LMP2231AMFX/NOPB - NATIONAL SEMICONDUCTOR

LMP2231

LMP2231 Single Micropower, 1.6V, Precision Operational Amplifier with CMOS

Inputs

Literature Number: SNOSB01D

LMP2231 Single

June 25, 2010

Micropower, 1.6V, Precision Operational Amplifier with

CMOS Inputs

General Description

The LMP2231 is a single 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 16 μW extend the battery life in portable

battery operated systems. The LMP2231 is part of the

LMP® precision amplifier family. The high impedance CMOS

input makes it ideal for instrumentation and other sensor in-

terface applications.

The LMP2231 has a maximum offset of 150 µV and maximum

offset voltage drift of only 0.4 µV/°C along with low bias cur-

rent of only ±20 fA. These precise specifications make the

LMP2231 a great choice for maintaining system accuracy and

long term stability.

The LMP2231 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 LMP2231 is ideal for use in

single supply applications with ground sensing.

The LMP2231 is offered in 5-Pin SOT23 and 8-pin SOIC

packages.

The dual and quad versions of this product are also available.

The dual, LMP2232 is offered in 8-pin SOIC and MSOP. The

quad, LMP2234 is offered in 14-pin SOIC and TSSOP.

Features

(For VS = 5V, TA = 25°C, Typical unless otherwise noted)

■Supply current 10 µA

■Operating voltage range 1.6V to 5.5V

■TCVOS (LMP2231A) ±0.4 µV/°C (max)

■TCVOS (LMP2231B) ±2.5µV/°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

30033874

Strain Gauge Bridge Amplifier

LMP® is a registered trademark of National Semiconductor Corporation.

LMP2231 Single Micropower, 1.6V, Precision, Operational Amplifier with CMOS Inputs

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

For soldering specifications:

see product folder at www.national.com and

www.national.com/ms/MS/MS-SOLDERING.pdf

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)

5-Pin SOT23 160.6 °C/W

8-Pin SOIC 116.2 °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 LMP2231A ±0.3 ±0.4 μV/°C

LMP2231B ±0.3 ±2.5

IBIAS Input Bias Current 0.02 ±1

±50 pA

IOS Input Offset Current 5 fA

CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 4V 81

97 dB

PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V

V− = 0V, VCM = 0V

120 dB

CMVR Common Mode Voltage Range CMRR ≥ 80 dB

CMRR ≥ 79 dB

−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

IOOutput Current (Note 7) Sourcing, VO to V–

VIN(diff) = 100 mV

Sinking, VO to V+

VIN(diff) = −100 mV

ISSupply Current 10 16

18 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

V/ms

Rising Edge 33

θ m Phase Margin CL = 20 pF, RL = 10 kΩ 78 deg

GmGain Margin CL = 20 pF, RL = 10 kΩ 27 dB

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LMP2231 Single

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

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

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

VOS Input Offset Voltage ±10 ±160

±250 μV

TCVOS Input Offset Voltage Drift LMP2231A ±0.3 ±0.4 μV/°C

LMP2231B ±0.3 ±2.5

IBIAS Input Bias Current 0.02 ±1

±50 pA

IOS Input Offset Current 5 fA

CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 2.3V 79

92 dB

PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V

V− = 0V, VCM = 0V

120 dB

CMVR Common Mode Voltage Range CMRR ≥ 78 dB

CMRR ≥ 77 dB

−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

IOOutput Current (Note 7) Sourcing, VO to V–

VIN(diff) = 100 mV

Sinking, VO to V+

VIN(diff) = −100 mV

ISSupply Current 10 15

16 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 76 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

inInput-Referred Current Noise f = 1 kHz 10 fA/

THD+N Total Harmonic Distortion + Noise f = 100 Hz, RL = 10 kΩ 0.003 %

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LMP2231 Single

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 LMP2231A ±0.3 ±0.4 μV/°C

LMP2231B ±0.3 ±2.5

IBIAS Input Bias Current 0.02 ±1.0

±50 pA

IOS Input Offset Current 5 fA

CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 1.5V 77

91 dB

PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V

V− = 0V, VCM = 0V

120 dB

CMVR Common Mode Voltage Range CMRR ≥ 77 dB

CMRR ≥ 76 dB

−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

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

IOOutput Current (Note 7) Sourcing, VO to V−

VIN(diff) = 100 mV

Sinking, VO to V+

VIN(diff) = −100 mV

3.5

2.5

ISSupply Current 10 14

15 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 74 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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LMP2231 Single

1.8V DC Electrical Characteristics (Note 4) Unless otherwise 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

VOS Input Offset Voltage ±10 ±230

±325 μV

TCVOS Input Offset Voltage Drift LMP2231A ±0.3 ±0.4 μV/°C

LMP2231B ±0.3 ±2.5

IBIAS Input Bias Current 0.02 ±1.0

±50 pA

IOS Input Offset Current 5 fA

CMRR Common Mode Rejection Ratio 0V ≤ VCM ≤ 0.8V 76

92 dB

PSRR Power Supply Rejection Ratio 1.6V ≤ V+ ≤ 5.5V

V− = 0V, VCM = 0V

120 dB

CMVR Common Mode Voltage Rang CMRR ≥ 76 dB

CMRR ≥ 75 dB

−0.2

1.0

1.0 V

AVOL Large Signal Voltage Gain VO = 0.3V to 1.5V

RL = 10 kΩ to V+/2

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

IOOutput Current (Note 7) Sourcing, VO to V−

VIN(diff) = 100 mV

2.5

Sinking, VO to V+

VIN(diff) = −100 mV

1.5

ISSupply Current 10 14

15 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 70 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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LMP2231 Single

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. 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 Diagrams

5-Pin SOT23

30033802

Top View

8-Pin SOIC

30033842

Top View

Ordering Information

Package Part Number Temperature

Range

Package Marking Transport Media NSC Drawing

5-Pin SOT23

LMP2231AMF

−40°C to 125°C

AL5A

1k Units Tape and Reel

MF05A

LMP2231AMFE 250 Units Tape and Reel

LMP2231AMFX 3k Units Tape and Reel

LMP2231BMF

AL5B

1k Units Tape and Reel

LMP2231BMFE 250 Units Tape and Reel

LMP2231BMFX 3k Units Tape and Reel

8-Pin SOIC

LMP2231AMA

95 Units/Rail

M08A

LMP2231AMAE 250 Units Tape and Reel

LMP2231AMAX 2.5k Units Tape and Reel

LMP2231BMA

95 Units/Rail

LMP2231BMAE 250 Units Tape and Reel

LMP2231BMAX 2.5k Units Tape and Reel

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LMP2231 Single

Typical Performance Characteristics Unless otherwise Specified: TA = 25°C, VS = 5V, VCM = VS/2, where

VS = V+ - V−

Offset Voltage Distribution

30033807

TCVOS Distribution

30033811

Offset Voltage Distribution

30033806

TCVOS Distribution

30033810

Offset Voltage Distribution

30033805

TCVOS Distribution

30033809

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LMP2231 Single

Offset Voltage Distribution

30033873

TCVOS Distribution

30033869

Offset Voltage vs. VCM

30033818

Offset Voltage vs. VCM

30033865

Offset Voltage vs. VCM

30033864

Offset Voltage vs. VCM

30033872

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LMP2231 Single

Offset Voltage vs. Temperature

30033871

Offset Voltage vs. Supply Voltage

30033870

Time Domain Voltage Noise

30033833

Time Domain Voltage Noise

30033834

Time Domain Voltage Noise

30033832

Time Domain Voltage Noise

30033831

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LMP2231 Single

Input Bias Current vs. VCM

30033855

Input Bias Current vs. VCM

30033856

Input Bias Current vs. VCM

30033857

Input Bias Current vs. VCM

30033858

Input Bias Current vs. VCM

30033859

Input Bias Current vs. VCM

30033860

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LMP2231 Single

Input Bias Current vs. VCM

30033861

Input Bias Current vs. VCM

30033862

PSRR vs. Frequency

30033866

Supply Current vs. Supply Voltage

30033812

Sinking Current vs. Supply Voltage

30033813

Sourcing Current vs. Supply Voltage

30033814

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LMP2231 Single

Output Swing High vs. Supply Voltage

30033815

Output Swing Low vs. Supply Voltage

30033816

Open Loop Frequency Response

30033821

Open Loop Frequency Response

30033822

Phase Margin vs. Capacitive Load

30033863

Slew Rate vs. Supply Voltage

30033830

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LMP2231 Single

THD+N vs. Amplitude

30033828

THD+N vs. Frequency

30033829

Large Signal Step Response

30033824

Small Signal Step Response

30033823

Large Signal Step Response

30033826

Small Signal Step Response

30033825

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LMP2231 Single

CMRR vs. Frequency

30033867

Input Voltage Noise vs. Frequency

30033819

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LMP2231 Single

Application Information

LMP2231

The LMP2231 is a single CMOS precision amplifier that offer

low offset voltage and low offset voltage drift, and high gain

while only consuming 10 μA of current per channel.

The LMP2231 is a micropower op amp, consuming only

10 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 LMP2231 has an 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

amplification in precision applications.

The low input bias current of only ±20 fA gives the LMP2231

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 LMP2231 to interface with

high impedance sensors while generating negligible voltage

noise. Thus the LMP2231 provides better signal fidelity and

a higher signal-to-noise ration when interfacing with high

impedance sensors.

National Semiconductor is heavily committed to precision

amplifiers and the market segment they serve. Technical sup-

port and extensive characterization data is available for sen-

sitive applications or applications with a constrained error

budget.

The operating supply voltage range of 1.8V to 5.5V over the

extensive temperature range of −40°C to 125°C makes the

LMP2231 an excellent choice for low voltage precision appli-

cations with extensive temperature requirements.

The LMP2231 is offered in the space saving 5-Pin SOT23 and

8-pin SOIC package. These small packages are ideal solu-

tions for area constrained PC boards and portable electron-

ics.

TOTAL NOISE CONTRIBUTION

The LMP2231 has a very low input bias current, very low input

current noise, and low input voltage noise for micropower

amplifier. As a result, this amplifier makes a great choice for

circuits with high impedance sensor applications.

Figure 1 shows the typical input noise of the LMP2231 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 LMP2231 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.

30033848

FIGURE 1. Total Input Noise

VOLTAGE NOISE REDUCTION

The LMP2231 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 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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LMP2231 Single

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.

30033846

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 overall 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. 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.

30033836

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, 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 LMP2231.

(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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LMP2231 Single

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 a the deflection on a

previously calibrated scale. Pressure is often measured using

the same technique; however this pressure needs to be con-

verted 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 volt-

age detector across the other diagonal measures the output

voltage.

Bridges are mainly used as null circuits or to measure a dif-

ferential 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 el-

ements or human interfaces. Null systems force an active

resistor, such as a strain gauge, to balance the bridge by in-

fluencing 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.

30033850

30033851

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 the LMP2231 is used to buffer the LM4140's precision

output voltage. The LM4140A is a precision voltage refer-

ence. The other three LMP2231s are used to form an instru-

mentation amplifier. This instrumentation amplifier uses the

LMP2231'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.

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LMP2231 Single

30033874

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 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 traces of oxygen in ppm.

Figure 6 shows a typical circuit used to amplify the output of

an oxygen detector. The LMP2231 makes an excellent choice

for this application as it only draws 10 µA of current and op-

erates 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 LMP2231 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 LMP2231, such as its very low offset

voltage, low TCVOS , low input bias current, low CMRR, and

low PSRR are other factors which make the LMP2231 a great

choice for this application.

30033849

FIGURE 6. Precision Oxygen Sensor

www.national.com 18

LMP2231 Single

Physical Dimensions inches (millimeters) unless otherwise noted

5-Pin SOT23

NS Package Number MF0A5

8-Pin SOIC

NS Package Number M08A

19 www.national.com

LMP2231 Single

Notes

LMP2231 Single Micropower, 1.6V, Precision, Operational Amplifier with CMOS Inputs

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