for power on and off. This lastcase is particularly important since the signal source and ampli-fier may be powered separately.
If the overvoltage is expected to exceed this value, the currentthrough these diodes should be limited to about 10 mA usingexternal current limiting resistors. This is shown in Figure 42.The size of this resistor is defined by the supply voltage and therequired overvoltage protection.
VOVER
VOVER
RLIM
RLIM RLIM = VOVER2VS +0.7V
1 = 10mA MAX
+VS
2VS
OUTPUT
RGAD623
10mA
Figure 42. Input Protection
RF INTERFERENCEAll instrumentation amplifiers can rectify high frequency out-of-band signals. Once rectified, these signals appear as dc offseterrors at the output. As shown in Figure 43, a low-pass filter canbe used to prevent unwanted noise from reaching the differen-tial inputs. A capacitor is connected across the inputs of theinstrumentation amplifier and forms a differential low-pass filterwith the two resistors. An additional benefit of using a differen-tially connected capacitor is that it reduces common-modecapacitive imbalance, which helps to preserve high frequencycommon-mode rejection. In applications where the sensor is anRTD or a resistive strain gauge, the filter resistors can be omit-ted if the sensor is physically close to the amplifier inputs. It isimportant to note that resistor tolerance or mismatch, poorlayout and excessive resistor thermal noise (caused by largeresistor values) can all contribute to degrading the effectivenessof this filter.
RG
RG
RG
VIN
+VS
+2.5V TO +6V
0.1mF 10mF
VOUT
REF (INPUT)
OUTPUTREF
–VS
0.1mF 10mF
–2.5V TO –6V
+
RG
RG
RG
VIN
+VS +3V TO +12V
0.1mF 10mF
VOUT
REF (INPUT)
OUTPUTREF
+
a. Dual Supply b. Single Supply
Figure 41. Basic Connections
AD623
–13–REV. A
RG
50V
R–IN
1IN
1.5nFC VOUT
EXTERNALFILTER
LOCATE AS CLOSE TOTHE INPUT PINS AS POSSIBLE
1VS
2VS
AD623
REFERENCE50V
R
Figure 43. Circuit to Attenuate RF Interference
In many applications shielded cables are used to minimize noise;for best CMR over frequency the shield should be properlydriven. Figure 44 shows an active guard drive that is configuredto improve ac common-mode rejection by “bootstrapping” thecapacitances of input cable shields, thus minimizing the capaci-tance mismatch between the inputs.
RG2
–INPUT
+INPUT
RG2
100V
AD623
VOUT
AD8031
+VS
REFERENCE
–VS
+
–
+
Figure 44. Common-Mode Shield Driver
GROUNDINGSince the AD623 output voltage is developed with respect to thepotential on the reference terminal, many grounding problemscan be solved by simply by tying the REF pin to the appropri-ate “local ground.” The REF pin should, however, be tied to alow impedance point for optimal CMR.
The use of ground planes is recommended to minimize theimpedance of ground returns (and hence the size of dc errors).In order to isolate low level analog signals from a noisy digitalenvironment, many data-acquisition components have separateanalog and digital ground returns (Figure 45). All ground pinsfrom mixed signal components such as analog-to-digital con-verters should be returned through the “high quality” analogground plane. Maximum isolation between analog and digital isachieved by connecting the ground planes back at the supplies.The digital return currents from the ADC, which flow in theanalog ground plane will, in general, have a negligible effect onnoise performance.
If there is only a single power supply available, it must be sharedby both digital and analog circuitry. Figure 46 shows how tominimize interference between the digital and analog circuitry.As in the previous case, separate analog and digital groundplanes should be used (reasonably thick traces can be used as analternative to a digital ground plane). These ground planesshould be connected at the power supply’s ground pin. Separatetraces should be run from the power supply to the supply pins ofthe digital and analog circuits. Ideally, each device should haveits own power supply trace, but these can be shared by a num-ber of devices as long as a single trace is not used to route cur-rent to both digital and analog circuitry.
ANALOG POWER SUPPLY
+5V –5V GND GND +5V
DIGITAL POWER SUPPLY
0.1mF
VIN1
VIN2
VDD AGND DGND
AD7892-2ADC
12 AGND VDD
mPROCESSOR
0.1mF
0.1mF
0.1mF
AD623
Figure 45. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies
ADC
12 VDD
mPROCESSOR
0.1mF
VINVDD AGND DGND
AD7892-2
0.1mF
0.1mF
AD623DGND
POWER SUPPLY
GND15V
Figure 46. Optimal Ground Practice in a Single Supply Environment
AD623
–14– REV. A
Ground Returns for Input Bias CurrentsInput bias currents are those dc currents that must flow in orderto bias the input transistors of an amplifier. These are usuallytransistor base currents. When amplifying “floating” input sourcessuch as transformers or ac-coupled sources, there must be adirect dc path into each input in order that the bias current canflow. Figure 47 shows how a bias current path can be providedfor the cases of transformer coupling, capacitive ac-coupling andfor a thermocouple application. In dc-coupled resistive bridgeapplications, providing this path is generally not necessary as thebias current simply flows from the bridge supply through thebridge and into the amplifier. However, if the impedances thatthe two inputs see are large and differ by a large amount (>10 kΩ),the offset current of the input stage will cause dc errors propor-tional with the input offset voltage of the amplifier.
RG
VOUT
–INPUT
+INPUT
+VS
+VS
AD623
LOAD
TO POWERSUPPLYGROUND
REFERENCE
–
+
Figure 47a. Ground Returns for Bias Currents with Transformer Coupled Inputs
RG AD623VOUT
–INPUT
+INPUT
2VS
+VS
LOAD
TO POWERSUPPLYGROUND
REFERENCE+
–
Figure 47b. Ground Returns for Bias Currents with Thermocouple Inputs
100kV 100kV
+INPUT
–INPUT
RG AD623
–VS
+VS
VOUT
LOAD
TO POWERSUPPLYGROUND
REFERENCE+
–
Figure 47c. Ground Returns for Bias Currents with ACCoupled Inputs
Output BufferingThe AD623 is designed to drive loads of 10 kΩ or greater. If theload is less that this value, the AD623’s output should be buff-ered with a precision single supply op amp such as the OP113.
This op amp can swing from 0 V to 4 V on its output whiledriving a load as small as 600 Ω. Table III summarizes the per-formance of some other buffer op amps.
VOUT
+5V+5V
0.1mF 0.1mF
RGVIN OP113AD623
REF
++
– –
Figure 48. Output Buffering
Table III. Buffering Options
Op Amp Comments
OP113 Single Supply, High Output CurrentOP191 Rail-to-Rail Input and Output, Low Supply CurrentOP150 Rail-to-Rail Input and Output, High Output Current
A Single Supply Data Acquisition SystemInterfacing bipolar signals to single supply analog to digitalconverters (ADCs) presents a challenge. The bipolar signalmust be “mapped” into the input range of the ADC. Figure 49shows how this translation can be achieved.
0.1mF+5V+5V
0.1mF
RG
REFOUT
REFIN
AIN
AD7776
AD623 REF
+5V
1.02kV610mV
+
Figure 49. A Single Supply Data Acquisition System
The bridge circuit is excited by a +5 V supply. The full-scaleoutput voltage from the bridge (± 10 mV) therefore has acommon-mode level of 2.5 V. The AD623 removes the common-mode component and amplifies the input signal by a factor of100 (RGAIN = 1.02 kΩ). This results in an output signal of ±1 V.In order to prevent this signal from running into the AD623’sground rail, the voltage on the REF pin has to be raised to atleast 1 V. In this example, the 2 V reference voltage from theAD7776 ADC is used to bias the AD623’s output voltage to2 V ±1 V. This corresponds to the input range of the ADC.
Amplifying Signals with Low Common-Mode VoltageBecause the common-mode input range of the AD623 extends0.1 V below ground, it is possible to measure small differentialsignals which have low, or no, common mode component. Fig-ure 50 shows a thermocouple application where one side of theJ-type thermocouple is grounded.
AD623
–15–REV. A
RG1.02kV
+5V
0.1mF
AD623
REF
J-TYPETHERMOCOUPLE
2V
VOUT
+
–
Figure 50. Amplifying Bipolar Signals with Low Common-Mode Voltage
Over a temperature range from –200°C to +200°C, the J-typethermocouple delivers a voltage ranging from –7.890 mV to10.777 mV. A programmed gain on the AD623 of 100 (RG =1.02 kΩ) and a voltage on the AD623 REF pin of 2 V, results inthe AD623’s output voltage ranging from 1.110 V to 3.077 Vrelative to ground.
INPUT DIFFERENTIAL AND COMMON-MODE RANGEVS. SUPPLY AND GAINFigure 51 shows a simplified block diagram of the AD623. Thevoltages at the outputs of the amplifiers A1 and A2 are given bythe equations
VA1 = VCM + VDIFF/2 + 0.6 V + VDIFF × RF/RG
= VCM + 0.6 V + VDIFF × (1/2 + RF/RG)
= VCM + 0.6 V + VDIFF × Gain/2
VA2 = VCM – VDIFF /2 + 0.6 V – VDIFF × RF/RG
= VCM + 0.6 V – VDIFF × (1/2 + RF/RG)
= VCM + 0.6 V – VDIFF × Gain/2
The voltages on these internal nodes are critical in determiningwhether or not the output voltage will be clipped. The voltagesVA1 and VA2 can swing from about 10 mV above the negativesupply (V– or Ground) to within about 100 mV of the positiverail (V+) before clipping occurs. Based on this, the maximumand minimum input common-mode voltages are given by theequations
VCMMAX = V+ – 100 mV – 0.6 V – VDIFF × Gain/2
VCMMIN = V– + 10 mV – 0.6 V + VDIFF × Gain/2
Table IV. Maximum Gain and Resulting Output Swing for Different Input Conditions
Supply Max Closest 1% Resulting OutputVCM VDIFF REF Pin Voltages Gain Gain Resistor Gain Swing
0 V ±10 mV 2.5 V +5 V 118 845 119.3 ±1.2 V0 V ±100 mV 2.5 V +5 V 11.8 9.31K 10.7 ±1 V0 V ±10 mV 0 V ±5 V 430 232 432 ±4.32 V0 V ±100 mV 0 V ±5 V 43 2.37K 43.2 ±4.32 V0 V ±1 V 0 V ±5 V 4.3 30.1K 4.32 ±4.32 V1.9 V ±10 mV 2.5 V +5 V 240 422 237.9 ±2.37 V1.9 V ±100 mV 2.5 V +5 V 24 4.32K 24.1 ±2.4 V1.9 V ±1 V 2.5 V +5 V 2.4 71.5K 2.4 ±2.4 V2.5 V ±10 mV 2.5 V +5 V 180 562 178.9 ±1.79 V2.5 V ±100 mV 2.5 V +5 V 18 5.9K 17.95 ±1.79 V2.5 V ±1 V 2.5 V +5 V 1.8 124K 1.80 ±1.80 V
50kV
VOUT6
REF5
50kV
A3
50kV
50kV
50kV1
7POS SUPPLY
INVERTING2
4
8
4NEG SUPPLY
NON-INVERTING
3
7
GAIN
2
VDIFF
2
VDIFF
VCM 50kV
RF
RF
A2
A1
RG
+
–
+
+
–
–
–
+
+
–
Figure 51. Simplified Block Diagram
These equations can be rearranged to give the maximum posi-tive and negative differential voltage for a particular (fixed)common-mode voltage and gain, i.e.,
VDIFFMAX = (V+ – 100 mV – 0.6 V – VCM)/Gain
VDIFFMIN = (VCM – V– – 10 mV + 0.6 V)/Gain
Note that these equations assume that the differential inputvoltage is bipolar. So the REF pin would have to be biased atmidsupply for maximum swing. If the differential input signal isunipolar (say positive), the REF pin can be grounded, so thatthe signal size can be double the value indicated in the aboveequation.
We can also rearrange these equations to predict the maximumprogrammable gain for a fixed set of input conditions, i.e.,
GainMAX = (V+ – 100 mV – 0.6 V – VCM)/VDIFFMAX
GainMAX = (VCM – V– – 10 mV + 0.6 V)/VDIFFMIN
The maximum gain will be the lesser of these two results. Notethat these equations also assume a bipolar input signal and thatthe REF pin is at midsupply. The maximum gain and resultingoutput swing is given for different input conditions in Table IV.Note that output voltages are referenced to the voltage on theREF pin.
AD623
–16– REV. A
OUTLINE DIMENSIONSDimensions shown in inches and (mm).
C32
02a–
2–12
/97
PR
INT
ED
IN U
.S.A
.
8-Lead Plastic DIP(N-8)
8
1 4
5
0.430 (10.92)0.348 (8.84)
0.280 (7.11)0.240 (6.10)
PIN 1
SEATINGPLANE
0.022 (0.558)0.014 (0.356)
0.060 (1.52)0.015 (0.38)
0.210 (5.33)MAX 0.130
(3.30)MIN
0.070 (1.77)0.045 (1.15)
0.100(2.54)BSC
0.160 (4.06)0.115 (2.93)
0.325 (8.25)0.300 (7.62)
0.015 (0.381)0.008 (0.204)
0.195 (4.95)0.115 (2.93)
8-Lead SOIC(R-8)
8 5
41
0.1968 (5.00)0.1890 (4.80)
0.1574 (4.00)0.1497 (3.80)
0.2440 (6.20)0.2284 (5.80)
PIN 1
SEATINGPLANE
0.0098 (0.25)0.0040 (0.10)
0.0192 (0.49)0.0138 (0.35)
0.102 (2.59)0.094 (2.39)
0.0500(1.27)BSC
0.0098 (0.25)0.0075 (0.19)
0.0500 (1.27)0.0160 (0.41)
8°0°
0.0196 (0.50)0.0099 (0.25)
x 45°
8-Lead microSOIC(RM-8)
8 5
41
0.122 (3.10)0.114 (2.90)
0.199 (5.05)0.187 (4.75)
PIN 1
0.0256 (0.65) BSC
0.122 (3.10)0.114 (2.90)
SEATINGPLANE
0.006 (0.15)0.002 (0.05)
0.018 (0.46)0.008 (0.20)
0.043 (1.09)0.037 (0.94)
0.120 (3.05)0.112 (2.84)
0.011 (0.28)0.003 (0.08)
0.028 (0.71)0.016 (0.41)
33°27°
0.120 (3.05)0.112 (2.84)