Page 1
January 16, 2008
LMP2234 Quad
Micropower, Precision, RRO Amplifier with CMOS Input
General Description
The LMP2234 is a quad micropower precision amplifier designed for battery powered applications. The 1.8 to 5.5V
guaranteed supply voltage range and quiescent power consumption of only 56 μW extend the battery life in portable
systems. The LMP2234 is part of the LMP® precision amplifier
family. The high impedance CMOS input makes it ideal for
instrumentation and other sensor interface applications.
The LMP2234 has a maximum offset voltage of 150 μV and
0.3 μV/°C offset drift along with low bias current of only
±20 fA. These precise specifications make the LMP2234 a
great choice for maintaining system accuracy and long term
stability.
The LMP2234 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 LMP2234 is ideal for ground
sensing in single supply applications.
The LMP2234 is offered in 14-Pin SOIC and TSSOP packages.
Features
(For VS = 5V, Typical unless otherwise noted)
■
Supply current (per channel) 9 µA
■
Operating voltage range 1.6V to 5.5V
■
Low TCV
OS
±0.75 µV/°C (max)
■
V
OS
±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
Strain Gauge Bridge Amplifier
20203468
LMP® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 202034 www.national.com
LMP2234 Quad Micropower, Precision, RRO, Operational Amplifier with CMOS Input
Page 2
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)
14-Pin SOIC 101.5 °C/W
14-Pin TSSOP 121 °C/W
5V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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
V
OS
Input Offset Voltage ±10 ±150
±230
μV
TCV
OS
Input Offset Voltage Drift LMP2234A ±0.3 ±0.75
μV/°C
LMP2234B ±0.3 ±2.5
I
BIAS
Input Bias Current ±0.02 ±1
±50
pA
I
OS
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
VCM = 0V
83
82
120
dB
CMVR Common Mode Voltage Range
CMRR ≥ 80 dB
CMRR ≥ 79 dB
−0.2
−0.2
4.2
4.2
V
A
VOL
Large Signal Voltage Gain VO = 0.3V to 4.7V
RL = 10 kΩ to V+/2
110
108
120
dB
V
O
Output 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
I
O
Output 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
I
S
Supply Current 36 48
50
µA
5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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
GBWP 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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LMP2234 Quad
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Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
G
m
Gain Margin
CL = 20 pF, RL = 10 kΩ
27
dB
e
n
Input-Referred Voltage Noise Density f = 1 kHz 60
nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.3
μV
PP
i
n
Input-Referred Current Noise Density 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 are 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
V
OS
Input Offset Voltage ±10 ±160
±250
μV
TCV
OS
Input Offset Voltage Drift LMP2234A ±0.3 ±0.75
μV/°C
LMP2234B ±0.3 ±2.5
I
BIAS
Input Bias Current ±0.02 ±1
±50
pA
I
OS
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
VCM = 0V
83
82
120
dB
CMVR Common Mode Voltage Range
CMRR ≥ 78 dB
CMRR ≥ 77 dB
−0.2
−0.2
2.5
2.5
V
A
VOL
Large Signal Voltage Gain VO = 0.3V to 3V
RL = 10 kΩ to V+/2
108
107
120
dB
V
O
Output 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
I
O
Output 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
I
S
Supply Current 34 44
46
µA
3.3V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits are 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
GBWP 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
G
m
Gain Margin
CL = 20 pF, RL = 10 kΩ
26 dB
e
n
Input-Referred Voltage Noise Density f = 1 kHz 60
nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4
μV
PP
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LMP2234 Quad
Page 4
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
i
n
Input-Referred Current Noise Density 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 are 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
V
OS
Input Offset Voltage ±10 ±190
±275
μV
TCV
OS
Input Offset Voltage Drift LMP2234A ±0.3 ±0.75
μV/°C
LMP2234B ±0.3 ±2.5
I
BIAS
Input Bias Current ±0.02 ±1.0
±50
pA
I
OS
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
VCM = 0V
83
82
120
dB
CMVR Common Mode Voltage Range
CMRR ≥ 77 dB
CMRR ≥ 76 dB
−0.2
−0.2
1.7
1.7
V
A
VOL
Large Signal Voltage Gain VO = 0.3V to 2.2V
RL = 10 kΩ to V+/2
104
104
120
dB
V
O
Output 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
I
O
Output 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
I
S
Supply Current 32 44
46
µA
2.5V AC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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
GBWP 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
G
m
Gain Margin
CL = 20 pF, RL = 10 kΩ
26
dB
e
n
Input-Referred Voltage Noise Density f = 1 kHz 60
nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.5
μV
PP
i
n
Input-Referred Current Noise Density f = 1 kHz 10
fA/
THD+N Total Harmonic Distortion + Noise
f = 100 Hz, RL = 10 kΩ
0.005 %
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LMP2234 Quad
Page 5
1.8V DC Electrical Characteristics (Note 4) Unless otherwise specified, all limits are 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
V
OS
Input Offset Voltage ±10 ±230
±325
μV
TCV
OS
Input Offset Voltage Drift LMP2234A ±0.3 ±0.75
μV/°C
LMP2234B ±0.3 ±2.5
I
BIAS
Input Bias Current ±0.02 ±1.0
±50
pA
I
OS
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
VCM = 0V
83
82
120
dB
CMVR Common Mode Voltage Range
CMRR ≥ 76 dB
CMRR ≥ 75 dB
-0.2
0
1.0
1.0
V
A
VOL
Large Signal Voltage Gain VO = 0.3V to 1.5V
RL = 10 kΩ to V+/2
103
103
120
dB
V
O
Output 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
I
O
Output 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
I
S
Supply Current 31 42
44
µA
1.8V AC Electrical Characteristics (Note 4) Unless otherwise is specified, all limits are 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
GBWP 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
70 deg
G
m
Gain Margin
CL = 20 pF, RL = 10 kΩ
25
dB
e
n
Input-Referred Voltage Noise Density f = 1 kHz 60
nV/
Input-Referred Voltage Noise 0.1 Hz to 10 Hz 2.4
μV
PP
i
n
Input-Referred Current Noise Density f = 1 kHz 10
fA/
THD+N Total Harmonic Distortion + Noise
f = 100 Hz, RL = 10 kΩ
0.005 %
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LMP2234 Quad
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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 T
J(MAX)
, θJA. The maximum allowable power dissipation at any ambient temperature is
PD = (T
J(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 as determined 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
14-Pin TSSOP/SOIC
20203404
Ordering Information
Package Part Number Temperature
Range
Package Marking Transport Media NSC Drawing
14-Pin SOIC
LMP2234AMA
-40°C to 125°C
LMP2234AMA
55 Units/Rail
M14A
LMP2234AMAE 250 Units Tape and Reel
LMP2234AMAX 2.5k Units Tape and Reel
LMP2234BMA
LMP2234BMA
55 Units/Rail
LMP2234BMAE 250 Units Tape and Reel
LMP2234BMAX 2.5k Units Tape and Reel
14-Pin TSSOP
LMP2234AMT
LMP2234AMT
94 Units/Rail
MTC14
LMP2234AMTE 250 Units Tape and Reel
LMP2234AMTX 2.5k Units Tape and Reel
LMP2234BMT
LMP2234BMT
94 Units/Rail
LMP2234BMTE 250 Units Tape and Reel
LMP2234BMTX 2.5k Units Tape and Reel
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Typical Performance Characteristics Unless otherwise Specified: T
A
= 25°C, VS = 5V, VCM = VS/2, where
VS = V+ - V
−
Offset Voltage Distribution
20203407
TCVOS Distribution
20203411
Offset Voltage Distribution
20203406
TCVOS Distribution
20203410
Offset Voltage Distribution
20203405
TCVOS Distribution
20203409
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LMP2234 Quad
Page 8
Offset Voltage Distribution
20203473
TCVOS Distribution
20203469
Offset Voltage vs. V
CM
20203418
Offset Voltage vs. V
CM
20203465
Offset Voltage vs. V
CM
20203464
Offset Voltage vs. V
CM
20203472
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LMP2234 Quad
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Offset Voltage vs. Temperature
20203471
Offset Voltage vs. Supply Voltage
20203470
0.1 Hz to 10 Hz Voltage Noise
20203433
0.1 Hz to 10 Hz Voltage Noise
20203434
0.1 Hz to 10 Hz Voltage Noise
20203432
0.1 Hz to 10 Hz Voltage Noise
20203431
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LMP2234 Quad
Page 10
Input Bias Current vs. V
CM
20203455
Input Bias Current vs. V
CM
20203456
Input Bias Current vs. V
CM
20203457
Input Bias Current vs. V
CM
20203458
Input Bias Current vs. V
CM
20203459
Input Bias Current vs. V
CM
20203460
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LMP2234 Quad
Page 11
Input Bias Current vs. V
CM
20203461
Input Bias Current vs. V
CM
20203462
PSRR vs. Frequency
20203466
Supply Current vs. Supply Voltage (per channel)
20203412
Sinking Current vs. Supply Voltage
20203413
Sourcing Current vs. Supply Voltage
20203414
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LMP2234 Quad
Page 12
Output Swing High vs. Supply Voltage
20203415
Output Swing Low vs. Supply Voltage
20203416
Open Loop Frequency Response
20203421
Open Loop Frequency Response
20203422
Phase Margin vs. Capacitive Load
20203463
Slew Rate vs. Supply Voltage
20203430
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LMP2234 Quad
Page 13
THD+N vs. Amplitude
20203428
THD+N vs. Frequency
20203429
Large Signal Step Response
20203424
Small Signal Step Response
20203423
Large Signal Step Response
20203426
Small Signal Step Response
20203425
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LMP2234 Quad
Page 14
CMRR vs. Frequency
20203467
Input Voltage Noise vs. Frequency
20203419
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LMP2234 Quad
Page 15
Application Information
LMP2234
The LMP2234 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 LMP2234 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 5.5V along with the ultra-low supply current extend
the battery run time in two ways. The extended power supply
voltage range of 1.8V to 5.5V 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 LMP2234 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 LMP2234
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 LMP2234 to interface with
high impedance sensors while generating negligible voltage
noise. Thus the LMP2234 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 1.8V to 5.5V over the extensive temperature range of −40°C to 125°C makes the
LMP2234 an excellent choice for low voltage precision applications with extensive temperature requirements.
The LMP2234 is offered in the 14-pin TSSOP and 14-pin
SOIC package. These small packages are ideal solutions for
area constrained PC boards and portable electronics.
TOTAL NOISE CONTRIBUTION
The LMP2234 has very low input bias current, very low input
current noise, and low input voltage noise for micropower
amplifiers. As a result, this amplifier makes a great choice for
circuits with high impedance sensor applications.
shows the typical input noise of the LMP2234 as a function of
source resistance at f = 1 kHz 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* i
n
•
et shows the thermal noise of the source resistance
•
eni shows the total noise on the input.
Where:
The input current noise of the LMP2234 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 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.
20203448
FIGURE 1. Total Input Noise
VOLTAGE NOISE REDUCTION
The LMP2234 has an input voltage noise of 60 nV/
. While
this value is very low for micropower amplifiers, this input
voltage noise can be further reduced by placing multiple 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:
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Page 16
Figure 2 shows a schematic of this input voltage noise reduction circuit using the LMP2234. Typical resistor values are:
RG = 10Ω, RF = 1 kΩ, and RO = 1 kΩ.
20203446
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, the
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 interest and the common signal is considered noise. A classic circuit implementation that is used 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.
20203436
FIGURE 3. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, the output stage, is a differential amplifier. In an ideal case the two
amplifiers of the first stage, the input stage, would be configured as buffers to isolate the inputs. However they cannot be
connected as followers because of mismatch in 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 LMP2234.
(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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LMP2234 Quad
Page 17
SINGLE SUPPLY STRAIN GAUGE 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
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.
20203450
20203451
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 LMP2234 amplifiers is used to buffer the
LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three amplifiers in the
LMP2234 are used to form an instrumentation amplifier. This
instrumentation amplifier uses the LMP2234'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 www.national.com
LMP2234 Quad
Page 18
20203468
FIGURE 5. Strain Gauge 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 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 LMP2234 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
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 LMP2234 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 LMP2234, such as its very low offset
voltage, low TCVOS, low input bias current, low CMRR, and
low PSRR are other factors which make the LMP2234 a great
choice for this application.
20203449
FIGURE 6. Precision Oxygen Sensor
www.national.com 18
LMP2234 Quad
Page 19
Physical Dimensions inches (millimeters) unless otherwise noted
14-Pin SOIC
NS Package Number M14A
14-Pin TSSOP
NS Package Number MTC14
19 www.national.com
LMP2234 Quad
Page 20
Notes
LMP2234 Quad Micropower, Precision, RRO, Operational Amplifier with CMOS Input
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