December 18, 2008
LMP2232 Dual
Micropower, 1.6V, Precision, Operational Amplifier with
CMOS Input
General Description
The LMP2232 is a dual micropower precision amplifier de-
signed for battery powered applications. The 1.6V to 5.5V
operating supply voltage range and quiescent power con-
sumption of only 26 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 be-
low 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.
Features
(For VS = 5V, Typical unless otherwise noted)
■Supply current at 1.8V 16 µA
■Operating voltage range 1.6V to 5.5V
■Low TCVOS ±0.5 µV/°C (max)
■VOS ±150 µV (max)
■Input bias current 20 fA
■PSRR 120 dB
■CMRR 97 dB
■Open loop gain 120 dB
■Gain bandwidth product 130 kHz
■Slew rate 58 V/ms
■Input voltage noise, f = 1 kHz 60 nV/√Hz
■Temperature range –40°C to 125°C
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, Micropower, 1.6V, Precision, Operational Amplifier with CMOS Input
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 2000V
Machine Model 100V
Differential Input Voltage ±300 mV
Supply Voltage (VS = V+ - V–)6V
Voltage on Input/Output Pins V+ + 0.3V, V– – 0.3V
Storage Temperature Range −65°C to 150°C
Junction Temperature (Note 3) 150°C
Mounting Temperature
Infrared or Convection (20
sec.) +235°C
Wave Soldering Lead
Temperature (10 sec.) +260°C
Operating Ratings (Note 1)
Operating Temperature Range (Note 3) −40°C to 125°C
Supply Voltage (VS = V+ - V–)1.6V to 5.5V
Package Thermal Resistance (θJA)(Note 3)
8-Pin SOIC 111.2 °C/W
8-Pin MSOP 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 Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
VOS Input Offset Voltage ±10 ±150
±230 μV
TCVOS Input Offset Voltage Drift LMP2232A ±0.3 ±0.5 μV/°C
LMP2232B ±0.3 ±2.5
IBIAS Input Bias Current 0.02 ±3
±125 pA
IOS Input Offset Current 5 fA
CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 4V 81
80
97 dB
PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120 dB
CMVR Common Mode Voltage Range CMRR ≥ 80 dB
CMRR ≥ 79 dB
−0.2
−0.2
4.2
4.2 V
AVOL Large Signal Voltage Gain VO = 0.3V to 4.7V
RL = 10 kΩ to V+/2
110
108
120 dB
VOOutput Swing High RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
17 50
50 mV
from either
rail
Output Swing Low RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
17 50
50
IOOutput Current (Note 7) Sourcing, VO to V−
VIN(diff) = 100 mV
27
19
30
mA
Sinking, VO to V+
VIN(diff) = −100 mV
17
12
22
ISSupply Current 19 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 Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 130 kHz
SR Slew Rate AV = +1 Falling Edge 33
32
58
V/ms
Rising Edge 33
32
48
θ m Phase Margin CL = 20 pF, RL = 10 kΩ 68 deg
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LMP2232
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
GmGain Margin CL = 20 pF, RL = 10 kΩ 27 dB
enInput-Referred Voltage Noise Density f = 1 kHz 60 nV/
Input Referred Voltage Noise 0.1 Hz to 10 Hz 2.3 μVPP
inInput-Referred Current Noise f = 1 kHz 10 fA/
THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 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 Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
VOS Input Offset Voltage ±10 ±160
±250 μV
TCVOS Input Offset Voltage Drift LMP2232A ±0.3 ±0.5 μV/°C
LMP2232B ±0.3 ±2.5
IBIAS Input Bias Current 0.02 ±3
±125 pA
IOS Input Offset Current 5 fA
CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 2.3V 79
77
92 dB
PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120 dB
CMVR Common Mode Voltage Range CMRR ≥ 78 dB
CMRR ≥ 77 dB
−0.2
−0.2
2.5
2.5 V
AVOL Large Signal Voltage Gain VO = 0.3V to 3V
RL = 10 kΩ to V+/2
108
107
120 dB
VOOutput Swing High RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
14 50
50 mV
from either
rail
Output Swing Low RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
14 50
50
IOOutput Current (Note 7) Sourcing, VO to V−
VIN(diff) = 100 mV
11
8
14
mA
Sinking, VO to V+
VIN(diff) = −100 mV
8
5
11
ISSupply Current 17 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 Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 128 kHz
SR Slew Rate AV = +1, CL = 20 pF
RL = 10 kΩ
Falling Edge 58 V/ms
Rising Edge 48
θ m Phase Margin CL = 20 pF, RL = 10 kΩ 66 deg
GmGain Margin CL = 20 pF, RL = 10 kΩ 26 dB
enInput-Referred Voltage Noise Density f = 1 kHz 60 nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4 μVPP
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LMP2232
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
inInput-Referred Current Noise f = 1 kHz 10 fA/
THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 0.003 %
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 Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
VOS Input Offset Voltage ±10 ±190
±275 μV
TCVOS Input Offset Voltage Drift LMP2232A ±0.3 ±0.5 μV/°C
LMP2232B ±0.3 ±2.5
IBias Input Bias Current 0.02 ±3
±125 pA
IOS Input Offset Current 5 fA
CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 1.5V 77
76
91 dB
PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V
V– = 0V, VCM = 0V
83
83
120 dB
CMVR Common Mode Voltage Range CMRR ≥ 77 dB
CMRR ≥ 76 dB
−0.2
−0.2
1.7
1.7 V
AVOL Large Signal Voltage Gain VO = 0.3V to 2.2V
RL = 10 kΩ to V+/2
104
104
120 dB
VOOutput Swing High RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
12 50
50 mV
from either
rail
Output Swing Low RL = 10 kΩ to V+/2
VIN(diff) = –100 mV
13 50
50
IOOutput Current (Note 7) Sourcing, VO to V–
VIN(diff) = 100 mV
5
4
8
mA
Sinking, VO to V+
VIN(diff) = –100 mV
3.5
2.5
7
ISSupply Current 16 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
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 128 kHz
SR Slew Rate AV = +1, CL = 20 pF
RL = 10 kΩ
Falling Edge 58 V/ms
Rising Edge 48
θ m Phase Margin CL = 20 pF, RL = 10 kΩ 64 deg
GmGain Margin CL = 20 pF, RL = 10 kΩ 26 dB
enInput-Referred Voltage Noise Density f = 1 kHz 60 nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.5 μVPP
inInput-Referred Current Noise f = 1 kHz 10 fA/
THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 0.005 %
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LMP2232
1.8V DC Electrical Characteristics (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 Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
VOS Input Offset Voltage ±10 ±230
±325 μV
TCVOS Input Offset Voltage Drift LMP2232A ±0.3 ±0.5 μV/°C
LMP2232B ±0.3 ±2.5
IBIAS Input Bias Current 0.02 ±3
±125 pA
IOS Input Offset Current 5 fA
CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 0.8V 76
75
92 dB
PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V
V− = 0V, VCM = 0V
83
83
120 dB
CMVR Common Mode Voltage Range CMRR ≥ 76 dB
CMRR ≥ 75 dB
−0.2
0
1.0
1.0 V
AVOL Large Signal Voltage Gain VO = 0.3V to 1.5V
RL = 10 kΩ to V+/2
103
103
120 dB
VOOutput Swing High RL = 10 kΩ to V+/2
VIN(diff) = 100 mV
12 50
50 mV
from either
rail
Output Swing Low RL = 10 kΩ to V+/2
VIN(diff) = −100 mV
13 50
50
IOOutput Current (Note 7) Sourcing, VO to V–
VIN(diff) = 100 mV
2.5
2
5
mA
Sinking, VO to V+
VIN(diff) = −100 mV
2
1.5
5
ISSupply Current 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 Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
GBW Gain-Bandwidth Product CL = 20 pF, RL = 10 kΩ 127 kHz
SR Slew Rate AV = +1, CL = 20 pF
RL = 10 kΩ
Falling Edge 58 V/ms
Rising Edge 48
θ m Phase Margin CL = 20 pF, RL = 10 kΩ 60 deg
GmGain Margin CL = 20 pF, RL = 10 kΩ 25 dB
enInput-Referred Voltage Noise Density f = 1 kHz 60 nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4 μVPP
inInput-Referred Current Noise f = 1 kHz 10 fA/
THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 0.005 %
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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 NSC Drawing
8-Pin SOIC
LMP2232AMA
–40°C to 125°C
LMP2232AMA
95 Units/Rail
M08A
LMP2232AMAE 250 Units Tape and Reel
LMP2232AMAX 2.5k Units Tape and Reel
LMP2232BMA
LMP2232BMA
95 Units/Rail
LMP2232BMAE 250 Units Tape and Reel
LMP2232BMAX 2.5k Units Tape and Reel
8-Pin MSOP
LMP2232AMM
AK5A
1k Units Tape and Reel
MUA08A
LMP2232AMME 250 Units Tape and Reel
LMP2232AMMX 3.5k Units Tape and Reel
LMP2232BMM
AK5B
1k Units Tape and Reel
LMP2232BMME 250 Units Tape and Reel
LMP2232BMMX 3.5k Units Tape and Reel
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LMP2232
Typical Performance Characteristics Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where
VS = V+ - V−
Offset Voltage Distribution
30033907
TCVOS Distribution
30033911
Offset Voltage Distribution
30033906
TCVOS Distribution
30033910
Offset Voltage Distribution
30033905
TCVOS Distribution
30033909
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LMP2232
Offset Voltage Distribution
30033973
TCVOS Distribution
30033969
Offset Voltage vs. VCM
30033918
Offset Voltage vs. VCM
30033965
Offset Voltage vs. VCM
30033964
Offset Voltage vs. VCM
30033972
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LMP2232
Offset Voltage vs. Temperature
30033971
Offset Voltage vs. Supply Voltage
30033970
0.1 Hz to 10 Hz Voltage Noise
30033933
0.1 Hz to 10 Hz Voltage Noise
30033934
0.1 Hz to 10 Hz Voltage Noise
30033932
0.1 Hz to 10 Hz Voltage Noise
30033931
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LMP2232
Input Bias Current vs. VCM
30033955
Input Bias Current vs. VCM
30033956
Input Bias Current vs. VCM
30033957
Input Bias Current vs. VCM
30033958
Input Bias Current vs. VCM
30033959
Input Bias Current vs. VCM
30033960
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LMP2232
Input Bias Current vs. VCM
30033961
Input Bias Current vs. VCM
30033962
PSRR vs. Frequency
30033966
Supply Current vs. Supply Voltage (per channel)
30033912
Sinking Current vs. Supply Voltage
30033913
Sourcing Current vs. Supply Voltage
30033914
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LMP2232
Output Swing High vs. Supply Voltage
30033915
Output Swing Low vs. Supply Voltage
30033916
Open Loop Frequency Response
30033921
Open Loop Frequency Response
30033922
Phase Margin vs. Capacitive Load
30033963
Slew Rate vs. Supply Voltage
30033930
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LMP2232
THD+N vs. Amplitude
30033928
THD+N vs. Frequency
30033929
Large Signal Step Response
30033924
Small Signal Step Response
30033923
Large Signal Step Response
30033926
Small Signal Step Response
30033925
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LMP2232
CMRR vs. Frequency
30033967
Input Voltage Noise vs. Frequency
30033919
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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 guar-
anteed to be less than ±230 μV over temperature. This mini-
mal offset voltage along with very low TCVOS of only 0.3 µV/
°C typical allows more accurate signal detection and amplifi-
cation 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 exten-
sive temperature range of −40°C to 125°C makes the
LMP2232 an excellent choice for low voltage precision appli-
cations 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.
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:
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 unrealisti-
cally 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
The LMP2232 has an input voltage noise of 60nV/ . While
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 num-
ber 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:
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LMP2232
Figure 2 shows a schematic of this input voltage noise reduc-
tion circuit. Typical resistor values are: RG = 10Ω, RF = 1 kΩ,
and RO = 1 kΩ.
30033946
FIGURE 2. Noise Reduction Circuit
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 in-
strumentation amplifier. Instrumentation amplifiers have a
finite, accurate, and stable gain. They also have extremely
high input impedances and very low output impedances. Fi-
nally they have an extremely high CMRR so that the amplifier
can only respond to the differential signal. A typical instru-
mentation amplifier is shown in Figure 3.
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 am-
plifiers of the first stage, the input stage, would be set up as
buffers to isolate the inputs. However they cannot be con-
nected 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 in-
strumentation 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.
(1)
By Ohm’s Law:
(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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LMP2232
SINGLE SUPPLY STRAIN GAGE BRIDGE AMPLIFIER
Strain gauges are popular electrical elements used to mea-
sure force or pressure. Strain gauges are subjected to an
unknown force which is measured as the deflection on a pre-
viously calibrated scale. Pressure is often measured using the
same technique; however this pressure needs to be convert-
ed into force using an appropriate transducer. Strain gauges
are often resistors which are sensitive to pressure or to flex-
ing. 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 volt-
age detector across the other diagonal measures the output
voltage.
Bridges are mainly used as null circuits or to measure differ-
ential voltages. Bridges will have no output voltage if the ratios
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 el-
ement 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.
30033950
30033951
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. In-
strumentation amplifiers reject this common mode signal.
Figure 5 shows a strain gauge bridge amplifier. In this appli-
cation one of the LMP2232 amplifiers is used to buffer the
LM4140A's precision output voltage. The LM4140A is a pre-
cision 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 dif-
ferential 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 www.national.com
LMP2232
30033974
FIGURE 5. Strain Gage Bridge Amplifier
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 deliv-
ered 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 oxy-
gen. 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 oxy-
gen in ppm.
Figure 6 shows a typical circuit used to amplify the output
signal of an oxygen detector. The LMP2232 makes an excel-
lent choice for this application as it draws only 36 µA of current
and operates on supply voltages down to 1.8V. This applica-
tion 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
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 preci-
sion 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..
30033949
FIGURE 6. Precision Oxygen Sensor
www.national.com 18
LMP2232
Physical Dimensions inches (millimeters) unless otherwise noted
8-Pin MSOP
NS Package Number MUA08A
8-Pin SOIC
NS Package Number M08A
19 www.national.com
LMP2232
Notes
LMP2232 Dual, Micropower, 1.6V, Precision, Operational Amplifier with CMOS Input
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