NSC LMP2232AMA Micropower, 1.8v, precision, operational amplifier with cmos input Datasheet

LMP2232 Dual
Micropower, 1.8V, Precision, Operational Amplifier with
CMOS Input
General Description
Features
The LMP2232 is a dual micropower precision amplifier designed for battery powered applications. The 1.8V to 5.0V
guaranteed supply voltage range and quiescent power consumption of only 29 μW extend the battery life in portable
systems. The LMP2232 is part of the LMP® precision amplifier
family. The high impedance CMOS input makes it ideal for
instrumentation and other sensor interface applications.
The LMP2232 has a maximum offset voltage of 150 μV and
maximum offset voltage drift of only 0.5 μV/°C along with low
bias current of only ±20 fA. These precise specifications make
the LMP2232 a great choice for maintaining system accuracy
and long term stability.
The LMP2232 has a rail-to-rail output that swings 15 mV from
the supply voltage, which increases system dynamic range.
The common mode input voltage range extends 200 mV below the negative supply, thus the LMP2232 is ideal for ground
sensing in single supply applications.
The LMP2232 is offered in 8-pin SOIC and MSOP packages.
The LMP2231 is the single version of this product and the
LMP2234 is the quad version of this product. Both of these
products are available on National Semiconductor's website.
(For VS = 5V, Typical unless otherwise noted)
10 µA
■ Supply current (per channel)
1.6V to 5.5V
■ Operating voltage range
±0.5 µV/°C (max)
■ Low TCVOS
±150 µV (max)
■ VOS
20 fA
■ Input bias current
120 dB
■ PSRR
97 dB
■ CMRR
120 dB
■ Open loop gain
130 kHz
■ Gain bandwidth product
58 V/ms
■ Slew rate
60 nV/√Hz
■ Input voltage noise, f = 1 kHz
–40°C to 125°C
■ Temperature range
Applications
■
■
■
■
■
Precision instrumentation amplifiers
Battery powered medical instrumentation
High impedance sensors
Strain gauge bridge amplifier
Thermocouple amplifiers
Typical Application
30033974
Strain Gauge Bridge Amplifier
LMP® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation
300339
www.national.com
LMP2232 Dual 1.8V, Micropower, Precision, Operational Amplifier with CMOS Input
January 15, 2008
LMP2232
Mounting Temperature
Infrared or Convection (20 sec.)
Wave Soldering Lead
Temperature (10 sec.)
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 2)
Human Body Model
Machine Model
Differential Input Voltage
Supply Voltage (VS = V+ - V–)
Voltage on Input/Output Pins
Storage Temperature Range
Junction Temperature (Note 3)
Operating Ratings
2000V
100V
±300 mV
6V
V+ + 0.3V, V– – 0.3V
−65°C to 150°C
150°C
+235°C
+260°C
(Note 1)
Operating Temperature Range (Note 3)
Supply Voltage (VS = V+ - V–)
−40°C to 125°C
1.6V to 5.5V
Package Thermal Resistance (θJA)(Note 3)
8-Pin SOIC
8-Pin MSOP
111.2 °C/W
147.4 °C/W
5V DC Electrical Characteristics (Note 4)
Unless otherwise specified, all limits guaranteed for TA = 25°C,
V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Drift
Conditions
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
±10
±150
±230
μV
LMP2232A
±0.3
±0.5
LMP2232B
±0.3
±2.5
0.02
±3
±125
IBIAS
Input Bias Current
IOS
Input Offset Current
CMRR
Common Mode Rejection Ratio
0V ≤ VCM ≤ 4V
81
80
97
PSRR
Power Supply Rejection Ratio
1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120
CMVR
Common Mode Voltage Range
CMRR ≥ 80 dB
−0.2
−0.2
VO = 0.3V to 4.7V
110
108
5
CMRR ≥ 79 dB
AVOL
Large Signal Voltage Gain
RL = 10 kΩ to V+/2
VO
IO
IS
Output Swing High
Output Swing Low
RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
17
Output Current (Note 7)
Sourcing, VO to V−
VIN(diff) = 100 mV
27
19
30
Sinking, VO to V+
VIN(diff) = −100 mV
17
12
22
Supply Current
dB
dB
120
17
19
pA
fA
4.2
4.2
RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
μV/°C
V
dB
50
50
50
50
mV
from either
rail
mA
27
28
μA
5V AC Electrical Characteristics (Note 4)
Unless otherwise specified, all limits guaranteed for TA = 25°C,
V+ = 5V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
GBW
Gain-Bandwidth Product
CL = 20 pF, RL = 10 kΩ
SR
Slew Rate
AV = +1
θm
Phase Margin
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Typ
(Note 5)
130
Falling Edge
33
32
58
Rising Edge
33
32
48
CL = 20 pF, RL = 10 kΩ
2
Min
(Note 6)
68
Max
(Note 6)
Units
kHz
V/ms
deg
Parameter
Conditions
Min
(Note 6)
Typ
(Note 5)
Gm
Gain Margin
CL = 20 pF, RL = 10 kΩ
27
en
Input-Referred Voltage Noise Density
f = 1 kHz
60
Input Referred Voltage Noise
0.1 Hz to 10 Hz
2.3
in
Input-Referred Current Noise
f = 1 kHz
10
THD+N
Total Harmonic Distortion + Noise
f = 100 Hz, RL = 10 kΩ
Max
(Note 6)
Units
dB
nV/
μVPP
fA/
0.002
%
3.3V DC Electrical Characteristics
(Note 4) Unless otherwise specified, all limits guaranteed for
T A = 25°C, V+ = 3.3V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Drift
Conditions
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
±10
±160
±250
μV
LMP2232A
±0.3
±0.5
LMP2232B
±0.3
±2.5
0.02
±3
±125
IBIAS
Input Bias Current
IOS
Input Offset Current
CMRR
Common Mode Rejection Ratio
0V ≤ VCM ≤ 2.3V
79
77
92
PSRR
Power Supply Rejection Ratio
1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120
CMVR
Common Mode Voltage Range
CMRR ≥ 78 dB
−0.2
−0.2
VO = 0.3V to 3V
108
107
Large Signal Voltage Gain
RL = 10 kΩ to V+/2
VO
IO
IS
Output Swing High
pA
5
CMRR ≥ 77 dB
AVOL
μV/°C
dB
dB
2.5
2.5
V
120
RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
14
Output Swing Low
RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
14
Output Current (Note 7)
Sourcing, VO to V−
VIN(diff) = 100 mV
11
8
14
Sinking, VO to V+
VIN(diff) = −100 mV
8
5
11
Supply Current
fA
17
dB
50
50
mV
from either
rail
50
50
mA
25
26
μA
3.3V AC Electrical Characteristics
(Note 4) Unless otherwise is specified, all limits guaranteed for
TA = 25°C, V+ = 3.3V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
GBW
Gain-Bandwidth Product
CL = 20 pF, RL = 10 kΩ
128
SR
Slew Rate
AV = +1, CL = 20 pF Falling Edge
58
RL = 10 kΩ
48
Rising Edge
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
kHz
V/ms
θm
Phase Margin
CL = 20 pF, RL = 10 kΩ
66
deg
Gm
Gain Margin
CL = 20 pF, RL = 10 kΩ
26
dB
en
Input-Referred Voltage Noise Density
f = 1 kHz
60
Input-Referred Voltage Noise
0.1 Hz to 10 Hz
2.4
3
nV/
μVPP
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LMP2232
Symbol
LMP2232
Symbol
Parameter
in
Input-Referred Current Noise
THD+N
Total Harmonic Distortion + Noise
Conditions
Min
(Note 6)
f = 1 kHz
Typ
(Note 5)
Max
(Note 6)
Units
10
fA/
0.003
f = 100 Hz, RL = 10 kΩ
%
2.5V DC Electrical Characteristics
(Note 4) Unless otherwise specified, all limits guaranteed for
TA = 25°C, V+ = 2.5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Drift
Conditions
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
±10
±190
±275
μV
LMP2232A
±0.3
±0.5
LMP2232B
±0.3
±2.5
0.02
±3
±125
IBias
Input Bias Current
IOS
Input Offset Current
CMRR
Common Mode Rejection Ratio
0V ≤ VCM ≤ 1.5V
77
76
91
PSRR
Power Supply Rejection Ratio
1.6V ≤ V+ ≤ 5.5V
V– = 0V, VCM = 0V
83
83
120
CMVR
Common Mode Voltage Range
CMRR ≥ 77 dB
−0.2
−0.2
VO = 0.3V to 2.2V
104
104
Large Signal Voltage Gain
RL = 10 kΩ to V+/2
VO
IO
IS
Output Swing High
pA
5
CMRR ≥ 76 dB
AVOL
μV/°C
dB
dB
1.7
1.7
V
120
RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
12
Output Swing Low
RL = 10 kΩ to V+/2
VIN(diff) = –100 mV
13
Output Current (Note 7)
Sourcing, VO to V–
VIN(diff) = 100 mV
5
4
8
Sinking, VO to V+
VIN(diff) = –100 mV
3.5
2.5
7
Supply Current
fA
16
dB
50
50
50
50
mV
from either
rail
mA
24
25
µA
2.5V AC Electrical Characteristics
(Note 4) Unless otherwise specified, all limits guaranteed for
TA = 25°C, V+ = 2.5V, V− = 0V, VCM = VO = V+/2, and RL > 1MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
Min
Typ
(Note 6) (Note 5)
GBW
Gain-Bandwidth Product
CL = 20 pF, RL = 10 kΩ
128
SR
Slew Rate
AV = +1, CL = 20 pF
Falling Edge
58
RL = 10 kΩ
Rising Edge
48
Max
(Note 6)
Units
kHz
V/ms
θm
Phase Margin
CL = 20 pF, RL = 10 kΩ
64
deg
Gm
Gain Margin
CL = 20 pF, RL = 10 kΩ
26
dB
en
Input-Referred Voltage Noise Density f = 1 kHz
60
Input-Referred Voltage Noise
0.1 Hz to 10 Hz
2.5
in
Input-Referred Current Noise
f = 1 kHz
10
THD+N
Total Harmonic Distortion + Noise
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f = 100 Hz, RL = 10 kΩ
4
0.005
nV/
μVPP
fA/
%
(Note 4) Unless otherwise specified, all limits guaranteed for
T A = 25°C, V+ = 1.8V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
VOS
Input Offset Voltage
TCVOS
Input Offset Voltage Drift
IBIAS
Input Bias Current
IOS
Input Offset Current
CMRR
Common Mode Rejection Ratio
PSRR
CMVR
Conditions
Typ
(Note 5)
Max
(Note 6)
Units
±10
±230
±325
μV
LMP2232A
±0.3
±0.5
LMP2232B
±0.3
±2.5
0.02
±3
±125
0V ≤ VCM ≤ 0.8V
Power Supply Rejection Ratio
1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120
Common Mode Voltage Range
CMRR ≥ 76 dB
−0.2
0
VO = 0.3V to 1.5V
103
103
RL = 10 kΩ to V+/2
VO
IO
IS
Output Swing High
pA
fA
92
Large Signal Voltage Gain
μV/°C
5
76
75
CMRR ≥ 75 dB
AVOL
Min
(Note 6)
dB
1.0
1.0
V
120
RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
12
Output Swing Low
RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
13
Output Current (Note 7)
Sourcing, VO to V–
VIN(diff) = 100 mV
2.5
2
5
Sinking, VO to V+
VIN(diff) = −100 mV
2
1.5
5
Supply Current
dB
dB
50
50
mV
from either
rail
50
50
mA
16
24
25
µA
1.8V AC Electrical Characteristics
(Note 4) Unless otherwise is specified, all limits guaranteed for
TA = 25°C, V+ = 1.8V, V− = 0V, VCM = VO = V+/2, and RL > 1 MΩ. Boldface limits apply at the temperature extremes.
Symbol
Parameter
Conditions
GBW
Gain-Bandwidth Product
CL = 20 pF, RL = 10 kΩ
127
SR
Slew Rate
AV = +1, CL = 20 pF Falling Edge
58
RL = 10 kΩ
48
Rising Edge
Min
(Note 6)
Typ
(Note 5)
θm
Phase Margin
Gm
Gain Margin
en
Input-Referred Voltage Noise Density f = 1 kHz
60
Input-Referred Voltage Noise
0.1 Hz to 10 Hz
2.4
in
Input-Referred Current Noise
f = 1 kHz
10
THD+N
Total Harmonic Distortion + Noise
f = 100 Hz, RL = 10 kΩ
Max
(Note 6)
Units
kHz
V/ms
CL = 20 pF, RL = 10 kΩ
60
deg
CL = 20 pF, RL = 10 kΩ
25
dB
5
0.005
nV/
μVPP
fA/
%
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LMP2232
1.8V DC Electrical Characteristics
LMP2232
Note 1: Absolute Maximum Ratings indicate limits beyond which damage may occur. Operating Ratings indicate conditions for which the device is intended to
be functional, but specific performance is not guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC)
Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).
Note 3: 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) – TA)/ θJA. All numbers apply for packages soldered directly onto a PC board.
Note 4: 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. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ >
TA. Absolute Maximum Ratings indicate junction temperature limits beyond which the device may be permanently degraded, either mechanically or electrically.
Note 5: Typical values represent the most likely parametric norm at the time of characterization. Actual typical values may vary over time and will also depend
on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.
Note 6: All limits are guaranteed by testing, statistical analysis or design.
Note 7: The short circuit test is a momentary open loop test.
Connection Diagram
8-Pin MSOP/SOIC
30033938
Top View
Ordering Information
Package
Part Number
Temperature
Range
Package Marking
Transport Media
LMP2232AMA
250 Units Tape and Reel
LMP2232AMA
95 Units/Rail
LMP2232AMAE
8-Pin SOIC
LMP2232AMAX
2.5k Units Tape and Reel
LMP2232BMA
95 Units/Rail
LMP2232BMAE
LMP2232BMAX
LMP2232AMM
LMP2232BMA
–40°C to 125°C
250 Units Tape and Reel
1k Units Tape and Reel
AK5A
250 Units Tape and Reel
LMP2232AMMX
3.5k Units Tape and Reel
LMP2232BMM
1k Units Tape and Reel
LMP2232BMME
AK5B
LMP2232BMMX
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M08A
2.5k Units Tape and Reel
LMP2232AMME
8-Pin MSOP
NSC Drawing
250 Units Tape and Reel
3.5k Units Tape and Reel
6
MUA08A
Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where
Offset Voltage Distribution
TCVOS Distribution
30033907
30033911
Offset Voltage Distribution
TCVOS Distribution
30033906
30033910
Offset Voltage Distribution
TCVOS Distribution
30033905
30033909
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LMP2232
Typical Performance Characteristics
VS = V+ - V−
LMP2232
Offset Voltage Distribution
TCVOS Distribution
30033969
30033973
Offset Voltage vs. VCM
Offset Voltage vs. VCM
30033918
30033965
Offset Voltage vs. VCM
Offset Voltage vs. VCM
30033964
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30033972
8
LMP2232
Offset Voltage vs. Temperature
Offset Voltage vs. Supply Voltage
30033971
30033970
0.1 Hz to 10 Hz Voltage Noise
0.1 Hz to 10 Hz Voltage Noise
30033933
30033934
0.1 Hz to 10 Hz Voltage Noise
0.1 Hz to 10 Hz Voltage Noise
30033932
30033931
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LMP2232
Input Bias Current vs. VCM
Input Bias Current vs. VCM
30033955
30033956
Input Bias Current vs. VCM
Input Bias Current vs. VCM
30033957
30033958
Input Bias Current vs. VCM
Input Bias Current vs. VCM
30033959
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30033960
10
LMP2232
Input Bias Current vs. VCM
Input Bias Current vs. VCM
30033961
30033962
PSRR vs. Frequency
Supply Current vs. Supply Voltage (per channel)
30033966
30033912
Sinking Current vs. Supply Voltage
Sourcing Current vs. Supply Voltage
30033913
30033914
11
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LMP2232
Output Swing High vs. Supply Voltage
Output Swing Low vs. Supply Voltage
30033915
30033916
Open Loop Frequency Response
Open Loop Frequency Response
30033921
30033922
Phase Margin vs. Capacitive Load
Slew Rate vs. Supply Voltage
30033963
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30033930
12
LMP2232
THD+N vs. Amplitude
THD+N vs. Frequency
30033929
30033928
Large Signal Step Response
Small Signal Step Response
30033924
30033923
Large Signal Step Response
Small Signal Step Response
30033926
30033925
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LMP2232
CMRR vs. Frequency
Input Voltage Noise vs. Frequency
30033919
30033967
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LMP2232
Application Information
LMP2232
The LMP2232 is a quad CMOS precision amplifier that offers
low offset voltage, low offset voltage drift, and high gain while
consuming less than 10 μA of supply current per channel.
The LMP2232 is a micropower op amp, consuming only
36 μA of current. Micropower op amps extend the run time of
battery powered systems and reduce energy consumption in
energy limited systems. The guaranteed supply voltage range
of 1.8V to 5.0V along with the ultra-low supply current extend
the battery run time in two ways. The extended guaranteed
power supply voltage range of 1.8V to 5.0V enables the op
amp to function when the battery voltage has depleted from
its nominal value down to 1.8V. In addition, the lower power
consumption increases the life of the battery.
The LMP2232 has input referred offset voltage of only
±150 μV maximum at room temperature. This offset is guaranteed to be less than ±230 μV over temperature. This minimal offset voltage along with very low TCVOS of only 0.3 µV/
°C typical allows more accurate signal detection and amplification in precision applications.
The low input bias current of only ±20 fA gives the LMP2232
superiority for use in high impedance sensor applications.
Bias current of an amplifier flows through source resistance
of the sensor and the voltage resulting from this current flow
appears as a noise voltage on the input of the amplifier. The
low input bias current enables the LMP2232 to interface with
high impedance sensors while generating negligible voltage
noise. Thus the LMP2232 provides better signal fidelity and
a higher signal-to-noise ratio when interfacing with high
impedance sensors.
National Semiconductor is heavily committed to precision
amplifiers and the market segments they serve. Technical
support and extensive characterization data is available for
sensitive applications or applications with a constrained error
budget.
The operating voltage range of 1.6V to 5.5V over the extensive temperature range of −40°C to 125°C makes the
LMP2232 an excellent choice for low voltage precision applications with extensive temperature requirements.
The LMP2232 is offered in the 8-pin MSOP and 8-pin SOIC
packages. These small packages are ideal solutions for area
constrained PC boards and portable electronics.
The input current noise of the LMP2232 is so low that it will
not become the dominant factor in the total noise unless
source resistance exceeds 300 MΩ, which is an unrealistically high value. As is evident in Figure 1, at lower RS values,
total noise is dominated by the amplifier’s input voltage noise.
Once RS is larger than a 100 kΩ, then the dominant noise
factor becomes the thermal noise of RS. As mentioned before,
the current noise will not be the dominant noise factor for any
practical application.
30033948
FIGURE 1. Total Input Noise
VOLTAGE NOISE REDUCTION
. While
The LMP2232 has an input voltage noise of 60nV/
this value is very low for micropower amplifiers, this input
voltage noise can be further reduced by placing N amplifiers
in parallel as shown in Figure 2. The total voltage noise on the
output of this circuit is divided by the square root of the number of amplifiers used in this parallel combination. This is
because each individual amplifier acts as an independent
noise source, and the average noise of independent sources
is the quadrature sum of the independent sources divided by
the number of sources. For N identical amplifiers, this means:
TOTAL NOISE CONTRIBUTION
The LMP2232 has very low input bias current, very low input
current noise, and low input voltage noise for micropower
amplifiers. As a result, these amplifiers make great choices
for circuits with high impedance sensor applications.
Figure 1 shows the typical input noise of the LMP2232 as a
function of source resistance where:
en denotes the input referred voltage noise
ei is the voltage drop across source resistance due to input
referred current noise or ei = RS * in
et shows the thermal noise of the source resistance
eni shows the total noise on the input.
Where:
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LMP2232
Figure 2 shows a schematic of this input voltage noise reduction circuit. Typical resistor values are: RG = 10Ω, RF = 1 kΩ,
and RO = 1 kΩ.
30033936
FIGURE 3. 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, the input stage, would be set up as
buffers to isolate the inputs. However they cannot be connected as followers because of mismatch of amplifiers. 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 LMP2232.
30033946
(1)
FIGURE 2. Noise Reduction Circuit
By Ohm’s Law:
PRECISION INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires
close attention to the input impedance of the amplifier, gain
of the signal on the inputs, and the gain on each input of the
amplifier. This is because the difference of the input signal on
the two inputs is of the interest and the common signal is
considered noise. A classic circuit implementation is an instrumentation amplifier. Instrumentation amplifiers have a
finite, accurate, and stable gain. They also 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 3.
(2)
However:
(3)
So we have:
VO1–VO2 = (2a+1)(V1–V2)
(4)
Now looking at the output of the instrumentation amplifier:
(5)
Substituting from Equation 4:
(6)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1)
Typical values for this circuit can be obtained by setting:
a = 12 and K= 4. This results in an overall gain of −100.
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30033951
30033950
FIGURE 4. Bridge Sensor
Instrumentation amplifiers are great for interfacing with bridge
sensors. Bridge sensors often sense a very small differential
signal in the presence of a larger common mode voltage. Instrumentation amplifiers reject this common mode signal.
Figure 5 shows a strain gauge bridge amplifier. In this application one of the LMP2232 amplifiers is used to buffer the
LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three amplifiers in the
LMP2232 are used to form an instrumentation amplifier. This
instrumentation amplifier uses the LMP2232's high CMRR
and low VOS and TCVOS to accurately amplify the small differential signal generated by the output of the bridge sensor.
This amplified signal is then fed into the ADC121S021 which
is a 12-bit analog to digital converter. This circuit works on a
single supply voltage of 5V.
17
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LMP2232
of two adjacent resistor values are equal. This fact is used in
null circuit measurements. These are particularly used in
feedback systems which involve electrochemical elements or
human interfaces. Null systems force an active resistor, such
as a strain gauge, to balance the bridge by influencing the
measured parameter.
Often in sensor applications at lease one of the resistors is a
variable resistor, or a sensor. The deviation of this active element from its initial value is measured as an indication of
change in the measured quantity. A change in output voltage
represents the sensor value change. Since the sensor value
change is often very small, the resulting output voltage is very
small in magnitude as well. This requires an extensive and
very precise amplification circuitry so that signal fidelity does
not change after amplification.
Sensitivity of a bridge is the ratio of its maximum expected
output change to the excitation voltage change.
Figure 4(a) shows a typical bridge sensor and Figure 4(b)
shows the bridge with four sensors. R in Figure 4(b) is the
nominal value of the sense resistor and the deviations from R
are proportional to the quantity being measured.
SINGLE SUPPLY STRAIN GAGE BRIDGE AMPLIFIER
Strain gauges are popular electrical elements used to measure force or pressure. Strain gauges are subjected to an
unknown force which is measured as the deflection on a previously calibrated scale. Pressure is often measured using the
same technique; however this pressure needs to be converted into force using an appropriate transducer. Strain gauges
are often resistors which are sensitive to pressure or to flexing. Sense resistor values range from tens of ohms to several
hundred kilo-ohms. The resistance change which is a result
of applied force across the strain gauge might be 1% of its
total value. An accurate and reliable system is needed to
measure this small resistance change. Bridge configurations
offer a reliable method for this measurement.
Bridge sensors are formed of four resistors, connected as a
quadrilateral. A voltage source or a current source is used
across one of the diagonals to excite the bridge while a voltage detector across the other diagonal measures the output
voltage.
Bridges are mainly used as null circuits or to measure differential voltages. Bridges will have no output voltage if the ratios
LMP2232
30033974
FIGURE 5. Strain Gage Bridge Amplifier
or specify a range of acceptable values for the load resistor.
Oxygen sensors typically have a life of one to two years. The
use of the micropower LMP2232 means minimal power usage
by the op amp and it enhances the battery life. Depending on
other components present in the circuit design, the battery
could last for the entire life of the oxygen sensor. The precision specifications of the LMP2232, such as its very low offset
voltage, low TCVOS, low input bias current, low CMRR, and
low PSRR are other factors which make the LMP2232 a great
choice for this application..
PORTABLE GAS DETECTION SENSOR
Gas sensors are used in many different industrial and medical
applications. They generate a current which is proportional to
the percentage of a particular gas sensed in an air sample.
This current goes through a load resistor and the resulting
voltage drop is measured. Depending on the sensed gas and
sensitivity of the sensor, the output current can be in the order
of tens of microamperes to a few milliamperes. Gas sensor
datasheets often specify a recommended load resistor value
or they suggest a range of load resistors to choose from.
Oxygen sensors are used when air quality or oxygen delivered to a patient needs to be monitored. Fresh air contains
20.9% oxygen. Air samples containing less than 18% oxygen
are considered dangerous. Oxygen sensors are also used in
industrial applications where the environment must lack oxygen. An example is when food is vacuum packed. There are
two main categories of oxygen sensors, those which sense
oxygen when it is abundantly present (i.e. in air or near an
oxygen tank) and those which detect very small traces of oxygen in ppm.
Figure 6 shows a typical circuit used to amplify the output
signal of an oxygen detector. The LMP2232 makes an excellent choice for this application as it draws only 36 µA of current
and operates on supply voltages down to 1.8V. This application detects oxygen in air. The oxygen sensor outputs a
known current through the load resistor. This value changes
with the amount of oxygen present in the air sample. Oxygen
sensors usually recommend a particular load resistor value
www.national.com
30033949
FIGURE 6. Precision Oxygen Sensor
18
LMP2232
Physical Dimensions inches (millimeters) unless otherwise noted
8-Pin MSOP
NS Package Number MUA08A
8-Pin SOIC
NS Package Number M08A
19
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LMP2232 Dual 1.8V, Micropower, Precision, Operational Amplifier with CMOS Input
Notes
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