LME49724MRX/NOPB - National Semiconductor

LME49724

LME49724 High Performance, High Fidelity, Fully-Differential Audio

Operational Amplifier

Literature Number: SNAS438

November 12, 2008

LME49724

High Performance, High Fidelity, Fully-Differential Audio

Operational Amplifier

General Description

The LME49724 is an ultra-low distortion, low noise, high slew

rate fully-differential operational amplifier optimized and fully

specified for high performance, high fidelity applications.

Combining advanced leading-edge process technology with

state of the art circuit design, the LME49724 fully-differential

audio operational amplifier delivers superior audio signal am-

plification for outstanding audio performance. The LME49724

combines extremely low voltage noise density (2.1nV/√Hz)

with vanishingly low THD+N (0.00003%) to easily satisfy the

most demanding audio applications. To ensure that the most

challenging loads are driven without compromise, the

LME49724 has a high slew rate of ±18V/μs and an output

current capability of ±80mA. Further, dynamic range is max-

imized by an output stage that drives 600Ω loads to 52VP-P

while operating on a ±15V supply voltage.

The LME49724's outstanding CMRR (102dB), PSRR

(125dB), and VOS (0.2mV) results in excellent operational

amplifier DC performance.

The LME49724 has a wide supply range of ±2.5V to ±18V.

Over this supply range the LME49724’s input circuitry main-

tains excellent common-mode and power supply rejection, as

well as maintaining its low input bias current. The LME49724

is unity gain stable. This Fully-Differential Audio Operational

Amplifier achieves outstanding AC performance while driving

complex loads with capacitive values as high as 100pF.

Key Specifications

■ Power Supply Voltage Range ±2.5V to ±18V

■ THD+N

(AV = 1, VOUT = 3VRMS, fIN = 1kHz)

RL = 2kΩ0.00003% (typ)

RL = 600Ω 0.00003% (typ)

■ Input Noise Density 2.1nV/√Hz (typ)

■ Slew Rate ±18V/μs (typ)

■ Gain Bandwidth Product 50MHz (typ)

■ Open Loop Gain (RL = 600Ω) 125dB (typ)

■ Input Bias Current 60nA (typ)

■ Input Offset Voltage 0.2mV (typ)

■ DC Gain Linearity Error 0.000009%

Features

■Drives 600Ω loads with full output signal swing

■Optimized for superior audio signal fidelity

■Output short circuit protection

■PSRR and CMRR exceed 100dB (typ)

■Available in PSOP package

Applications

■Ultra high quality audio amplification

■High fidelity preamplifiers and active filters

■Simple single-ended to differential conversion

■State of the art D-to-A converters

■State of the art A-to-D input amplifiers

■Professional Audio

■High fidelity equalization and crossover networks

■High performance line drivers and receivers

LME49724 High Performance, High Fidelity, Fully-Differential Audio Operational Amplifier

Typical Application

300442w9

FIGURE 1. Typical Application Circuit

Connection Diagrams

300442r4

Order Number LME49724MR

See NS Package Number MRA08B

PSOP Marking

300442r6

Top View

XY — Date Code

TT — Die Traceability

L49724 — LME49724

MR — Package Code

Ordering Information

Order Number Package Package DWG

#Transport Media MSL

Level Green Status Features

LME49724MR 8 lead PSOP MRA08B

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LME49724

Pin Descriptions

Pin Name Pin Function Type

1VIN- Input pin Analog Input

2VOCM

Sets the output DC voltage. Internally set by a resistor divider to the

midpoint of the voltages on the VCC and VEE pins. Can be forced

externally to a different voltage (50kΩ input impedance).

Analog Input

3VCC Positive power supply pin. Power Supply

4VOUT+

Output pin. Signal is inverted relative to VIN- where the feedback loop

is connected. Analog Output

5VOUT-

Output pin. Signal is inverted relative to VIN+ where the feedback loop

is connected. Analog Output

6VEE

Negative power supply pin or ground for a single supply

configuration. Power Supply

7 ENABLE

Enables the LME49724 when the voltage is greater than 2.35V

above the voltage on the VEE pin. Disable the LME49724 by

connecting to the same voltage as on the VEE pin which will reduce

current consumption to less than 0.3mA (typ).

Analog Input

8VIN+ Input pin Analog Input

Exposed

Pad Exposed pad for improved thermal performance. Connect to the

same potential as the VEE pin or electrically isolate.

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LME49724

Absolute Maximum Ratings (Notes 1, 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Power Supply Voltage

(VS = VCC + |VEE |) 38V

Storage Temperature −65°C to 150°C

Input Voltage (VEE) – 0.7V to (VCC) + 0.7V

Output Short Circuit Continuous

Power Dissipation (Note 3) Internally Limited

ESD Rating (Note 4) 2000V

ESD Rating (Note 5) 200V

Junction Temperature (TJMAX)150°C

Soldering Information

Vapor Phase (60sec.) 215°C

Infrared (60sec.) 220°C

Thermal Resistance

θJA (MR) 49.6°C/W

Operating Ratings (Notes 1, 2)

Temperature Range

TMIN ≤ TA ≤ TMAX −40°C ≤ TA ≤ +85°C

Supply Voltage Range ±2.5V ≤ VS ≤ ±18V

Electrical Characteristics (Notes 1, 2) The following specifications apply for VS = ±15V, RL = 2kΩ, fIN = 1kHz,

and TA = 25°C, unless otherwise specified.

Symbol Parameter Conditions

LME49724 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

POWER SUPPLY

VSOperating Power Supply

±2.5V

±18V

V (min)

V (max)

ICCQ Total Quiescent Current

VO = 0V, IO = 0mA

Enable = GND

Enable = VEE

0.3

0.5

mA (max)

PSRR Power Supply Rejection Ratio VS = ±5V to ±15V (Note 8) 125 95 dB (min)

VENIH Enable High Input Voltage Device active, TA = 25°C (Note 9) VEE + 2.35 V

VENIL Enable Low Input Voltage Device disabled, TA = 25°C (Note 9) VEE + 1.75 V

DYNAMIC PERFORMANCE

THD+N Total Harmonic Distortion + Noise

AV = 1, VOUT = 3VRMS

RL = 2kΩ

RL = 600Ω

0.00003

0.00003 0.00009

% (max)

IMD Intermodulation Distortion AV = 1, VOUT = 3VRMS

Two-tone, 60Hz & 7kHz 4:1 0.0005 %

GBWP Gain Bandwidth Product 50 35 MHz (min)

FPBW Full Power Bandwidth

VOUT = 1VP-P, –3dB

referenced to output magnitude

at f = 1kHz

MHz

SR Sew Rate RL = 2kΩ±18 ±13 V/μs (min)

tSSettling time AV = –1, 10V step, CL = 100pF

settling time to 0.1% 0.2 μs

AVOL Open-Loop Voltage Gain

–10V < VOUT < 10V, RL = 600Ω 125 100 dB (min)

–10V < VOUT < 10V, RL = 2kΩ125 dB

–10V < VOUT < 10V, RL = 10kΩ125 dB

NOISE

Equivalent Input Noise Voltage fBW = 20Hz to 20kHz 0.30 0.64 μVRMS

(max)

Equivalent Input Noise Density f = 1kHz

f = 10Hz

2.1

3.7

nV/√Hz

(max)

INPUT CHARACTERISTICS

VOS Offset Voltage ±0.2 ±1 mV (max)

ΔVOS/ΔTemp Average Input Offset Voltage Drift vs

Temperature –40°C ≤ TA ≤ 85°C 0.5 μV/°C

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LME49724

Symbol Parameter Conditions

LME49724 Units

(Limits)

Typical Limit

(Note 6) (Note 7)

IBInput Bias Current VCM = 0V 60 200 nA (max)

IOS Input Offset Current VCM = 0V 10 65 nA (max)

ΔIOS/ΔTemp Input Bias Current Drift vs

Temperature –40°C ≤ TA ≤ 85°C 0.1 nA/°C

VIN-CM Common-Mode Input Voltage Range ±14 VCC – 1.5

VEE + 1.5

V (min)

CMRR Common-Mode Rejection –10V < VCM < 10V 102 95 dB (min)

ZIN

Differential Input Impedance 16 kΩ

Common-Mode Input Impedance –10V < VCM < 10V 500 MΩ

OUTPUT CHARACTERISTICS

VOUTMAX Maximum Output Voltage Swing

RL = 600Ω 52 50 VP-P (min)

RL = 2kΩ52 VP-P

RL = 10kΩ53 VP-P

IOUT-CC Instantaneous Short Circuit Current 80 mA

ROUT Output Impedance

fIN = 10kHz

Closed-Loop

Open-Loop

0.01

Ω

CLOAD Capacitive Load Drive Overshoot CL = 100pF 5 %

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability

and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in

the Recommended Operating Conditions is not implied. The Recommended Operating Conditions indicate conditions at which the device is functional and the

device should not be operated beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified.

Note 2: The Electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified

or specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed.

Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The maximum

allowable power dissipation is PDMAX = (TJMAX - TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower.

Note 4: Human body model, applicable std. JESD22-A114C.

Note 5: Machine model, applicable std. JESD22-A115-A.

Note 6: Typical values represent most likely parametric norms at TA = +25ºC, and at the Recommended Operation Conditions at the time of product

characterization and are not guaranteed.

Note 7: Datasheet min/max specification limits are guaranteed by test or statistical analysis.

Note 8: PSRR is measured as follows: VOS is measured at two supply voltages, ±5V and ±15V. PSRR = | 20log(ΔVOS/ΔVS) |.

Note 9: The ENABLE threshold voltage is determined by VBE voltages and will therefore vary with temperature. The typical values represent the most likely

parametric norms at TA = +25°C.

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LME49724

Typical Performance Characteristics

THD+N vs Frequency

VS = ±2.5V, VO = 0.5VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s3

THD+N vs Frequency

VS = ±2.5V, VO = 0.8VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s4

THD+N vs Frequency

VS = ±15V, VO = 3VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s5

THD+N vs Frequency

VS = ±15V, VO = 10VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s6

THD+N vs Frequency

VS = ±18V, VO = 3VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s7

THD+N vs Frequency

VS = ±18V, VO = 10VRMS, Differential Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442s8

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LME49724

THD+N vs Output Voltage

VS = ±2.5V, RL = 600Ω, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t1

THD+N vs Output Voltage

VS = ±15V, RL = 600Ω, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t4

THD+N vs Output Voltage

VS = ±18V, RL = 600Ω, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t7

THD+N vs Output Voltage

VS = ±2.5V, RL = 2kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442s9

THD+N vs Output Voltage

VS = ±15V, RL = 2kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t2

THD+N vs Output Voltage

VS = ±18V, RL = 2kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t5

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LME49724

THD+N vs Output Voltage

VS = ±2.5V, RL = 10kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t0

THD+N vs Output Voltage

VS = ±15V, RL = 10kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t3

THD+N vs Output Voltage

VS = ±18V, RL = 10kΩ, Differential Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442t6

THD+N vs Frequency

VS = ±2.5V, VO = 0.5VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442u8

THD+N vs Frequency

VS = ±2.5V, VO = 0.8VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442u9

THD+N vs Frequency

VS = ±15V, VO = 3VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442v0

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LME49724

THD+N vs Frequency

VS = ±15V, VO = 5VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442v1

THD+N vs Frequency

VS = ±18V, VO = 3VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442v2

THD+N vs Frequency

VS = ±18V, VO = 5VRMS, Single-ended Input

RL = 600Ω, 2kΩ, 10kΩ, 80kHz BW

300442v3

THD+N vs Output Voltage

VS = ±2.5V, RL = 600Ω, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v6

THD+N vs Output Voltage

VS = ±15V, RL = 600Ω, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v9

THD+N vs Output Voltage

VS = ±18V, RL = 600Ω, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442w2

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LME49724

THD+N vs Output Voltage

VS = ±2.5V, RL = 2kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v4

THD+N vs Output Voltage

VS = ±15V, RL = 2kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v7

THD+N vs Output Voltage

VS = ±18V, RL = 2kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442w0

THD+N vs Output Voltage

VS = ±2.5V, RL = 10kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v5

THD+N vs Output Voltage

VS = ±15V, RL = 10kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442v8

THD+N vs Output Voltage

VS = ±18V, RL = 10kΩ, Single-ended Input

f = 20Hz, 1kHz, 20kHz, 80kHz BW

300442w1

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LME49724

PSRR vs Frequency

VS = ±2.5V, RL = 600Ω, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u0

PSRR vs Frequency

VS = ±15V, RL = 600Ω, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u3

PSRR vs Frequency

VS = ±18V, RL = 600Ω, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u6

PSRR vs Frequency

VS = ±2.5V, RL = 2kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442t8

PSRR vs Frequency

VS = ±15V, RL = 2kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u1

PSRR vs Frequency

VS = ±18V, RL = 2kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u4

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LME49724

PSRR vs Frequency

VS = ±2.5V, RL = 10kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442t9

PSRR vs Frequency

VS = ±15V, RL = 10kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u2

PSRR vs Frequency

VS = ±18V, RL = 10kΩ, Inputs to GND

VRIPPLE = 200mVP-P, 80kHz BW

300442u5

CMRR vs Frequency

VS = ±2.5V, VCMRR = 1VP-P

RL = 600Ω, 2kΩ, 10kΩ

300442y0

CMRR vs Frequency

VS = ±15V, VCMRR = 1VP-P

RL = 600Ω, 2kΩ, 10kΩ

300442x8

CMRR vs Frequency

VS = ±18V, VCMRR = 1VP-P

RL = 600Ω, 2kΩ, 10kΩ

300442x9

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LME49724

Output Voltage vs Load Resistance

VS = ±2.5V, RL = 500Ω – 10kΩ

THD+N ≤ 1%, 80kHz BW

300442w3

Output Voltage vs Load Resistance

VS = ±15V, RL = 500Ω – 10kΩ

THD+N ≤ 1%, 80kHz BW

300442w4

Output Voltage vs Load Resistance

VS = ±18V, RL = 500Ω – 10kΩ

THD+N ≤ 1%, 80kHz BW

300442w5

Output Voltage vs Supply Voltage

RL = 600Ω, 2kΩ, 10kΩ, THD+N ≤ 1%

80kHz BW

300442w6

Supply Current vs Supply Voltage

VIN = 0V, RL = No Load

300442u7

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LME49724

Application Information

GENERAL OPERATION

The LME49724 is a fully differential amplifier with an integrat-

ed common-mode reference input (VOCM). Fully differential

amplification provides increased noise immunity, high dy-

namic range, and reduced harmonic distortion products.

Differential amplifiers typically have high CMRR providing im-

proved immunity from noise. When input, output, and supply

line trace pairs are routed together, noise pick up is common

and easily rejected by the LME49724. CMRR performance is

directly proportional to the tolerance and matching of the gain

configuring resistors. With 0.1% tolerance resistors the worst

case CMRR performance will be about 60dB (20LOG

(0.001)).

A differential output has a higher dynamic range than a single-

ended output because of the doubling of output voltage. The

dynamic range is increased by 6dB as a result of the outputs

being equal in magnitude but opposite in phase. As an ex-

ample, a single-ended output with a 1VPP signal will be two

1VPP signals with a differential output. The increase is 20LOG

(2) = 6dB. Differential amplifiers are ideal for low voltage ap-

plications because of the increase in signal amplitude relative

to a single-ended amplifier and the resulting improvement in

SNR.

Differential amplifiers can also have reduced even order har-

monics, all conditions equal, when compared to a single-

ended amplifier. The differential output causes even harmon-

ics to cancel between the two inverted outputs leaving only

the odd harmonics. In practice even harmonics do not cancel

completely, however there still is a reduction in total harmonic

distortion.

OUTPUT COMMON-MODE VOLTAGE (VOCM pin)

The output common-mode voltage is the DC voltage on each

output. The output common-mode voltage is set by the

VOCM pin. The VOCM pin can be driven by a low impedance

source. If no voltage is applied to the VOCM pin, the DC com-

mon-mode output voltage will be set by the internal resistor

divider to the midpoint of the voltages on the VCC and VEE

pins. The input impedance of the VOCM pin is 50kΩ. The

VOCM pin can be driven up to VCC - 1.5V and VEE + 1.5V. The

VOCM pin should be bypassed to ground with a 0.1μF to 1μF

capacitor. The VOCM pin should be connected to ground when

the desired output common-mode voltage is ground refer-

ence. The value of the external capacitor has an effect on the

PSRR performance of the LME49724. With the VOCM pin only

bypassed with a low value capacitor, the PSRR performance

of the LME49724 will be reduced, especially at low audio fre-

quencies. For best PSRR performance, the VOCM pin should

be connected to stable, clean reference. Increasing the value

of the bypass capacitor on the VOCM pin will also improve

PSRR performance.

ENABLE FUNCTION

The LME49724 can be placed into standby mode to reduce

system current consumption by driving the ENABLE pin be-

low VEE + 1.75V. The LME49724 is active when the voltage

on the ENABLE pin is above VEE + 2.35V. The ENABLE pin

should not be left floating. For best performance under all

conditions, drive the ENABLE pin to the VEE pin voltage to

enter standby mode and to ground for active operation when

operating from split supplies. When operating from a single

supply, drive the ENABLE pin to ground for standby mode and

to VCC for active mode.

FULLY DIFFERENTIAL OPERATION

The LME49724 performs best in a fully differential configura-

tion. The circuit shown in Figure 2 is the typical fully differential

configuration.

300442r9

FIGURE 2. Fully Differential Configuration

The closed-loop gain is shown in Equation 1 below.

AV = RF / Ri (V/V) (1)

Where RF1 = RF2, Ri1 = Ri2. Using low value resistors will give

the lowest noise performance.

SINGLE-ENDED TO DIFFERENTIAL CONVERSION

For many applications, it is required to convert a single-ended

signal to a differential signal. The LME49724 can be used for

a high performance, simple single-to-differential converter.

Figure 3 shows the typical single-to-differential converter cir-

cuit configuration.

300442s0

FIGURE 3. Single-Ended Input to Differential Output

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LME49724

SINGLE SUPPLY OPERATION

The LME49724 can be operated from a single power supply,

as shown in Figure 4. The supply voltage range is limited to

a minimum of 5V and a maximum of 36V. The common-mode

output DC voltage will be set to the midpoint of the supply

voltage. The VOCM pin can be used to adjust the common-

mode output DC voltage on the outputs, as described previ-

ously, if the supply voltage midpoint is not the desired DC

voltage.

300442s1

FIGURE 4. Single Supply Configuration

DRIVING A CAPACITIVE LOAD

The LME49724 is a high speed op amp with excellent phase

margin and stability. Capacitive loads up to 100pF will cause

little change in the phase characteristics of the amplifiers and

are therefore allowable.

Capacitive loads greater than 100pF must be isolated from

the output. The most straightforward way to do this is to put

a resistor in series with the output. This resistor will also pre-

vent excess power dissipation if the output is accidentally

shorted.

THERMAL PCB DESIGN

The LME49724's high operating supply voltage along with its

high output current capability can result in significant power

dissipation. For this reason the LME49724 is provided in the

exposed DAP MSOP (PSOP) package for improved thermal

dissipation performance compared to other surface mount

packages. The exposed pad is designed to be soldered to a

copper plane on the PCB which then acts as a heat sink. The

thermal plane can be on any layer by using multiple thermal

vias under and outside the IC package. The vias under the IC

should have solder mask openings for the entire pad under

the IC on the top layer but cover the vias on the bottom layer.

This method prevents solder from being pulled away from the

thermal vias during the reflow process resulting in optimum

thermal conductivity.

Heat radiation from the PCB plane area is best accomplished

when the thermal plane is on the top or bottom copper layers.

The LME49724 should always be soldered down to a copper

pad on the PCB for both optimum thermal performance as

well as mechanical stability.

The exposed pad is for heat transfer and the thermal plane

should either be electrically isolated or connected to the same

potential as the VEE pin. For high frequency applications (f >

1MHz) or lower impedance loads, the pad should be con-

nected to a plane that is connected to the VEE potential.

SUPPLY BYPASSING

The LME49724 should have its supply leads bypassed with

low-inductance capacitors such as leadless surface mount

(SMT) capacitors located as close as possible to the supply

pins. It is recommended that a 10μF tantalum or electrolytic

capacitor be placed in parallel with a 0.1μF ceramic or film

type capacitor on each supply pin. These capacitors should

be star routed with a dedicated ground return plane or large

trace for best THD performance. Placing capacitors too far

from the power supply pins, especially with thin connecting

traces, can lead to excessive inductance, resulting in degrad-

ed high-frequency bypassing. Poor high-frequency bypassing

can result in circuit instabilities. When using high bandwidth

power supplies, the value and number of supply bypass ca-

pacitors should be reduced for optimal power supply perfor-

mance.

BALANCE CABLE DRIVER

With high peak-to-peak differential output voltage and plenty

of low distortion drive current, the LME49724 makes an ex-

cellent balanced cable driver. Combining the single-to-differ-

ential configuration with a balanced cable driver results in a

high performance single-ended input to balanced line driver

solution.

Although the LME49724 can drive capacitive loads up to

100pF, cable loads exceeding 100pF can cause instability.

For such applications, series resistors are needed on the out-

puts before the capacitive load.

ANALOG-TO-DIGITAL CONVERTER (ADC)

APPLICATION

Figure 5 is a typical fully differential application circuit for driv-

ing an analog-to-digital converter (ADC). The additional com-

ponents of R5, R6, and C7 are optional components and are

for stability and proper ADC sampling. ADC's commonly use

switched capacitor circuitry at the input. When the ADC sam-

ples the signal the current momentarily increases and may

disturb the signal integrity at the sample point causing a signal

glitch. Component C7 is significantly larger than the input ca-

pacitance of a typical ADC and acts as a charge reservoir

greatly reducing the effect of the signal sample by the ADC.

Resistors R5 and R6 decouple the capacitive load, C7, for sta-

bility. The values shown are general values. Specific values

should be optimized for the particular ADC loading require-

ments.

The output reference voltage from the ADC can be used to

drive the VOCM pin to set the common-mode DC voltage on

the outputs of the LME49724. A buffer may be needed to drive

the LME49724's VOCM pin if the ADC cannot drive the 50kΩ

input impedance of the VOCM pin.

In order to minimize circuit distortion when using capacitors

in the signal path, the capacitors should be comprised of ei-

ther NPO ceramic, polystyrene, polypropylene or mica com-

position. Other types of capacitors may provide a reduced

distortion performance but for a cost improvement, so capac-

itor selection is dependent upon design requirements. The

performance/cost tradeoff for a specific application is left up

to the user.

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LME49724

300442x7

* Value is application and converted dependent.

FIGURE 5. Typical Analog-to-Digital Converter Circuit

DISTORTION MEASUREMENTS

The vanishing low residual distortion produced by the

LME49724 is below the capabilities of commercially available

equipment. This makes distortion measurements more diffi-

cult than simply connecting a distortion meter to the

amplifier’s inputs and outputs. The solution, however, is quite

simple: an additional resistor. Adding this resistor extends the

resolution of the distortion measurement equipment.

The LME49724’s low residual distortion is an input referred

internal error. As shown in Figure 6, adding a resistor con-

nected between the amplifier’s inputs changes the amplifier’s

noise gain. The result is that the error signal (distortion) is

increased. Although the amplifier’s closed-loop gain is unal-

tered, the feedback available to correct distortion errors is

reduced, which means that measurement resolution increas-

es. To ensure minimum effects on distortion measurements,

keep the value of R5 low. The distortion reading on the audio

analyzer must be divided by a factor of (R3 + R4)/R5, where

R1 = R2 and R3 = R4, to get the actual measured distortion of

the device under test. The values used for the LME49724

measurements were R1, R2, R3, R4 = 1kΩ and R5 = 20Ω.

This technique is verified by duplicating the measurements

with high closed-loop gain and/or making the measurements

at high frequencies. Doing so produces distortion compo-

nents that are within the measurement equipment’s capabili-

ties.

www.national.com 16

LME49724

300442r5

FIGURE 6. THD+N and IMD Distortion Test Circuit

PERFORMANCE VARIATIONS

The LME49724 has excellent performance with little variation

across different supply voltages, load impedances, and input

configuration (single-ended or differential). Inspection of the

THD+N vs Frequency and THD+N vs Output Voltage perfor-

mance graphs reveals only minimal differences with different

load values. Figures 7 and 8 below show the performance

across different supply voltages with the same output signal

level and load. Figure 7 has plots at ±5V, ±12V, ±15V, and

±18V with a 3VRMS output while Figure 8 has plots at ±12V,

±15V, and ±18V with a 10VRMS output. Both figures use a

600Ω load. The performance for each different supply voltage

under the same conditions is so similar it is nearly impossible

to discern the different plots lines.

300442x5

FIGURE 7. THD+N vs FREQUENCY with RL = 600Ω

VOUT = 3VRMS, Differential Input, 80kHz BW

VS = ±5V, ±12V, ±15V, and ±18V

300442x4

FIGURE 8. THD+N vs FREQUENCY with RL = 600Ω

VOUT = 10VRMS, Differential Input, 80kHz BW

VS = ±12V, ±15V, and ±18V

Whether the input configuration is single-ended or differential

has only a minimal affect on THD+N performance at higher

audio frequencies or higher signal levels. For easy compari-

son, Figures 9 and 10 are a combination of the performance

graphs found in the Typical Performance Characteristics sec-

tion above.

17 www.national.com

LME49724

300442x3

FIGURE 9. THD+N vs FREQUENCY with RL = 10kΩ

VOUT = 3VRMS, VS = ±15V, 80kHz BW

Single-ended and Differential Input

300442x6

FIGURE 10. THD+N vs OUTPUT VOLTAGE with RL =

10kΩ

f = 20Hz, 1kHz, 20kHz, VS = ±15V, 80kHz BW

Single-ended and Differential Input

Power Supply Rejection Ratio does not vary with load value

nor supply voltage. For easy comparison, Figures 11 and 12

below are created by combining performance graphs found

in the Typical Performance Characteristics section above.

300442x1

FIGURE 11. PSRR vs FREQUENCY with RL = 600Ω

VS = ±2.5V, ±15V, and ±18V, 80kHz BW

300442x0

FIGURE 12. PSRR vs FREQUENCY with VS = ±15V

RL = 600Ω, 2kΩ, and 10kΩ, 80kHz BW

Although supply current may not be a critical specification for

many applications, there is also no real variation in supply

current with no load or with a 600Ω load. This is a result of the

extremely low offset voltage, typically less than 1mV. Figure

13 shows the supply current under the two conditions with no

real difference discernable.

300442x2

FIGURE 13. Supply Current vs Supply Voltage

RL = No Load and 600Ω

www.national.com 18

LME49724

Demo Board Schematic

300442w8

FIGURE 14. Demonstration Board Circuit

19 www.national.com

LME49724

Build of Materials

TABLE 1. Reference Demo Board Bill of Materials

Designator Value Tolerance Part Description Comment

R1, R2, R3, R41kΩ1% 1/8W, 0603 Resistor

R5, R640.2Ω 1% 1/8W, 0603 Resistor

C1, C21000pF 10% 0603, NPO Ceramic Capacitor, 50V

C3, C4, C8, C90.1μF–20%, +80% 0603, Y5V Ceramic Capacitor, 25V

C5, C610μF20% Size C (6032), Tantalum Capacitor, 25V

C72700pF 10% 0805, NPO Ceramic Capacitor, 50V

U1 LME49724MR

J1, J2, J3, J4 SMA coaxial connector Inputs & Outputs

J5 0.100" 1x3 header, vertical mount VDD, VEE, GND

J6, J7, J8, J9, J10,

J11

0.100" 1x2 header, vertical mount Inputs, Outputs, VOCM,

Enable

www.national.com 20

LME49724

Revision History

Rev Date Description

1.0 11/12/08 Initial release.

21 www.national.com

LME49724

Physical Dimensions inches (millimeters) unless otherwise noted

8 – Lead PSOP Package

Order Number LME49724MR

NS Package Number MRA08B

www.national.com 22

LME49724

Notes

23 www.national.com

LME49724

Notes

LME49724 High Performance, High Fidelity, Fully-Differential Audio Operational Amplifier

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