SM73308
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SNOSB90B – JUNE 2011 – REVISED APRIL 2013
Low Offset, Low Noise, RRO Operational Amplifier
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FEATURES
DESCRIPTION
1
The SM73308 is a single low noise precision
operational amplifier intended for use in a wide range
of applications. Other important characteristics
include: an extended operating temperature range of
−40°C to 125°C, the tiny SC70-5 package, and low
input bias current.
(Unless Otherwise Noted, Typical Values at VS
= 2.7V)
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Renewable Energy Grade
Ensured 2.7V and 5V Specifications
Maximum VOS 850μV (Limit)
Voltage noiseN
– f = 100 Hz 12.5nV/√Hz
– f = 10 kHz 7.5nV/√Hz
Rail-to-Rail Output Swing
– RL = 600Ω 100mV From Rail
– RL = 2kΩ 50mV From Rail
Open Loop Gain With RL = 2kΩ 100dB
VCM 0 to V+ −0.9V
Supply Current 550µA
Gain Bandwidth Product 3.5MHz
Temperature Range −40°C to 125°C
The extended temperature range of −40°C to 125°C
allows the SM73308 to accommodate a broad range
of applications. The SM73308 expands TI’s Silicon
Dust™ amplifier portfolio offering enhancements in
size, speed, and power savings. The SM73308 is
ensured to operate over the voltage range of 2.7V to
5.0V and has rail-to-rail output.
The SM73308 is designed for precision, low noise,
low voltage, and miniature systems. This amplifier
provides rail-to-rail output swing into heavy loads.
The maximum input offset is 850 μV at room
temperature and the input common mode voltage
range includes ground.
The SM73308 is offered in the tiny SC70-5 package.
APPLICATIONS
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Transducer Amplifier
Instrumentation Amplifier
Precision Current Sensing
Data Acquisition Systems
Active Filters and Buffers
Sample and Hold
Portable/battery Powered Electronics
Automotive
Connection Diagram
1
5
+IN
V
2
+
+
GND
-
-IN
3
4
VOUT
Figure 1. SC70-5 – Top View
See Package Number DCK
1
2
3
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
Silicon Dust is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2011–2013, Texas Instruments Incorporated
SM73308
SNOSB90B – JUNE 2011 – REVISED APRIL 2013
www.ti.com
Instrumentation Amplifier
V1
+
V01
-
R2
KR2
R1
R1
R11 = a
+
R1
V2
+
VOUT
V02
R2
KR2
VO = -K (2a + 1) (V1 - V2)
(1)
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
Machine Model
ESD Tolerance (3)
200V
Human Body Model
2000V
Differential Input Voltage
± Supply Voltage
+
Voltage at Input Pins
(V ) + 0.3V, (V–) – 0.3V
Current at Input Pins
±10 mA
Supply Voltage (V+–V −)
5.75V
+
See (4)
−
See (5)
Output Short Circuit to V
Output Short Circuit to V
Mounting Temperture
Infrared or Convection (20 sec)
235°C
Wave Soldering Lead Temp (10 sec)
260°C
−65°C to 150°C
Storage Temperature Range
Junction Temperature (6)
(1)
(2)
(3)
(4)
(5)
(6)
150°C
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For ensured specifications and the test
conditions, see the Electrical Characteristics.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
Human Body Model is 1.5 kΩ in series with 100 pF. Machine Model is 0Ω in series with 20 pF.
Shorting output to V+ will adversely affect reliability.
Shorting output to V− will adversely affect reliability.
The maximum power dissipation is a function of TJ(MAX) , θJA, and TA. The maximum allowable power dissipation at any ambient
temperature is PD = (TJ(MAX)–T A) / θJA. All numbers apply for packages soldered directly into a PC board.
Operating Ratings (1)
Supply Voltage
2.7V to 5.5V
−40°C to 125°C
Temperature Range
Thermal Resistance (θJA)
(1)
2
440 °C/W
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For ensured specifications and the test
conditions, see the Electrical Characteristics.
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SNOSB90B – JUNE 2011 – REVISED APRIL 2013
2.7V DC Electrical Characteristics (1)
Unless otherwise specified, all limits are ensured for TA = 25°C. V+ = 2.7V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol
Parameter
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Average Drift
(4)
IB
Input Bias Current
IOS
Input Offset Current (4)
Condition
Min (2)
Typ (3)
Max (2)
Units
0.3
0.85
1.0
mV
−0.45
VCM = 1V
100
250
pA
0.004
100
pA
550
900
910
µA
IS
Supply Current
CMRR
Common Mode Rejection Ratio
0.5 ≤ VCM ≤ 1.2V
74
72
80
PSSR
Power Supply Rejection Ratio
2.7V ≤ V+ ≤ 5V
82
76
90
VCM
Input Common-Mode Voltage Range For CMRR ≥ 50dB
Large Signal Voltage Gain (5)
AV
VO
Output Swing
IO
(1)
(2)
(3)
(4)
(5)
Output Short Circuit Current
µV/°C
−0.1
0
dB
dB
1.8
RL = 600Ω to 1.35V,
VO = 0.2V to 2.5V
92
80
100
RL = 2kΩ to 1.35V,
VO = 0.2V to 2.5V
98
86
100
RL = 600Ω to 1.35V
VIN = ± 100mV
0.11
0.14
0.084 to
2.62
2.59
2.56
RL = 2kΩ to 1.35V
VIN = ± 100mV
0.05
0.06
0.026 to
2.68
2.65
2.64
Sourcing, VO = 0V
VIN = 100mV
18
11
24
Sinking, VO = 2.7V
VIN = −100mV
18
11
22
V
dB
V
mA
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA.
All limits are ensured by testing or statistical analysis.
Typical values represent the most likely parametric norm.
Limits ensured by design.
RL is connected to mid-supply. The output voltage is set at 200mV from the rails. VO = GND + 0.2V and VO = V+ −0.2V
2.7V AC Electrical Characteristics (1)
Unless otherwise specified, all limits are ensured for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol
Parameter
(4)
Conditions
SR
Slew Rate
GBW
Gain-Bandwidth Product
Φm
Gm
en
Input-Referred Voltage Noise
(Flatband)
f = 10kHz
en
Input-Referred Voltage Noise (l/f)
in
Input-Referred Current Noise
Total Harmonic Distortion
THD
(1)
(2)
(3)
(4)
AV = +1, RL = 10 kΩ
Min (2)
Typ (3)
Max (2)
Units
1.4
V/µs
3.5
MHz
Phase Margin
79
Deg
Gain Margin
−15
dB
7.5
nV/√Hz
f = 100Hz
12.5
nV/√Hz
f = 1kHz
0.001
pA/√Hz
f = 1kHz, AV = +1
RL = 600Ω, VIN = 1 VPP
0.007
%
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA.
All limits are ensured by testing or statistical analysis.
Typical values represent the most likely parametric norm.
The number specified is the slower of positive and negative slew rates.
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SM73308
SNOSB90B – JUNE 2011 – REVISED APRIL 2013
5.0V DC Electrical Characteristics
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(1)
Unless otherwise specified, all limits are ensured for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol
Parameter
Condition
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Average Drift
(4)
IB
Input Bias Current
IOS
Input Offset Current (4)
Min (2)
Typ (3)
Max (2)
Units
0.25
0.85
1.0
mV
−0.35
VCM = 1V
µV/°C
−0.23
100
250
pA
0.017
100
pA
600
950
960
µA
IS
Supply Current
CMRR
Common Mode Rejection Ratio
0.5 ≤ VCM ≤ 3.5V
80
79
90
PSRR
Power Supply Rejection Ratio
2.7V ≤ V+ ≤ 5V
82
76
90
VCM
Input Common-Mode Voltage Range For CMRR ≥ 50dB
0
RL = 600Ω to 2.5V,
VO = 0.2V to 4.8V
92
89
100
RL = 2kΩ to 2.5V,
VO = 0.2V to 4.8V
98
95
100
RL = 600Ω to 2.5V
VIN = ± 100mV
0.15
0.23
0.112 to
4.9
4.85
4.77
RL = 2kΩ to 2.5V
VIN = ± 100mV
0.06
0.07
0.035 to
4.97
4.94
4.93
Sourcing, VO = 0V
VIN = 100mV
35
35
75
Sinking, VO = 2.7V
VIN = −100mV
35
35
66
Large Signal Voltage Gain (5)
AV
VO
Output Swing
IO
(1)
(2)
(3)
(4)
(5)
(6)
Output Short Circuit Current
(4) (6)
dB
dB
4.1
V
dB
V
mA
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA.
All limits are ensured by testing or statistical analysis.
Typical values represent the most likely parametric norm.
Limits ensured by design.
RL is connected to mid-supply. The output voltage is set at 200mV from the rails. VO = GND + 0.2V and VO = V+ −0.2V
Continuous operation of the device with an output short circuit current larger than 35mA may cause permanent damage to the device.
5.0V AC Electrical Characteristics (1)
Unless otherwise specified, all limits are ensured for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol
Parameter
SR
Slew Rate (4)
GBW
Φm
Conditions
Typ (3)
Max (2)
Units
1.4
V/µs
Gain-Bandwidth Product
3.5
MHz
Phase Margin
79
Deg
Gm
Gain Margin
−15
dB
en
Input-Referred Voltage Noise
(Flatband)
f = 10kHz
6.5
nV/√Hz
en
Input-Referred Voltage Noise (l/f)
f = 100Hz
12
nV/√Hz
in
Input-Referred Current Noise
f = 1kHz
0.001
pA/√Hz
THD
Total Harmonic Distortion
f = 1kHz, AV = +1
RL = 600Ω, VIN = 1 VPP
0.007
%
(1)
(2)
(3)
(4)
4
AV = +1, RL = 10 kΩ
Min (2)
Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device such that TJ = TA.
All limits are ensured by testing or statistical analysis.
Typical values represent the most likely parametric norm.
The number specified is the slower of positive and negative slew rates.
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Typical Performance Characteristics
VOS
vs.
VCM Over Temperature
VOS
vs.
VCM Over Temperature
4
3
VS = 2.7V
-40°C
2.5
-40°C
25°C
3
25°C
85°C
2
2.5
85°C
125°C
1.5
VOS (mV)
VOS (mV)
VS = 5V
3.5
125°C
1
0.5
2
1.5
1
0.5
0
0
-0.5
-1
-0.5
-0.5
0
0.5
1.5
1
2
-1
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
2.5
4.5 5
VCM (V)
VCM (V)
Figure 2.
Figure 3.
Output Swing
vs.
VS
Output Swing
vs.
VS
40
120
RL = 2k:
NEGATIVE SWING
VOUT FROM VSUPPLY (mV)
VOUT FROM VSUPPLY (mV)
110
100
90
80
POSITIVE SWING
70
60
50
40
2.5
RL = 600:
TA = 25°C
35
NEGATIVE SWING
30
POSITIVE SWING
25
TA = 25°C
3
3.5
4.5
4
5
20
2.5
5.5
3
3.5
Output Swing
vs.
VS
IS
vs.
VS Over Temperature
-40°C
0.6
SUPPLY CURRENT (mA)
-
VOUT FROM V (mV)
0.6
POSITIVE SWING
0.5
0.4
0.3
0
2.5
5.5
0.7
NEGATIVE SWING
0.7
0.1
5
Figure 5.
0.8
0.2
4.5
Figure 4.
1
0.9
4
VS (V)
VS (V)
RL = 100k:
25°C
0.4
85°C
125°C
0.3
0.2
0.1
TA = 25°C
3
0.5
3.5
4
4.5
VS (V)
5
5.5
0
2.5
3
3.5
4
4.5
5
5.5
SUPPLY VOLTAGE (V)
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
VIN
vs.
VOUT
VIN
vs.
VOUT
500
500
VS = ±1.35V
400
RL = 2k:
100
0
RL = 600:
-100
-200
200
RL = 2k:
100
0
RL = 600:
-100
-200
-300
-300
-400
-400
-500
-1.5
TA = 25°C
300
INPUT VOLTAGE (PV)
INPUT VOLTAGE (PV)
200
VS = ±2.5V
400
TA = 25°C
300
-500
-1
0.5
0
0.5
1
1.5
-3
-2
-1
OUTPUT VOLTAGE (V)
1
2
Figure 8.
Figure 9.
Sourcing Current
vs.
VOUT (1)
Sourcing Current
vs.
VOUT (1)
3
0
0
VS = 2.7V
-5
-10
VS = 5V
-20
-15
ISOURCE (mA)
-10
ISOURCE (mA)
0
OUTPUT VOLTAGE (V)
125°C
-20
85°C
-25
-30
-30
-50
25°C
85°C
-60
-70
-35
125°C
-40
25°C
-80
-40
-40°C
-90
-40°C
-100
-45
0
1
0.5
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
3
-
-
VOUT FROM V (V)
VOUT FROM V (V)
Figure 10.
Figure 11.
Sinking Current
vs.
VOUT (1)
Sinking Current
vs.
VOUT (1)
100
40
VS = 2.7V
-40°C
-40°C
90
80
25°C
25°C
70
ISINK (mA)
ISINK (mA)
30
85°C
20
60
50
125°C
40
85°C
125°C
30
10
20
10
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
+
(1)
6
VS = 5V
0
0
VOUT REFERENCED TO V (V)
VOUT FROM V
Figure 12.
Figure 13.
+
Continuous operation of the device with an output short circuit current larger than 35mA may cause permanent damage to the device.
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Typical Performance Characteristics (continued)
Input Voltage Noise
vs.
Frequency
Input Bias Current Over Temperature
INPUT VOLTAGE NOISE (nV/ Hz)
35
30
25
20
15
VS = 2.7V
10
VS = 5V
5
0
10
100
1k
10k
FREQUENCY (Hz)
Figure 14.
Figure 15.
Input Bias Current Over Temperature
Input Bias Current Over Temperature
500
50
T = 25°C
300
200
100
VS = 2.7V
0
-100
VS = 5V
-200
-300
T = -40°C
40
INPUT BIAS CURRENT (fA)
INPUT BIAS CURRENT (fA)
400
30
20
VS = 2.7V
10
0
-10
-20
VS = 5V
-30
-400
-40
-500
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
-50
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
VCM (V)
VCM (V)
Figure 16.
Figure 17.
THD+N
vs.
Frequency
THD+N
vs.
VOUT
1
10
RL = 600:
AV = +10
VS = 5V, VO = 2.5VPP
VS = 2.7V, VO = 1VPP
0.1
AV = +1
AV = +10
0.1
THD+N (%)
THD+N (%)
1
VS = 2.7V
0.01
0.01
AV = +1
VS = 5V, VO = 1VPP
VS = 5V
VS = 2.7V, VO = 1VPP
0.001
10
100
1k
10k
100k
0.001
0.1
FREQUENCY (Hz)
1
10
VOUT (VPP)
Figure 18.
Figure 19.
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Typical Performance Characteristics (continued)
Slew Rate
vs.
Supply Voltage
Open Loop Frequency Response Over Temperature
SLEW RATE (V/Ps)
1.8
90
70
RL = 10k:
80
60
VIN = 2VPP
25°C
70
50
1.6
GAIN (dB)
1.7
RISING EDGE
1.5
1.4
125°C
GAIN
40
60
50
30
-40°C
40
20
125°C
30
10
FALLING EDGE
1.2
0
1.1
-10
3.5
4
4.5
25°C
RL = 2k:
-20
1k
10k
5
SUPPLY VOLTAGE (V)
RL = 600:
60
RL = 100k:
GAIN
40
90
70
80
60
60
RL = 2k:
50
RL = 100k:
RL = 600:
10
0
VS = 2.7V
-20
1k
10k
40
100k
1M
0
10
-10
0
-20
1k
CL = 100pF
GAIN
80
90
70
10
VS = 5V
0
10k
100k
80
60
70
50
60
30
50
20
40
CL = 1000pF
CL = 500pF
0
CL = 0pF
VS = 5V
RL = 600:
-20
1k
10k
CL = 100pF
1M
40
100
10
0
10M
-10
70
CL = 100pF
GAIN
60
50
40
CL = 1000pF
30
CL = 500pF
VS = 5V
20
CL = 0pF
RL = 100k:
-20
1k
10k
CL = 100pF
10
0
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 24.
Figure 25.
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90
80
20
20
10
10M
CL = 0pF
30
30
0
100k
1M
PHASE
GAIN (dB)
GAIN (dB)
50
20
Open Loop Gain & Phase with Cap. Loading
100
PHASE (°)
CL = 0pF
40
30
RL = 2k:
20
Figure 23.
60
-10
RL = 600:
Figure 22.
PHASE
10
50
RL = 100k:
FREQUENCY (Hz)
70
60
RL = 2k:
20
Open Loop Gain & Phase with Cap. Loading
90
70
30
FREQUENCY (Hz)
80
40
GAIN
40
10
10M
100
80
RL = 600:
50
30
RL = 2k:
-10
RL = 100k:
PHASE
70
30
20
80
GAIN (dB)
GAIN (dB)
50
0
10M
Open Loop Frequency Response
100
PHASE (°)
70
1M
Figure 21.
Open Loop Frequency Response
PHASE
100k
FREQUENCY (Hz)
Figure 20.
80
10
PHASE (°)
3
20
VS = 5V
PHASE (°)
1.3
1
2.5
8
-40°C
PHASE
AV = +1
PHASE (°)
1.9
100
80
2
1M
10M
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Typical Performance Characteristics (continued)
Non-Inverting Large Signal Pulse Response
INPUT SIGNAL
VS = ±2.5V
TA = -40°C
RL = 2k:
TA = -40°C
RL = 2k:
OUTPUT SIGNAL
(1 V/div)
VS = ±2.5V
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
Non-Inverting Small Signal Pulse Response
TIME (10 Ps/div)
Figure 26.
Figure 27.
Non-Inverting Small Signal Pulse Response
Non-Inverting Large Signal Pulse Response
VS = ±2.5V
INPUT SIGNAL
VS = ±2.5V
RL = 2k:
TA = 25°C
RL = 2k:
OUTPUT SIGNAL
(1 V/div)
TA = 25°C
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
TIME (10 Ps/div)
TIME (10 Ps/div)
Figure 29.
Non-Inverting Small Signal Pulse Response
Non-Inverting Large Signal Pulse Response
VS = ±2.5V
TA = 125°C
RL = 2k:
INPUT SIGNAL
OUTPUT SIGNAL
(1 V/div)
Figure 28.
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
TIME (10 Ps/div)
TIME (10 Ps/div)
VS = ±2.5V
TA = 125°C
RL = 2k:
TIME (10 Ps/div)
Figure 30.
Figure 31.
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Typical Performance Characteristics (continued)
INPUT SIGNAL
Inverting Large Signal Pulse Response
TA = -40°C
RL = 2k:
OUTPUT SIGNAL
VS = ±2.5V
TA = -40°C
RL = 2k:
(1 V/div)
VS = ±2.5V
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
Inverting Small Signal Pulse Response
TIME (10 Ps/div)
Figure 33.
Inverting Small Signal Pulse Response
Inverting Large Signal Pulse Response
VS = ±2.5V
TA = 25°C
RL = 2k:
OUTPUT SIGNAL
VS = ±2.5V
TA = 25°C
RL = 2k:
(1 V/div)
INPUT SIGNAL
Figure 32.
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
TIME (10 Ps/div)
TIME (10 Ps/div)
Figure 35.
Inverting Small Signal Pulse Response
Inverting Large Signal Pulse Response
TA = 125°C
RL = 2k:
OUTPUT SIGNAL
TIME (10 Ps/div)
TA = 125°C
RL = 2k:
TIME (10 Ps/div)
Figure 36.
10
VS = ±2.5V
(1 V/div)
VS = ±2.5V
INPUT SIGNAL
Figure 34.
(50 mV/div)
OUTPUT SIGNAL
INPUT SIGNAL
TIME (10 Ps/div)
Figure 37.
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Typical Performance Characteristics (continued)
Stability
vs.
VCM
Stability
vs.
VCM
500
250
400
CAPACITIVE LOAD (pF)
CAPACITIVE LOAD (pF)
450
350
25% OVERSHOOT
300
250
200
VS = ±2.5V
150
AV = +1
100
200
25% OVERSHOOT
150
100
VS = ±2.5V
AV = +1
50
RL = 2k:
50
RL = 1M:
VO = 100mV
VO = 100mV
0
0
-2
-1.5
-1
-0.5
0
0.5
1
-2
1.5
-1.5
-1
VCM (V)
0
0.5
1
1.5
VCM (V)
Figure 38.
Figure 39.
PSRR
vs.
Frequency
CMRR
vs.
Frequency
100
140
RL = 100k:
RL = 5 k:
90
120
80
VS = 2.7V, -PSRR
70
100
VS = 2.7V, +PSRR
80
CMRR (dB)
PSRR (dB)
-0.5
VS = 5V, +PSRR
60
VS = 5V, -PSRR
50
40
30
40
VS = 5V
60
VS = 2.7V
20
20
10
0
100
1k
10k
100k
1M
0
100
1k
10k
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 40.
Figure 41.
1M
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APPLICATION NOTE
SM73308
The SM73308 is a precision amplifier with very low noise and ultra low offset voltage. SM73308's extended
temperature range of −40°C to 125°C enables the user to design a variety of applications including automotive.
The SM73308 has a maximum offset voltage of 1mV over the extended temperature range. This makes the
SM73308 ideal for applications where precision is important.
INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires close attention to the input impedance of the
amplifier, gain of the overall signal on the inputs, and the gain on each input since we are only interested in the
difference of the two inputs and the common signal is considered noise. A classic solution is an instrumentation
amplifier. Instrumentation amplifiers have a finite, accurate, and stable gain. Also they have extremely high input
impedances and very low output impedances. Finally they have an extremely high CMRR so that the amplifier
can only respond to the differential signal. A typical instrumentation amplifier is shown in Figure 42.
V1
+
V01
-
R2
KR2
R1
R1
R11 = a
+
R1
V2
+
VOUT
V02
R2
KR2
Figure 42. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, output stage, is a differential amplifier. In an ideal case the
two amplifiers of the first stage, input stage, would be set up as buffers to isolate the inputs. However they
cannot be connected as followers because of real amplifier's mismatch. That is why there is a balancing resistor
between the two. The product of the two stages of gain will give the gain of the instrumentation amplifier. Ideally,
the CMRR should be infinite. However the output stage has a small non-zero common mode gain which results
from resistor mismatch.
In the input stage of the circuit, current is the same across all resistors. This is due to the high input impedance
and low input bias current of the SM73308. With the node equations we have:
GIVEN: I R = I R
1
11
(2)
By Ohm’s Law:
VO1 - VO2 = (2R1 + R11) IR
11
= (2a + 1) R11 x IR
11
= (2a + 1) V R
11
(3)
However:
VR
11 = V1 - V2
(4)
So we have:
12
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(5)
Now looking at the output of the instrumentation amplifier:
KR2
VO =
R2
(VO2 - VO1)
= -K (VO1 - VO2)
(6)
Substituting from Equation 5:
VO = -K (2a + 1) (V1 - V2)
(7)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1)
(8)
Typical values for this circuit can be obtained by setting: a = 12 and K= 4. This results in an overall gain of −100.
Figure 43 shows typical CMRR characteristics of this Instrumentation amplifier over frequency. Three SM73308
amplifiers are used along with 1% resistors to minimize resistor mismatch. Resistors used to build the circuit are:
R1 = 21.6kΩ, R11 = 1.8kΩ, R2 = 2.5kΩ with K = 40 and a = 12. This results in an overall gain of −1000, −K(2a+1)
= −1000.
0
VS = ±2.5V
-20
VCM = 0V
VIN = 3VPP
CMRR (dB)
-40
-60
-80
-100
-120
-140
10
100
1k
10k
FREQUENCY (Hz)
Figure 43. CMRR vs. Frequency
ACTIVE FILTER
Active filters are circuits with amplifiers, resistors, and capacitors. The use of amplifiers instead of inductors,
which are used in passive filters, enhances the circuit performance while reducing the size and complexity of the
filter.
The simplest active filters are designed using an inverting op amp configuration where at least one reactive
element has been added to the configuration. This means that the op amp will provide "frequency-dependent"
amplification, since reactive elements are frequency dependent devices.
LOW PASS FILTER
The following shows a very simple low pass filter.
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C
R2
R1
Vi
VOUT
+
Figure 44. Lowpass Filter
The transfer function can be expressed as follows:
By KCL:
-Vi
VO
VO
-
R1
1
jwc
-
R2
=O
(9)
Simplifying this further results in:
-R2
1
R1
jwcR2 +1
VO =
Vi
(10)
or
VO
Vi
-R2
1
R1
jwcR2 +1
=
(11)
Now, substituting ω=2πf, so that the calculations are in f(Hz) and not ω(rad/s), and setting the DC gain HO =
−R2/R1 and H = VO/Vi
H = HO
1
j2SfcR2 +1
(12)
Set: fo = 1/(2πR1C)
H = HO
1
1 + j (f/fo)
(13)
Low pass filters are known as lossy integrators because they only behave as an integrator at higher frequencies.
Just by looking at the transfer function one can predict the general form of the bode plot. When the f/fO ratio is
small, the capacitor is in effect an open circuit and the amplifier behaves at a set DC gain. Starting at fO, −3dB
corner, the capacitor will have the dominant impedance and hence the circuit will behave as an integrator and
the signal will be attenuated and eventually cut. The bode plot for this filter is shown in the following picture:
14
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dB
|H|
|HO|
-20dB/dec
0
f = fo
f (Hz)
Figure 45. Lowpass Filter Transfer Function
HIGH PASS FILTER
In a similar approach, one can derive the transfer function of a high pass filter. A typical first order high pass filter
is shown below:
C
R1
R2
Vi
VOUT
+
Figure 46. Highpass FIlter
Writing the KCL for this circuit :
(V1 denotes the voltage between C and R1)
V1 - V
V1 - Vi
1
jwC
=
R1
(14)
-
-
V + VO
V + V1
R1
-
=
R2
(15)
Solving these two equations to find the transfer function and using:
fO =
1
2SR1C
(16)
VO
-R2
HO =
(high frequency gain)
R1
H=
and
Vi
Which results:
H = HO
j (f/fo)
1 + j (f/fo)
(17)
Looking at the transfer function, it is clear that when f/fO is small, the capacitor is open and hence no signal is
getting in to the amplifier. As the frequency increases the amplifier starts operating. At f = fO the capacitor
behaves like a short circuit and the amplifier will have a constant, high frequency, gain of HO. Figure 47 shows
the transfer function of this high pass filter:
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dB
|H|
|HO|
-20dB/dec
0
f = fo
f (Hz)
Figure 47. Highpass Filter Transfer Function
BAND PASS FILTER
C2
C1
R2
R1
Vi
VOUT
+
Figure 48. Bandpass Filter
Combining a low pass filter and a high pass filter will generate a band pass filter. In this network the input
impedance forms the high pass filter while the feedback impedance forms the low pass filter. Choosing the
corner frequencies so that f1 < f2, then all the frequencies in between, f1 ≤ f ≤ f2, will pass through the filter while
frequencies below f1 and above f2 will be cut off.
The transfer function can be easily calculated using the same methodology as before.
H = HO
j (f/f1)
[1 + j (f/f1)] [1 + j (f/f2)]
where
f1 =
1
2SR1C1
f2 =
1
2SR2C2
HO =
•
-R2
R1
(18)
The transfer function is presented in the following figure.
16
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|H
|
dB
|HO|
-20dB/dec
20dB/dec
0
f1
f (Hz)
f2
Figure 49. Bandpass filter Transfer Function
STATE VARIABLE ACTIVE FILTER
State variable active filters are circuits that can simultaneously represent high pass, band pass, and low pass
filters. The state variable active filter uses three separate amplifiers to achieve this task. A typical state variable
active filter is shown in Figure 50. The first amplifier in the circuit is connected as a gain stage. The second and
third amplifiers are connected as integrators, which means they behave as low pass filters. The feedback path
from the output of the third amplifier to the first amplifier enables this low frequency signal to be fed back with a
finite and fairly low closed loop gain. This is while the high frequency signal on the input is still gained up by the
open loop gain of the 1st amplifier. This makes the first amplifier a high pass filter. The high pass signal is then
fed into a low pass filter. The outcome is a band pass signal, meaning the second amplifier is a band pass filter.
This signal is then fed into the third amplifiers input and so, the third amplifier behaves as a simple low pass
filter.
R4
R1
C2
VIN
R2
-
A1
R5
C3
R3
VHP
+
-
A2
VBP
+
A3
+
VLP
R6
Figure 50. State Variable Active Filter
The transfer function of each filter needs to be calculated. The derivations will be more trivial if each stage of the
filter is shown on its own.
The three components are:
R4
R1
VO
R5
VIN
A1
+
VO1
R6
VO2
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C2
R2
VO1
A2
VO2
+
C3
R3
VO2
A3
V
O
+
For A1 the relationship between input and output is:
R6
-R4
VO1 =
R1
V0 +
R1 + R4
R1
R5 + R6
VIN +
R5
R1 + R4
R5 + R6
R1
VO2
(19)
This relationship depends on the output of all the filters. The input-output relationship for A2 can be expressed
as:
VO2 =
-1
VO1
s C 2R 2
(20)
And finally this relationship for A3 is as follows:
VO =
-1
s C 3R 3
VO2
(21)
Re-arranging these equations, one can find the relationship between VO and VIN (transfer function of the lowpass
filter), VO1 and VIN (transfer function of the highpass filter), and VO2 and VIN (transfer function of the bandpass
filter) These relationships are as follows:
Lowpass Filter
R 1 + R4
R1
VO
VIN
R6
1
R5 + R6 C2C3R2R3
=
2
s +s
1
R5
R1 + R4
C 2R 2
R5 + R6
R1
1
+
C2C3R2R3
(22)
Highpass Filter
s
VO1
VIN
2
R1 + R 4
R6
R1
R5 + R6
=
2
s +s
1
R5
R1 + R4
C 2R 2
R5 + R6
R1
1
+
C2C3R2R3
(23)
Bandpass Filter
1
R1 + R 4
R6
C 2R 2
R1
R5 + R6
s
VO2
VIN
=
2
s +s
18
1
R5
R1 + R4
C 2R 2
R5 + R6
R1
1
+
C2C3R2R3
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The center frequency and Quality Factor for all of these filters is the same. The values can be calculated in the
following manner:
1
Zc =
C 2 C 3R 2R 3
and
Q=
C 2R 2
R5 + R6
R1
C 3R 3
R6
R1 + R 4
(25)
A design example is shown here:
Designing a bandpass filter with center frequency of 10kHz and Quality Factor of 5.5
To do this, first consider the Quality Factor. It is best to pick convenient values for the capacitors. C2 = C3 =
1000pF. Also, choose R1 = R4 = 30kΩ. Now values of R5 and R6 need to be calculated. With the chosen values
for the capacitors and resistors, Q reduces to:
Q=
1
11
=
2
2
R5 + R6
R6
(26)
or
R5 = 10R6 R6 = 1.5kΩ R5 = 15kΩ
(27)
Also, for f = 10kHz, the center frequency is ωc = 2πf = 62.8kHz.
Using the expressions above, the appropriate resistor values will be R2 = R3 = 16kΩ.
The following graphs show the transfer function of each of the filters. The DC gain of this circuit is:
DC GAIN =
R1 + R4
R6
R1
R 5 + R6
= -14.8 dB
(28)
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REVISION HISTORY
Changes from Revision A (April 2013) to Revision B
•
20
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 19
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
SM73308MG/NOPB
ACTIVE
SC70
DCK
5
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
S08
SM73308MGE/NOPB
ACTIVE
SC70
DCK
5
250
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
S08
SM73308MGX/NOPB
ACTIVE
SC70
DCK
5
3000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
S08
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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