LMV854EVAL - National Semiconductor

October 2007

LMV851/LMV852/LMV854

8 MHz Low Power CMOS, EMI Hardened Operational

Amplifiers

General Description

National’s LMV851/LMV852/LMV854 are CMOS input, low

power op amp ICs, providing a low input bias current, a wide

temperature range of −40°C to +125°C and exceptional per-

formance, making them robust general purpose parts. Addi-

tionally, the LMV851/LMV852/LMV854 are EMI hardened to

minimize any interference so they are ideal for EMI sensitive

applications. The unity gain stable LMV851/LMV852/LMV854

feature 8 MHz of bandwidth while consuming only 0.4 mA of

current per channel. These parts also maintain stability for

capacitive loads as large as 200 pF. The LMV851/LMV852/

LMV854 provide superior performance and economy in terms

of power and space usage. This family of parts has a maxi-

mum input offset voltage of 1 mV, a rail-to-rail output stage

and an input common-mode voltage range that includes

ground. Over an operating supply range from 2.7V to 5.5V the

LMV851/LMV852/LMV854 provide a CMRR of 92 dB, and a

PSRR of 93 dB. The LMV851/LMV852/LMV854 are offered

in the space saving 5-Pin SC70 package, the 8-Pin MSOP

and the 14-Pin TSSOP package.

Features

Unless otherwise noted, typical values at TA = 25°C,

VSUPPLY = 3.3V

■Supply voltage 2.7V to 5.5V

■Supply current (per channel) 0.4 mA

■Input offset voltage 1 mV max

■Input bias current 0.1 pA

■GBW 8 MHz

■EMIRR at 1.8 GHz 87 dB

■Input noise voltage at 1 kHz 11 nV/√Hz

■Slew rate 4.5 V/µs

■Output voltage swing Rail-to-Rail

■Output current drive 30 mA

■Operating ambient temperature range −40°C to 125°C

Applications

■Photodiode preamp

■Piezoelectric sensors

■Portable/battery-powered electronic equipment

■Filters/buffers

■PDAs/phone accessories

■Medical diagnosis equipment

Typical Application

Sensor Amplifiers Close to RF Sources

20202101

LMV851 Single/ LMV852 Dual/ LMV854 Quad 8 MHz Low Power CMOS, EMI Hardened Operational

Amplifiers

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 2 kV

Charge-Device Model 1 kV

Machine Model 200V

VINDifferential ± Supply Voltage

Supply Voltage (V+ – V−)6V

Voltage at Input/Output Pins V+ +0.4V

V− −0.4V

Storage Temperature Range −65°C to +150°C

Junction Temperature (Note 3) +150°C

Soldering Information

Infrared or Convection (20 sec) +260°C

Operating Ratings (Note 1)

Temperature Range (Note 3) −40°C to +125°C

Supply Voltage (V+ – V−)2.7V to 5.5V

Package Thermal Resistance (θJA (Note 3))

5-Pin SC70 313 °C/W

8-Pin MSOP 217 °C/W

14-Pin TSSOP 135 °C/W

3.3V Electrical Characteristics (Note 4)

Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = V+/2, and RL = 10 kΩ to V+/2.

Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

VOS Input Offset Voltage ±0.26

(Note 9)

±1

±1.2 mV

TCVOS Input Offset Voltage Drift

(Note 10)

±0.4

(Note 9)

±2 μV/°C

IBInput Bias Current

(Note 10)

0.1 10

500 pA

IOS Input Offset Current 1 pA

CMRR Common Mode Rejection Ratio −0.2V < VCM < V+ - 1.2V 76

(Note 9)

PSRR Power Supply Rejection Ratio 2.7V ≤ V+ ≤ 5.5V,

VOUT = 1V

(Note 9)

EMIRR EMI Rejection Ratio, IN+ and IN−

(Note 8)

VRFpeak = 100 mVP (−20 dBVP),

f = 400 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 900 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 1800 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 2400 MHz

CMVR Input Common-Mode Voltage Range CMRR ≥ 76 dB −0.2 2.1 V

AVOL Large Signal Voltage Gain

(Note 11)

RL = 2 kΩ,

VOUT = 0.15V to 1.65V,

VOUT = 3.15V to 1.65V

100

114

RL = 10 kΩ,

VOUT = 0.1V to 1.65V,

VOUT = 3.2V to 1.65V

100

115

VOOutput Swing High,

(measured from V+)

RL = 2 kΩ to V+/2 31 35

43 mV

RL = 10 kΩ to V+/2 7 10

Output Swing Low,

(measured from V−)

RL = 2 kΩ to V+/2 26 32

43 mV

RL = 10 kΩ to V+/2 6 11

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

IOOutput Short Circuit Current Sourcing, VOUT = VCM,

VIN = 100 mV

Sinking, VOUT = VCM,

VIN = −100 mV

ISSupply Current LMV851 0.42 0.50

0.58

LMV852 0.79 0.90

1.06

LMV854 1.54 1.67

1.99

SR Slew Rate (Note 7) AV = +1, VOUT = 1 VPP,

10% to 90%

4.5 V/μs

GBW Gain Bandwidth Product 8 MHz

ΦmPhase Margin 62 deg

enInput-Referred Voltage Noise f = 1 kHz 11 nV/

f = 10 kHz 10

inInput-Referred Current Noise f = 1 kHz 0.005 pA/

ROUT Closed Loop Output Impedance f = 6 MHz 400 Ω

CIN Common-Mode Input Capacitance 11 pF

Differential-Mode Input Capacitance 6

THD+N Total Harmonic Distortion + Noise f = 1 kHz, AV = 1, BW = >500 kHz 0.006 %

5V Electrical Characteristics (Note 4)

Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2, and RL = 10 kΩ to V+/2.

Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

VOS Input Offset Voltage ±0.26

(Note 9)

±1

±1.2 mV

TCVOS Input Offset Voltage Drift

(Note 10)

±0.4

(Note 9)

±2 μV/°C

IBInput Bias Current

(Note 10)

0.1 10

500 pA

IOS Input Offset Current 1 pA

CMRR Common Mode Rejection Ratio −0.2V ≤ VCM ≤ V+ −1.2V 77

(Note 9)

PSRR Power Supply Rejection Ratio 2.7V ≤ V+ ≤ 5.5V,

VOUT = 1V

(Note 9)

EMIRR EMI Rejection Ratio, IN+ and IN−

(Note 8)

VRFpeak = 100 mVP (−20 dBVP),

f = 400 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 900 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 1800 MHz

VRFpeak = 100 mVP (−20 dBVP),

f = 2400 MHz

CMVR Input Common-Mode Voltage Range CMRR ≥ 77 dB −0.2 3.8 V

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

AVOL Large Signal Voltage Gain

(Note 11)

RL = 2 kΩ,

VOUT = 0.15V to 2.5V,

VOUT = 4.85V to 2.5V

105

102

118

RL = 10 kΩ,

VOUT = 0.1V to 2.5V,

VOUT = 4.9V to 2.5V

105

102

120

VOOutput Swing High,

(measured from V+)

RL = 2 kΩ to V+/2 34 39

47 mV

RL = 10 kΩ to V+/2 7 11

Output Swing Low,

(measured from V−)

RL = 2 kΩ to V+/2 31 38

50 mV

RL = 10 kΩ to V+/2 7 12

IOOutput Short Circuit Current Sourcing, VOUT = VCM,

VIN = 100 mV

Sinking, VOUT = VCM,

VIN = −100 mV

ISSupply Current LMV851 0.43 0.52

0.60

LMV852 0.82 0.93

1.09

LMV854 1.59 1.73

2.05

SR Slew Rate (Note 7) AV = +1, VOUT = 2 VPP,

10% to 90%

4.5 V/μs

GBW Gain Bandwidth Product 8 MHz

ΦmPhase Margin 64 deg

enInput-Referred Voltage Noise f = 1 kHz 11 nV/

f = 10 kHz 10

inInput-Referred Current Noise f = 1 kHz 0.005 pA/

ROUT Closed Loop Output Impedance f = 6 MHz 400 Ω

CIN Common-Mode Input Capacitance 11 pF

Differential-Mode Input Capacitance 6

THD+N Total Harmonic Distortion + Noise f = 1 kHz, AV = 1, BW = >500 kHz 0.003 %

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics

Tables.

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.

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: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using statistical quality

control (SQC) method.

Note 7: Number specified is the slower of positive and negative slew rates.

Note 8: The EMI Rejection Ratio is defined as EMIRR = 20log ( VRFpeak/ΔVOS).

Note 9: The typical value is calculated by applying absolute value transform to the distribution, then taking the statistical average of the resulting distribution

Note 10: This parameter is guaranteed by design and/or characterization and is not tested in production.

Note 11: The specified limits represent the lower of the measured values for each output range condition.

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Connection Diagrams

5-Pin SC70

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Top View

8-Pin MSOP

20202103

Top View

14-Pin TSSOP

20202104

Top View

Ordering Information

Package Part Number Package Marking Transport Media NSC Drawing

5-Pin SC70 LMV851MG A98 1k Units Tape and Reel MAA05A

LMV851MGX 3k Units Tape and Reel

8-Pin MSOP LMV852MM AB5A 1k Units Tape and Reel MUA08A

LMV852MMX 3.5k Units Tape and Reel

14-Pin TSSOP LMV854MT LMV854MT 94 Units/Rail MTC14

LMV854MTX 2.5k Units Tape and Reel

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Typical Performance Characteristics At TA = 25°C, RL = 10 kΩ, VS = 3.3V, unless otherwise specified.

VOS vs. VCM at 3.3V

20202110

VOS vs. VCM at 5.0V

20202111

VOS vs. Supply Voltage

20202112

VOS vs. Temperature

20202113

VOS vs. VOUT

20202114

Input Bias Current vs. VCM at 25°C

20202115

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Input Bias Current vs. VCM at 85°C

20202116

Input Bias Current vs. VCM at 125°C

20202117

Supply Current vs. Supply Voltage Single LMV851

20202118

Supply Current vs. Supply Voltage Dual LMV852

20202119

Supply Current vs. Supply Voltage Quad LMV854

20202120

Supply Current vs. Temperature Single LMV851

20202121

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Supply Current vs. Temperature Dual LMV852

20202122

Supply Current vs. Temperature Quad LMV854

20202123

Sinking Current vs. Supply Voltage

20202124

Sourcing Current vs. Supply Voltage

20202125

Output Swing High vs. Supply Voltage RL = 2 kΩ

20202126

Output Swing High vs. Supply Voltage RL = 10 kΩ

20202127

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Output Swing Low vs. Supply Voltage RL = 2 kΩ

20202128

Output Swing Low vs. Supply Voltage RL = 10 kΩ

20202129

Output Voltage Swing vs. Load Current at 3.3V

20202155

Output Voltage Swing vs. Load Current at 5.0V

20202156

Open Loop Frequency Response vs. Temperature

20202131

Open Loop Frequency Response vs. Load Conditions

20202132

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Phase Margin vs. Capacitive Load

20202133

PSRR vs. Frequency

20202135

CMRR vs. Frequency

20202136

Channel Separation vs. Frequency

20202137

Large Signal Step Response with Gain = 1

20202138

Large Signal Step Response with Gain = 10

20202139

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Small Signal Step Response with Gain = 1

20202140

Small Signal Step Response with Gain = 10

20202141

Slew Rate vs. Supply Voltage

20202142

Overshoot vs. Capacitive Load

20202143

Input Voltage Noise vs. Frequency

20202144

THD+N vs. Frequency

20202145

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

THD+N vs. Amplitude

20202146

ROUT vs. Frequency

20202148

EMIRR IN+ vs. Power at 400 MHz

20202149

EMIRR IN+ vs. Power at 900 MHz

20202150

EMIRR IN+ vs. Power at 1800 MHz

20202151

EMIRR IN+ vs. Power at 2400 MHz

20202152

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

EMIRR IN+ vs. Frequency at 3.3V

20202153

EMIRR IN+ vs. Frequency at 5.0V

20202154

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

Application Information

INTRODUCTION

The LMV851/LMV852/LMV854 are operational amplifiers

with very good specifications, such as low offset, low noise

and a rail-to-rail output. These specifications make the

LMV851/LMV852/LMV854 great choices to use in areas such

as medical and instrumentation. The low supply current is

perfect for battery powered equipment. The small packages,

SC-70 package for the LMV851, the MSOP package for the

dual LMV852 and the TSSOP package for the quad LMV854,

make any of these parts a perfect choice for portable elec-

tronics. Additionally, the EMI hardening makes the LMV851/

LMV852 or LMV854 a must for almost all op amp applications.

Most applications are exposed to Radio Frequency (RF) sig-

nals such as the signals transmitted by mobile phones or

wireless computer peripherals. The LMV851/LMV852/

LMV854 will effectively reduce disturbances caused by RF

signals to a level that will be hardly noticeable. This again

reduces the need for additional filtering and shielding. Using

this EMI resistant series of op amps will thus reduce the num-

ber of components and space needed for applications that are

affected by EMI, and will help applications, not yet identified

as possible EMI sensitive, to be more robust for EMI.

INPUT CHARACTERISTICS

The input common mode voltage range of the LMV851/

LMV852/LMV854 includes ground, and can even sense well

below ground. The CMRR level does not degrade for input

levels up to 1.2V below the supply voltage. For a supply volt-

age of 5V, the maximum voltage that should be applied to the

input for best CMRR performance is thus 3.8V.

When not configured as unity gain, this input limitation will

usually not degrade the effective signal range. The output is

rail-to-rail and therefore will introduce no limitations to the

signal range.

The typical offset is only 0.26 mV, and the TCVOS is

0.4 μV/°C, specifications close to precision op amps.

CMRR MEASUREMENT

The CMRR measurement results may need some clarifica-

tion. This is because different setups are used to measure the

AC CMRR and the DC CMRR.

The DC CMRR is derived from ΔVOS versus ΔVCM. This value

is stated in the tables, and is tested during production testing.

The AC CMRR is measured with the test circuit shown in

Figure 1.

20202164

FIGURE 1. AC CMRR Measurement Setup

The configuration is largely the usually applied balanced con-

figuration. With potentiometer P1, the balance can be tuned

to compensate for the DC offset in the DUT. The main differ-

ence is the addition of the buffer. This buffer prevents the

open-loop output impedance of the DUT from affecting the

balance of the feedback network. Now the closed-loop output

impedance of the buffer is a part of the balance. But as the

closed-loop output impedance is much lower, and by careful

selection of the buffer also has a larger bandwidth, the total

effect is that the CMRR of the DUT can be measured much

more accurately. The differences are apparent in the larger

measured bandwidth of the AC CMRR.

One artifact from this test circuit is that the low frequency CM-

RR results appear higher than expected. This is because in

the AC CMRR test circuit the potentiometer is used to com-

pensate for the DC mismatches. So, mainly AC mismatch is

all that remains. Therefore, the obtained DC CMRR from this

AC CMRR test circuit tends to be higher than the actual DC

CMRR based on DC measurements.

The CMRR curve in Figure 2 shows a combination of the AC

CMRR and the DC CMRR.

20202136

FIGURE 2. CMRR Curve

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

OUTPUT CHARACTERISTICS

As already mentioned the output is rail to rail. When loading

the output with a 10 kΩ resistor the maximum swing of the

output is typically 7 mV from the positive and negative rail

The LMV851/LMV852/LMV854 can be connected as non-in-

verting unity gain amplifiers. This configuration is the most

sensitive to capacitive loading. The combination of a capaci-

tive load placed at the output of an amplifier along with the

amplifier’s output impedance creates a phase lag, which re-

duces the phase margin of the amplifier. If the phase margin

is significantly reduced, the response will be under damped

which causes peaking in the transfer and, when there is too

much peaking, the op amp might start oscillating. The

LMV851/LMV852/LMV854 can directly drive capacitive loads

up to 200 pF without any stability issues. In order to drive

heavier capacitive loads, an isolation resistor, RISO, should be

used, as shown in Figure 3. By using this isolation resistor,

the capacitive load is isolated from the amplifier’s output, and

hence, the pole caused by CL is no longer in the feedback

loop. The larger the value of RISO, the more stable the ampli-

fier will be. If the value of RISO is sufficiently large, the feed-

back loop will be stable, independent of the value of CL.

However, larger values of RISO result in reduced output swing

and reduced output current drive.

20202163

FIGURE 3. Isolating Capacitive Load

EMIRR

With the increase of RF transmitting devices in the world, the

electromagnetic interference (EMI) between those devices

and other equipment becomes a bigger challenge. The

LMV851/LMV852/LMV854 are EMI hardened op amps which

are specifically designed to overcome electromagnetic inter-

ference. Along with EMI hardened op amps, the EMIRR pa-

rameter is introduced to unambiguously specify the EMI

performance of an op amp. This section presents an overview

of EMIRR. A detailed description on this specification for EMI

hardened op amps can be found in Application Note AN-1698.

The dimensions of an op amp IC are relatively small com-

pared to the wavelength of the disturbing RF signals. As a

result the op amp itself will hardly receive any disturbances.

The RF signals interfering with the op amp are dominantly

received by the PCB and wiring connected to the op amp. As

a result the RF signals on the pins of the op amp can be rep-

resented by voltages and currents. This representation sig-

nificantly simplifies the unambiguous measurement and

specification of the EMI performance of an op amp.

RF signals interfere with op amps via the non-linearity of the

op amp circuitry. This non-linearity results in the detection of

the so called out-of-band signals. The obtained effect is that

the amplitude modulation of the out-of-band signal is down-

converted into the base band. This base band can easily

overlap with the band of the op amp circuit. As an example

Figure 4 depicts a typical output signal of a unity-gain con-

nected op amp in the presence of an interfering RF signal.

Clearly the output voltage varies in the rhythm of the on-off

keying of the RF carrier.

20202165

FIGURE 4. Offset Voltage Variation Due to an Interfering

RF Signal

EMIRR Definition

To identify EMI hardened op amps, a parameter is needed

that quantitatively describes the EMI performance of op

amps. A quantitative measure enables the comparison and

the ranking of op amps on their EMI robustness. Therefore

the EMI Rejection Ratio (EMIRR) is introduced. This param-

eter describes the resulting input-referred offset voltage shift

of an op amp as a result of an applied RF carrier (interference)

with a certain frequency and level. The definition of EMIRR is

given by:

In which VRF_PEAK is the amplitude of the applied un-modu-

lated RF signal (V) and ΔVOS is the resulting input-referred

offset voltage shift (V). The offset voltage depends quadrati-

cally on the applied RF level, and therefore, the RF level at

which the EMIRR is determined should be specified. The

standard level for the RF signal is 100 mVP. Application Note

AN-1698 addresses the conversion of an EMIRR measured

for an other signal level than 100 mVP. The interpretation of

the EMIRR parameter is straightforward. When two op amps

have an EMIRR which differ by 20 dB, the resulting error sig-

nals when used in identical configurations, differs by 20 dB as

well. So, the higher the EMIRR, the more robust the op amp.

Coupling an RF Signal to the IN+ Pin

Each of the op amp pins can be tested separately on EMIRR.

In this section the measurements on the IN+ pin (which, based

on symmetry considerations, also apply to the IN− pin) are

discussed. In Application Note AN-1698 the other pins of the

op amp are treated as well. For testing the IN+ pin the op amp

is connected in the unity gain configuration. Applying the RF

signal is straightforward as it can be connected directly to the

IN+ pin. As a result the RF signal path has a minimum of com-

ponents that might affect the RF signal level at the pin. The

circuit diagram is shown in Figure 5. The PCB trace from

RFIN to the IN+ pin should be a 50Ω stripline in order to match

the RF impedance of the cabling and the RF generator. On

the PCB a 50Ω termination is used. This 50Ω resistor is also

used to set the bias level of the IN+ pin to ground level. For

determining the EMIRR, two measurements are needed: one

is measuring the DC output level when the RF signal is off;

and the other is measuring the DC output level when the RF

signal is switched on. The difference of the two DC levels is

the output voltage shift as a result of the RF signal. As the op

amp is in the unity gain configuration, the input referred offset

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LMV851 Single/ LMV852 Dual/ LMV854 Quad

voltage shift corresponds one-to-one to the measured output

voltage shift.

20202167

FIGURE 5. Circuit for Coupling the RF Signal to IN+

Cell Phone Call

The effect of electromagnetic interference is demonstrated in

a setup where a cell phone interferes with a pressure sensor

application (Figure 7). This application needs two op amps

and therefore a dual op amp is used. The experiment is per-

formed on two different dual op amps: a typical standard op

amp and the LMV852, EMI hardened dual op amp. The op

amps are placed in a single supply configuration. The cell

phone is placed on a fixed position a couple of centimeters

from the op amps.

When the cell phone is called, the PCB and wiring connected

to the op amps receive the RF signal. Subsequently, the op

amps detect the RF voltages and currents that end up at their

pins. The resulting effect on the output of the second op amp

is shown in Figure 6.

20202168

FIGURE 6. Comparing EMI Robustness

The difference between the two types of dual op amps is

clearly visible. The typical standard dual op amp has an output

shift (disturbed signal) larger than 1V as a result of the RF

signal transmitted by the cell phone. The LMV852, EMI hard-

ened op amp does not show any significant disturbances.

DECOUPLING AND LAYOUT

Care must be given when creating a board layout for the op

amp. For decoupling the supply lines it is suggested that

10 nF capacitors be placed as close as possible to the op

amp. For single supply, place a capacitor between V+ and

V−. For dual supplies, place one capacitor between V+ and

the board ground, and a second capacitor between ground

and V−. Even with the LMV851/LMV852/LMV854 inherent

hardening against EMI, it is still recommended to keep the

input traces short and as far as possible from RF sources.

Then the RF signals entering the chip are as low as possible,

and the remaining EMI can be, almost, completely eliminated

in the chip by the EMI reducing features of the LMV851/

LMV852/LMV854.

PRESSURE SENSOR APPLICATION

The LMV851/LMV852/LMV854 can be used for pressure sen-

sor applications. Because of their low power the LMV851/

LMV852/LMV854 are ideal for portable applications, such as

blood pressure measurement devices, or portable barome-

ters. This example describes a universal pressure sensor that

can be used as a starting point for different types of sensors

and applications.

Pressure Sensor Characteristics

The pressure sensor used in this example functions as a

Wheatstone bridge. The value of the resistors in the bridge

change when pressure is applied to the sensor. This change

of the resistor values will result in a differential output voltage,

depending on the sensitivity of the sensor and the applied

pressure. The difference between the output at full scale

pressure and the output at zero pressure is defined as the

span of the pressure sensor. A typical value for the span is

100 mV. A typical value for the resistors in the bridge is

5 kΩ. Loading of the resistor bridge could result in incorrect

output voltages of the sensor. Therefore the selection of the

circuit configuration, which connects to the sensor, should

take into account a minimum loading of the sensor.

Pressure Sensor Example

The configuration shown in Figure 7 is simple, and is very

useful for the read out of pressure sensors. With two op amps

in this application, the dual LMV852 fits very well.

The op amp configured as a buffer and connected at the neg-

ative output of the pressure sensor prevents the loading of the

bridge by resistor R2. The buffer also prevents the resistors

of the sensor from affecting the gain of the following gain

stage. Given the differential output voltage VS of the pressure

sensor, the output signal of this op amp configuration, VOUT,

equals:

To align the pressure range with the full range of an ADC, the

power supply voltage and the span of the pressure sensor are

needed. For this example a power supply of 5V is used and

the span of the sensor is 100 mV.

When a 100Ω resistor is used for R2, and a 2.4 kΩ resistor is

used for R1, the maximum voltage at the output is 4.95V and

the minimum voltage is 0.05V. This signal is covering almost

the full input range of the ADC. Further processing can take

place in the microprocessor following the ADC.

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FIGURE 7. Pressure Sensor Application

THERMOCOUPLE AMPLIFIER

The following circuit is a typical example for a thermocouple

amplifier application using an LMV851/LMV852, or LMV854.

A thermocouple converts a temperature into a voltage. This

signal is then amplified by the LMV851/LMV852, or LMV854.

An ADC can convert the amplified signal to a digital signal.

For further processing the digital signal can be processed by

a microprocessor and used to display or log the temperature.

The temperature data can for instance be used in a fabrication

process.

Characteristics of a Thermocouple

A thermocouple is a junction of two different metals. These

metals produce a small voltage that increases with tempera-

ture.

The thermocouple used in this application is a K-type ther-

mocouple. A K-type thermocouple is a junction between Nick-

el-Chromium and Nickel-Aluminum. This is one of the most

commonly used thermocouples. There are several reasons

for using the K-type thermocouple, these include: tempera-

ture range, the linearity, the sensitivity, and the cost.

A K-type thermocouple has a wide temperature range. The

range of this thermocouple is from approximately −200°C to

approximately 1200°C, as can be seen in Figure 8. This cov-

ers the generally used temperature ranges.

Over the main part of the temperature range the output volt-

age depends linearly on the temperature. This is important for

easily converting the measured signal levels to a temperature

reading.

The K-type thermocouple has good sensitivity when com-

pared to many other types; the sensitivity is about 41 uV/°C.

Lower sensitivity requires more gain and makes the applica-

tion more sensitive to noise.

In addition, a K-type thermocouple is not expensive, many

other thermocouples consist of more expensive materials or

are more difficult to produce.

20202162

FIGURE 8. K-Type Thermocouple Response

Thermocouple Example

For this example, suppose the range of interest is 0°C to

500°C, and the resolution needed is 0.5°C. The power supply

for both the LMV851/LMV852, or LMV854 and the ADC is

3.3V.

The temperature range of 0°C to 500°C results in a voltage

range from 0 mV to 20.6 mV produced by the thermocouple.

This is indicated in Figure 8 by the dotted lines.

To obtain the highest resolution, the full ADC range of 0 to

3.3V is used. The gain needed for the full range can be cal-

culated as follows:

AV = 3.3V / 0.0206V = 160

If RG is 2 kΩ, then the value for RF can be calculated for a gain

of 160. Since AV = RF / RG, RF can be calculated as follows:

RF = AV x RG = 160 x 2 kΩ = 320 kΩ

To get a resolution of 0.5°C, the LSB of the ADC should be

smaller then 0.5°C / 500°C = 1/1000. A 10-bit ADC would be

sufficient as this gives 1024 steps. A 10-bit ADC such as the

two channel 10-bit ADC102S021 can be used.

17 www.national.com

LMV851 Single/ LMV852 Dual/ LMV854 Quad

Unwanted Thermocouple Effect

At the point where the thermocouple wires are connected to

the circuit, usually copper wires or traces, an unwanted ther-

mocouple effect will occur.

At this connection, this could be the connector on a PCB, the

thermocouple wiring forms a second thermocouple with the

connector. This second thermocouple disturbs the measure-

ments from the intended thermocouple.

Using an isothermal block as a reference enables correction

for this unwanted thermocouple effect. An isothermal block is

a good heat conductor. This means that the two thermocouple

connections both have the same temperature. The tempera-

ture of the isothermal block can be measured, and thereby

the temperature of the thermocouple connections. This is

usually called the cold junction reference temperature.

In the example, an LM35 is used to measure this temperature.

This semiconductor temperature sensor can accurately mea-

sure temperatures from −55°C to 150°C.

The two channel ADC in this example also converts the signal

from the LM35 to a digital signal. Now the microprocessor can

compensate the amplified thermocouple signal, for the un-

wanted thermocouple effect.

20202161

FIGURE 9. Thermocouple Read Out Circuit

www.national.com 18

LMV851 Single/ LMV852 Dual/ LMV854 Quad

Physical Dimensions inches (millimeters) unless otherwise noted

5-Pin SC70

NS Package Number MAA05A

8-Pin MSOP

NS Package Number MUA08A

19 www.national.com

LMV851 Single/ LMV852 Dual/ LMV854 Quad

14-Pin TSSOP

NS Package Number MTC14

www.national.com 20

LMV851 Single/ LMV852 Dual/ LMV854 Quad

Notes

21 www.national.com

LMV851 Single/ LMV852 Dual/ LMV854 Quad

Notes

LMV851 Single/ LMV852 Dual/ LMV854 Quad 8 MHz Low Power CMOS, EMI Hardened Operational

Amplifiers

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