LMH6552SDEVAL - National Semiconductor

September 2007

LMH6552

1.5 GHz Fully Differential Amplifier

General Description

The LMH6552 is a high performance fully differential amplifier

designed to provide the exceptional signal fidelity and wide

large-signal bandwidth necessary for driving 8 to 14 bit high

speed data acquisition systems. Using National's proprietary

differential current mode input stage architecture, the

LMH6552 allows operation at gains greater than unity without

sacrificing response flatness, bandwidth, harmonic distortion,

or output noise performance.

With external gain set resistors and integrated common mode

feedback, the LMH6552 can be configured as either a differ-

ential input to differential output or single ended input to

differential output gain block. The LMH6552 can be AC or DC

coupled at the input which makes it suitable for a wide range

of applications including communication systems and high

speed oscilloscope front ends. The performance of the

LMH6552 driving an ADC14DS105 is 86 dBc SFDR and 74

dBc SNR up to 40 MHz.

The LMH6552 is available in an 8-pin SOIC package as well

as a space saving, thermally enhanced 8-Pin LLP package

for higher performance.

Features

■1.5 GHz −3 dB small signal bandwidth @ AV = 1

■1.25 GHz −3 dB large signal bandwidth @ AV = 1

■800 MHz bandwidth @ AV = 4

■450 MHz 0.1 dB flatness

■3800 V/µs slew rate

■10 ns settling time to 0.1%

■−90 dB THD @ 20 MHz

■−74 dB THD @ 70 MHz

■20 ns enable/shutdown pin

■5 to 12V operation

Applications

■Differential ADC driver

■Video over twisted pair

■Differential line driver

■Single end to differential converter

■High speed differential signaling

■IF/RF amplifier

■Level shift amplifier

■SAW filter buffer/driver

Typical Application

Single-Ended Input Differential Output ADC Driver

30003566

LMH™ is a trademark of National Semiconductor Corporation.

LMH6552 1.5 GHz Fully Differential Amplifier

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 5)

Human Body Model 2000V

Machine Model 200V

Supply Voltage 13.2V

Common Mode Input Voltage ±VS

Maximum Input Current (pins 1, 2, 7, 8) 30 mA

Maximum Output Current (pins 4, 5) (Note 4)

Soldering Information

Infrared or Convection (20 sec) 235°C

Wave Soldering (10 sec) 260°C

Operating Ratings (Note 1)

Operating Temperature Range

(Note 3) −40°C to +85°C

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

Total Supply Voltage 4.5V to 12V

Package Thermal Resistance (θJA)

8-Pin SOIC 150°C/W

8-Pin LLP 58°C/W

±5V Electrical Characteristics (Note 2)

Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = +5V, V− = −5V, AV= 1, VCM = 0V, RF = RG = 357Ω,

RL = 500Ω, for single ended in, differential out. Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 8)

Typ

(Note 7)

Max

(Note 8)

Units

AC Performance (Differential)

SSBW Small Signal −3 dB Bandwidth

(Note 8)

VOUT = 0.2 VPP, AV = 1, RL = 1 kΩ 1500

MHz

VOUT = 0.2 VPP, AV = 1 1000

VOUT = 0.2 VPP, AV = 2 930

VOUT = 0.2 VPP, AV = 4 810

VOUT = 0.2 VPP, AV = 8 590

LSBW Large Signal −3 dB Bandwidth VOUT = 2 VPP, AV = 1, RL = 1 kΩ 1250

MHz

VOUT = 2 VPP, AV = 1 950

VOUT = 2 VPP, AV = 2 820

VOUT = 2 VPP, AV = 4 740

VOUT = 2 VPP, AV = 8 590

0.1 dB Bandwidth VOUT = 0.2 VPP, AV = 1 450 MHz

Slew Rate 4V Step, AV = 1 3800 V/μs

Rise/Fall Time, 10%-90% 2V Step 600 ps

0.1% Settling Time 2V Step 10 ns

Overdrive Recovery Time VIN = 1.8V to 0V Step, AV = 5 V/V 6 ns

Distortion and Noise Response

HD2 2nd Harmonic Distortion VOUT = 2 VPP, f = 20 MHz, RL = 800Ω −92

dBc

VOUT = 2 VPP, f = 70 MHz, RL = 800Ω −74

HD3 3rd Harmonic Distortion VOUT = 2 VPP, f = 20 MHz, RL = 800Ω −93

dBc

VOUT = 2 VPP, f = 70 MHz, RL = 800Ω −84

IMD3 Two-Tone Intermodulation f >70 MHz, Third Order Products, VOUT

= 2 VPP Composite

−87 dBc

Input Noise Voltage f ≥ 1 MHz 1.1 nV/

Input Noise Current f ≥ 1 MHz 19.5 pA/

Noise Figure (See Figure 5)50Ω System, AV = 9, 10 MHz 10.3 dB

Input Characteristics

IBI Input Bias Current (Note 10) 60 110 µA

IBoffset Input Bias Current Differential

(Note 7)

VCM = 0V, VID = 0V, IBoffset = (IB− - IB+)/2 2.5 18 µA

CMRR Common Mode Rejection Ratio

(Note 7)

DC, VCM = 0V, VID = 0V 80 dBc

RIN Input Resistance Differential 15 Ω

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LMH6552

Symbol Parameter Conditions Min

(Note 8)

Typ

(Note 7)

Max

(Note 8)

Units

CIN Input Capacitance Differential 0.5 pF

CMVR Input Common Mode Voltage

Range

CMRR > 38 dB ±3.5 ±3.8 V

Output Performance

Output Voltage Swing (Note 7) Differential Output 14.8 15.4 VPP

IOUT Linear Output Current (Note 7) VOUT = 0V ±70 ±80 mA

ISC Short Circuit Current One Output Shorted to Ground VIN = 2V

Single Ended (Note 6)

±141 mA

Output Balance Error ΔVOUT Common Mode /ΔVOUT

Differential , ΔVOD = 1V, f < 1 MHz

−60 dB

Miscellaneous Performance

ZTOpen Loop Transimpedance Differential 108 dBΩ

PSRR Power Supply Rejection Ratio DC, ΔVS = ±1V 80 dB

ISSupply Current (Note 7) RL = ∞19 22.5 25

28 mA

Enable Voltage Threshold 3.0 V

Disable Voltage Threshold 2.0 V

Enable/Disable time 15 ns

ISD Disable Shutdown Current 500 600 μA

Output Common Mode Control Circuit

Common Mode Small Signal

Bandwidth

VIN+ = VIN− = 0 400 MHz

Slew Rate VIN+ = VIN− = 0 607 V/μs

VOSCM Input Offset Voltage Common Mode, VID = 0, VCM = 0 1.5 ±16.5 mV

Input Bias Current (Note 9) −3.2 ±8 µA

Voltage Range ±3.7 ±3.8 V

CMRR Measure VOD, VID = 0V 80 dB

Input Resistance 200 kΩ

Gain ΔVO,CM/ΔVCM 0.995 1.0 1.012 V/V

±2.5V Electrical Characteristics (Note 2)

Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = +2.5V, V− = −2.5V, AV = 1, VCM = 0V, RF = RG = 357Ω,

RL = 500Ω, for single ended in, differential out. Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 8)

Typ

(Note 7)

Max

(Note 8)

Units

SSBW Small Signal −3 dB Bandwidth

(Note 8)

VOUT = 0.2 VPP, AV = 1, RL = 1 kΩ 1100

MHz

VOUT = 0.2 VPP, AV = 1 800

VOUT = 0.2 VPP, AV = 2 740

VOUT = 0.2 VPP, AV = 4 660

VOUT = 0.2 VPP, AV = 8 498

LSBW Large Signal −3 dB Bandwidth VOUT = 2 VPP, AV = 1, RL = 1 kΩ 820

MHz

VOUT = 2 VPP, AV = 1 690

VOUT = 2 VPP, AV = 2 620

VOUT = 2 VPP, AV = 4 589

VOUT = 2 VPP, AV = 8 480

0.1 dB Bandwidth VOUT = 0.2 VPP, AV = 1 300 MHz

Slew Rate 2V Step, AV = 1 2100 V/μs

Rise/Fall Time, 10% to 90% 2V Step 700 ps

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LMH6552

Symbol Parameter Conditions Min

(Note 8)

Typ

(Note 7)

Max

(Note 8)

Units

0.1% Settling Time 2V Step 10 ns

Overdrive Recovery Time VIN = 0.7 V to 0 V Step, AV = 5 V/V 6 ns

Distortion and Noise Response

HD2 2nd Harmonic Distortion VOUT = 2 VPP, f = 20 MHz, RL = 800Ω 82

dBc

VOUT = 2 VPP, f = 70 MHz, RL = 800Ω 65

HD3 3rd Harmonic Distortion VOUT = 2 VPP, f = 20 MHz, RL = 800Ω 79

dBc

VOUT = 2 VPP, f = 70 MHz, RL = 800Ω 67

IMD3 Two-Tone Intermodulation f ≥ 70 MHz, Third Order Products,

VOUT = 2 VPP Composite

−77 dBc

Input Noise Voltage f ≥ 1 MHz 1.1 nV/

Input Noise Current f ≥ 1 MHz 19.5 pA/

Noise Figure (See Figure 5)50Ω System, AV = 9, 10 MHz 10.2 dB

Input Characteristics

IBI Input Bias Current (Note 10) 54 90 µA

IBoffset Input Bias Current Differential

(Note 7)

VCM = 0V, VID = 0V, IBoffset = (IB− - IB+ )/2 2.3 18 μA

CMRR Common-Mode Rejection Ratio

(Note 7)

DC, VCM = 0V, VID = 0V 75 dBc

RIN Input Resistance Differential 15 Ω

CIN Input Capacitance Differential 0.5 pF

CMVR Input Common Mode Range CMRR > 38 dB ±1.0 ±1.3 V

Output Performance

Output Voltage Swing (Note 7) Differential Output 5.6 6.0 VPP

IOUT Linear Output Current (Note 7) VOUT = 0V ±55 ±65 mA

ISC Short Circuit Current One Output Shorted to Ground, VIN = 2V

Single Ended (Note 6)

±131 mA

Output Balance Error ΔVOUT Common Mode /ΔVOUT

Differential , ΔVOD = 1V, f < 1 MHz

60 dB

Miscellaneous Performance

ZT Open Loop Transimpedance Differential 107 dBΩ

PSRR Power Supply Rejection Ratio DC, ΔVS = ±1V 80 dB

ISSupply Current (Note 7) RL = ∞17 20.4 24

27 mA

Enable Voltage Threshold 3.0 V

Disable Voltage Threshold 2.0 V

Enable/Disable Time 15 ns

ISD Disable Shutdown Current 500 600 µA

Output Common Mode Control Circuit

Common Mode Small Signal

Bandwidth

VIN+ = VIN− = 0 310 MHz

Slew Rate VIN+ = VIN− = 0 430 V/μs

VOSCM Input Offset Voltage Common Mode, VID = 0, VCM = 0 1.65 ±15 mV

Input Bias Current (Note 9) −2.9 µA

Voltage Range ±1.19 ±1.25 V

CMRR Measure VOD, VID = 0V 80 dB

Input Resistance 200 kΩ

Gain ΔVO,CM/ΔVCM 0.995 1.0 1.012 V/V

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LMH6552

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, see the Electrical Characteristics tables.

Note 2: 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. See Applications Section for information on temperature de-rating of this device." Min/Max ratings are based on product characterization and simulation.

Individual parameters are tested as noted.

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: The maximum output current (IOUT) is determined by device power dissipation limitations. See the Power Dissipation section of the Application Section

for more details.

Note 5: Human Body Model, applicable std. MIL-STD-883, Method 30157. 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 6: Short circuit current should be limited in duration to no more than 10 seconds. See the Power Dissipation section of the Application Information for more

details.

Note 7: 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 8: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality

Control (SQC) methods.

Note 9: Negative input current implies current flowing out of the device.

Note 10: IBI is referred to a differential output offset voltage by the following relationship: VOD(offset) = IBI*2RF

Connection Diagrams

8-Pin SOIC

30003508

Top View

8-Pin LLP

30003564

Top View

Ordering Information

Package Part Number Package Marking Transport Media NSC Drawing

8-Pin SOIC LMH6552MA LMH6552MA 95/Rails M08A

LMH6552MAX 2.5k Units Tape and Reel

8-Pin LLP LMH6552SD 6552 1k Units Tape and Reel SDA08C

LMH6552SDX 4.5k Units Tape and Reel

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LMH6552

Typical Performance Characteristics V+ = +5V, V− = −5V (TA = 25°C, RF = RG =

357Ω, RL = 500Ω, AV = 1, for single ended in, differential out, unless specified).

Frequency Response vs. Gain

30003547

Frequency Response vs. Gain

30003534

Frequency Response vs. VOUT

30003548

Frequency Response vs. VOUT

30003516

Frequency Response vs. Supply Voltage

30003562

Frequency Response vs. Supply Voltage

30003563

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LMH6552

Frequency Response vs. Capacitive Load

30003521

Suggested ROUT vs. Capacitive Load

30003522

Frequency Response vs. Resistive Load

30003559

Frequency Response vs. Resistive Load

30003560

Frequency Response vs. RF

30003561

1 VPP Pulse Response Single Ended Input

30003526

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LMH6552

2 VPP Pulse Response Single Ended Input

30003527

Large Signal Pulse Response

30003525

Output Common Mode Pulse Response

30003524

Distortion vs. Frequency Single Ended Input

30003529

Distortion vs. Supply Voltage

30003543

Distortion vs. Supply Voltage

30003537

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LMH6552

Distortion vs. Output Common Mode Voltage

30003538

Distortion vs. Output Common Mode Voltage

30003567

Maximum VOUT vs. IOUT

30003530

Minimum VOUT vs. IOUT

30003531

Open Loop Transimpedance

30003541

Open Loop Transimpedance

30003542

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LMH6552

Closed Loop Output Impedance

30003517

Closed Loop Output Impedance

30003518

Overdrive Recovery

30003557

Overdrive Recovery

30003558

PSRR

30003519

PSRR

30003520

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LMH6552

CMRR

30003533

Balance Error

30003513

Noise Figure

30003545

Noise Figure

30003546

Input Noise vs. Frequency

30003549

Differential S-Parameter Magnitude vs. Frequency

30003555

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LMH6552

Differential S-Parameter Phase vs. Frequency

30003556

3rd Order Intermodulation Products vs. VOUT

30003551

3rd Order Intermodulation Products vs. VOUT

30003552

3rd Order Intermodulation Products vs. Center Frequency

30003565

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LMH6552

Application Information

The LMH6552 is a fully differential current feedback amplifier

with integrated output common mode control, designed to

provide low distortion amplification to wide bandwidth differ-

ential signals. The common mode feedback circuit sets the

output common mode voltage independent of the input com-

mon mode, as well as forcing the V+ and V− outputs to be

equal in magnitude and opposite in phase, even when only

one of the inputs is driven as in single to differential conver-

sion.

The proprietary current feedback architecture of the

LMH6552 offers gain and bandwidth independence with ex-

ceptional gain flatness and noise performance, even at high

values of gain, simply with the appropriate choice of RF1 and

RF2. Generally RF1 is set equal to RF2, and RG1 equal to

RG2, so that the gain is set by the ratio RF/RG. Matching of

these resistors greatly affects CMRR, DC offset error, and

output balance. A minimum of 0.1% tolerance resistors are

recommended for optimal performance, and the amplifier is

internally compensated to operate with optimum gain flatness

with values of RF between 270Ω and 390Ω depending on

package selection, PCB layout, and load resistance.

The output common mode voltage is set by the VCM pin with

a fixed gain of 1 V/V. This pin should be driven by a low

impedance reference and should be bypassed to ground with

a 0.1 µF ceramic capacitor. Any unwanted signal coupling into

the VCM pin will be passed along to the outputs, reducing the

performance of the amplifier. This pin must not be left floating.

The LMH6552 can be operated on a supply range as either a

single 5V supply or as a split +5V and −5V. Operation on a

single 5V supply, depending on gain, is limited by the input

common mode range; therefore, AC coupling may be re-

quired. For example, in a DC coupled input application on a

single 5V supply, with a VCM of 1.5V, the input common volt-

age at a gain of 1 will be 0.75V which is outside the minimum

1.2V to 3.8V input common mode range of the amplifier. The

minimum VCM for this application should be greater than 2.5V

depending on output signal swing. Alternatively, AC coupling

of the inputs in this example results in equal input and output

common mode voltages, so a 1.5V VCM would be achievable.

Split supplies will allow much less restricted AC and DC cou-

pled operation with optimum distortion performance.

The LMH6552 is equipped with an ENABLE pin to reduce

power consumption when not in use. The ENABLE pin, when

not driven, floats high (on). When the ENABLE pin is pulled

low the amplifier is disabled and the amplifier output stage

goes into a high impedance state so the feedback and gain

set resistors determine the output impedance of the circuit.

For this reason input to output isolation will be poor in the

disabled state and the part is not recommended in multiplexed

applications where outputs are all tied together.

LLP PACKAGE

Due to it's size and lower parasitics, the LLP requires the low-

er optimum value of 275Ω for RF. This will give a flat frequency

response with minimal peaking. With a lower RF value the LLP

package will have a reduction in noise compared to the SOIC

with its optimum RF = 360Ω.

FULLY DIFFERENTIAL OPERATION

The LMH6552 will perform best in a fully differential configu-

ration. The circuit shown in Figure 1 is a typical fully differential

application circuit as might be used to drive an analog to dig-

ital converter (ADC). In this circuit the closed loop gain

AV = VOUT/ VIN = RF/RG, where the feedback is symmetric.

The series output resistors, RO, are optional and help keep

the amplifier stable when presented with a capacitive load.

Refer to the Driving Capacitive Loads section for details.

30003504

FIGURE 1. Typical Application

When driven from a differential source, the LMH6552 pro-

vides low distortion, excellent balance, and common mode

rejection. This is true provided the resistors RF, RG and RO

are well matched and strict symmetry is observed in board

layout. With an intrinsic device CMRR of 80 dB, using 0.1%

resistors will give a worst case CMRR of around 60 dB for

most circuits.

30003553

FIGURE 2. Differential S-Parameter Test Circuit

The circuit configuration shown in Figure 2 was used to mea-

sure differential S parameters in a 50Ω environment at a gain

of 1 V/V. Refer to the Differential S-Parameter vs. Frequency

plots in the Typical Performance Characteristics section for

measurement results.

SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT

OPERATION

In many applications, it is required to drive a differential input

ADC from a single ended source. Traditionally, transformers

have been used to provide single to differential conversion,

but these are inherently bandpass by nature and cannot be

used for DC coupled applications. The LMH6552 provides

excellent performance as a single-to-differential converter

down to DC. Figure 3 shows a typical application circuit where

an LMH6552 is used to produce a differential signal from a

single ended source.

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LMH6552

30003510

FIGURE 3. Single Ended Input with Differential Output

When using the LMH6552 in single-to-differential mode, the

complimentary output is forced to a phase inverted replica of

the driven output by the common mode feedback circuit as

opposed to being driven by its own complimentary input. Con-

sequently, as the driven input changes, the common mode

feedback action results in a varying common mode voltage at

the amplifier's inputs, proportional to the driving signal. Due

to the non-ideal common mode rejection of the amplifier's in-

put stage, a small common mode signal appears at the out-

puts which is superimposed on the differential output signal.

The ratio of the change in output common mode voltage to

output differential voltage is commonly referred to as output

balance error. The output balance error response of the

LMH6552 over frequency is shown in the Typical Perfor-

mance Characteristics section.

To match the input impedance of the circuit in Figure 3 to a

specified source resistance, RS, requires that RT || RIN = RS.

The equations governing RIN and AV for single-to-differential

operation are also provided in Figure 3. These equations,

along with the source matching condition, must be solved it-

eratively to achieve the desired gain with the proper input

termination. Component values for several common gain con-

figurations in a 50Ω environment are given in Table 1.

Table 1. Gain Component Values for 50Ω System SOIC

Package

Gain RFRGRTRM

0 dB 357Ω 348Ω 56.2Ω 26.4Ω

6 dB 357Ω 169Ω 61.8Ω 27.6Ω

12 dB 357Ω 76.8Ω 76.8Ω 30.9Ω

Table 2. Gain Component Values for 50Ω System LLP

Package

Gain RFRGRTRM

0 dB 275Ω 255Ω 59Ω 26.7Ω

6 dB 275Ω 127Ω 68.1Ω 28.7Ω

12 dB 275Ω 54.9Ω 107Ω 34Ω

30003554

FIGURE 4. Single Ended Input S-Parameter Test Circuit

(50Ω System)

The circuit shown in Figure 4 was used to measure S-param-

eters for a single-to-differential configuration. The S-parame-

ter plots in the Typical Performance Curves are taken using

the recommended component values for 0 dB gain.

SINGLE SUPPLY OPERATION

Single supply operation is possible on supplies from 5V to

10V; however, as discussed earlier, AC input coupling is rec-

ommended for low supplies such as 5V due to input common

mode limitations. An example of an AC coupled, single sup-

ply, single-to-differential circuit is shown in Figure 5. Note that

when AC coupling, both inputs need to be AC coupled irre-

spective of single-to-differential or differential-to-differential

configuration. For higher supply voltages DC coupling of the

inputs may be possible provided that the output common

mode DC level is set high enough so that the amplifier's inputs

and outputs are within their specified operating ranges.

30003509

FIGURE 5. AC Coupled for Single Supply Operation

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LMH6552

SPLIT SUPPLY OPERATION

For optimum performance, split supply operation is recom-

mended using +5V and −5V supplies; however, operation is

possible on split supplies as low as +2.25V and −2.25V and

as high as +6V and −6V. Provided the total supply voltage

does not exceed the 4.5V to 12V operating specification, non-

symmetric supply operation is also possible and in some

cases advantageous. For example , if a 5V DC coupled op-

eration is required for low power dissipation but the amplifier

input common mode range prevents this operation, it is still

possible with split supplies of (V+) and (V−). Where

(V+) - (V−) = 5V and V+ and V− are selected to center the am-

plifier input common mode range to suit the application.

OUTPUT NOISE PERFORMANCE AND MEASUREMENT

Unlike differential amplifiers based on voltage feedback ar-

chitectures, noise sources internal to the LMH6552 refer to

the inputs largely as current sources, hence the low input re-

ferred voltage noise and relatively higher input referred cur-

rent noise. The output noise is therefore more strongly

coupled to the value of the feedback resistor and not to the

closed loop gain, as would be the case with a voltage feed-

back differential amplifier. This allows operation of the

LMH6552 at much higher gain without incurring a substantial

noise performance penalty, simply by choosing a suitable

feedback resistor.

Figure 6 shows a circuit configuration used to measure noise

figure for the LMH6552 in a 50Ω system. An RF value of

275Ω is chosen for the SOIC package to minimize output

noise while simultaneously allowing both high gain (9 V/V)

and proper 50Ω input termination. Refer to the section titled

Single Ended Input Operation for calculation of resistor and

gain values. Noise figure values at various frequencies are

shown in the plot titled Noise Figure in the Typical Perfor-

mance Characteristics section.

30003550

FIGURE 6. Noise Figure Circuit Configuration

DRIVING ANALOG TO DIGITAL CONVERTERS

Analog-to-digital converters present challenging load condi-

tions. They typically have high impedance inputs with large

and often variable capacitive components. As well, there are

usually current spikes associated with switched capacitor or

sample and hold circuits. Figure 7 shows a combination circuit

of the LMH6552 driving the ADC12DL080. The two 125Ω re-

sistors serve to isolate the capacitive loading of the ADC from

the amplifier and ensure stability. In addition, the resistors,

along with a 2.2 pF capacitor across the outputs (in parallel

with the ADC input capacitance), form a low pass anti-aliasing

filter with a pole frequency of about 60 MHz. For switched

capacitor input ADCs, the input capacitance will vary based

on the clock cycle, as the ADC switches between the sample

and hold mode. See your particular ADC's datasheet for de-

tails.

30003505

FIGURE 7. Driving a 12-bit ADC

Figure 8 shows the SFDR and SNR performance vs. frequen-

cy for the LMH6552 and ADC12DL080 combination circuit

with the ADC input signal level at −1 dBFS. The ADC12DL080

is a dual 12-bit ADC with maximum sampling rate of 80 MSPS.

The amplifier is configured to provide a gain of 2 V/V in single

to differential mode. An external band-pass filter is inserted in

series between the input signal source and the amplifier to

reduce harmonics and noise from the signal generator. In or-

der to properly match the input impedance seen at the

LMH6552 amplifier inputs, RM is chosen to match ZS || RT for

proper input balance.

30003540

FIGURE 8. LMH6552/ADC12DL080 SFDR and SNR

Performance vs. Frequency

Figure 9 shows a combination circuit of the LMH6552 driving

the ADC14DS105. The ADC14DS105 is a dual channel 14-

bit ADC with a sampling rate of 105 MSPS. The circuit in

Figure 9 has a 2nd order low-pass LC filter formed by the 620

nH inductor along with the 22 pF capacitor across the differ-

ential outputs of the LMH6552. The filter has a pole frequency

of about 50 MHz. Figure 10 shows the combined SFDR and

SNR performance over frequency with a −1 dBFs input signal

and a sampling rate of 1000 MSPS.

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LMH6552

30003566

FIGURE 9. Driving a 14-bit ADC

The amplifier is configured to provide a gain of 2 V/V in a sin-

gle-to-differential mode. The LMH6552 common mode volt-

age is set by the ADC14DS105. Circuit testing is the same as

described for the LMH6552 and ADC12DL080 combination

circuit. The 0.1 µF capacitor, in series with the 49.9Ω resistor,

is inserted to ground across the 68.1Ω resistor to balance the

amplifier inputs.

30003568

FIGURE 10. LMH6552/ADC14DS105 SFDR and SNR

Performance vs. Frequency

The amplifier and ADC should be located as close as possi-

ble. Both devices require that the filter components be in close

proximity to them. The amplifier needs to have minimal par-

asitic loading on the output traces and the ADC is sensitive to

high frequency noise that may couple in on its input lines.

Some high performance ADCs have an input stage that has

a bandwidth of several times its sample rate. The sampling

process results in all input signals presented to the input stage

mixing down into the first Nyquist zone (DC to Fs/2).

The LMH6552 is capable of driving a variety of National Semi-

conductor Analog-to-Digital Converters. This is shown in Ta-

ble 3, which offers a list of possible signal path ADC and

amplifier combinations. The use of the LMH6552 to drive an

ADC is determined by the application and the desired sam-

pling process (Nyquist operation, sub-sampling or over-sam-

pling). See application note AN-236 for more details on the

sampling processes and application note AN-1393 'Using

High Speed Differential Amplifiers to Drive ADCs. For more

information regarding a particular ADC, refer to the particular

ADC datasheet for details.

TABLE 3. DIFFERENTIAL INPUT ADC's COMPATIBLE

WITH LMH6552 DRIVER

Product Number Max

Sampling

Rate

(MSPS)

Resolution Channels

ADC1173 15 8 SINGLE

ADC1175 20 8 SINGLE

ADC08351 42 8 SINGLE

ADC1175-50 50 8 SINGLE

ADC08060 60 8 SINGLE

ADC08L060 60 8 SINGLE

ADC08100 100 8 SINGLE

ADC08200 200 8 SINGLE

ADC08500 500 8 SINGLE

ADC081000 1000 8 SINGLE

ADC08D1000 1000 8 DUAL

ADC10321 20 10 SINGLE

ADC10D020 20 10 DUAL

ADC10030 27 10 SINGLE

ADC10040 40 10 DUAL

ADC10065 65 10 SINGLE

ADC10DL065 65 10 DUAL

ADC10080 80 10 SINGLE

ADC11DL066 66 11 DUAL

ADC11L066 66 11 SINGLE

ADC11C125 125 11 SINGLE

ADC11C170 170 11 SINGLE

ADC12010 10 12 SINGLE

ADC12020 20 12 SINGLE

ADC12040 40 12 SINGLE

ADC12D040 40 12 DUAL

ADC12DL040 40 12 DUAL

ADC12DL065 65 12 DUAL

ADC12DL066 66 12 DUAL

ADC12L063 63 12 SINGLE

ADC12C080 80 12 SINGLE

ADC12DS080 80 12 DUAL

ADC12L080 80 12 SINGLE

ADC12C105 105 12 SINGLE

ADC12DS105 105 12 DUAL

ADC12C170 170 12 SINGLE

ADC14L020 20 14 SINGLE

ADC14L040 40 14 SINGLE

ADC14C080 80 14 SINGLE

ADC14DS080 80 14 DUAL

ADC14C105 105 14 SINGLE

ADC14DS105 105 14 DUAL

ADC14155 155 14 SINGLE

www.national.com 16

LMH6552

DRIVING CAPACITIVE LOADS

As noted previously, capacitive loads should be isolated from

the amplifier output with small valued resistors. This is par-

ticularly the case when the load has a resistive component

that is 500Ω or higher. A typical ADC has capacitive compo-

nents of around 10 pF and the resistive component could be

1000Ω or higher. If driving a transmission line, such as 50Ω

coaxial or 100Ω twisted pair, using matching resistors will be

sufficient to isolate any subsequent capacitance. For other

applications see the Suggested ROUT vs. Capacitive Load

charts in the Typical Performance Characteristics section.

BALANCED CABLE DRIVER

With up to 15 VPP differential output voltage swing and 80 mA

of linear drive current the LMH6552 makes an excellent cable

driver as shown in Figure 11. The LMH6552 is also suitable

for driving differential cables from a single ended source.

30003502

FIGURE 11. Fully Differential Cable Driver

POWER SUPPLY BYPASSING

The LMH6552 requires supply bypassing capacitors as

shown in Figure 12 and Figure 13. The 0.01 µF and 0.1 µF

capacitors should be leadless SMT ceramic capacitors and

should be no more than 3 mm from the supply pins. These

capacitors should be star routed with a dedicated ground re-

turn plane or trace for best harmonic distortion performance.

A small capacitor, ~0.01 µF, placed across the supply rails,

and as close to the chip's supply pins as possible, can further

improve HD2 performance. Thin traces or small vias will re-

duce the effectiveness of bypass capacitors. Also shown in

both figures is a capacitor from the VCM and ENABLE pins to

ground. These inputs are high impedance and can provide a

coupling path into the amplifier for external noise sources,

possibly resulting in loss of dynamic range, degraded CMRR,

degraded balance and higher distortion.

30003501

FIGURE 12. Split Supply Bypassing Capacitors

30003512

FIGURE 13. Single Supply Bypassing Capacitors

POWER DISSIPATION

The LMH6552 is optimized for maximum speed and perfor-

mance in the small form factor of the standard SOIC package,

and is essentially a dual channel amplifier. To ensure maxi-

mum output drive and highest performance, thermal shut-

down is not provided. Therefore, it is of utmost importance to

make sure that the TJMAX is never exceeded due to the overall

power dissipation.

Follow these steps to determine the maximum power dissi-

pation for the LMH6552:

1. Calculate the quiescent (no-load) power: PAMP = ICC*

(VS), where VS = V+ - V−. (Be sure to include any current

through the feedback network if VOCM is not mid-rail.)

2. Calculate the RMS power dissipated in each of the output

stages: PD (rms) = rms ((VS - V+OUT) * I+OUT) + rms ((VS

− V−OUT) * I−OUT) , where VOUT and IOUT are the voltage

and the current measured at the output pins of the

differential amplifier as if they were single ended

amplifiers and VS is the total supply voltage.

3. Calculate the total RMS power: PT = PAMP + PD.

The maximum power that the LMH6552 package can dissi-

pate at a given temperature can be derived with the following

equation:

PMAX = (150° – TAMB)/ θJA, where TAMB = Ambient temperature

(°C) and θJA = Thermal resistance, from junction to ambient,

17 www.national.com

LMH6552

for a given package (°C/W). For the SOIC package θJA is

150°C/W; LLP package θJA is 58°C/W.

NOTE: If VCM is not 0V then there will be quiescent current

flowing in the feedback network. This current should be in-

cluded in the thermal calculations and added into the quies-

cent power dissipation of the amplifier.

ESD PROTECTION

The LMH6552 is protected against electrostatic discharge

(ESD) on all pins. The LMH6552 will survive 2000V Human

Body model and 200V Machine model events. Under normal

operation the ESD diodes have no affect on circuit perfor-

mance. There are occasions, however, when the ESD diodes

will be evident. If the LMH6552 is driven by a large signal while

the device is powered down the ESD diodes will conduct . The

current that flows through the ESD diodes will either exit the

chip through the supply pins or will flow through the device,

hence it is possible to power up a chip with a large signal

applied to the input pins. Using the shutdown mode is one

way to conserve power and still prevent unexpected opera-

tion.

BOARD LAYOUT

The LMH6552 is a very high performance amplifier. In order

to get maximum benefit from the differential circuit architec-

ture board layout and component selection is very critical. The

circuit board should have a low inductance ground plane and

well bypassed broad supply lines. External components

should be leadless surface mount types. The feedback net-

work and output matching resistors should be composed of

short traces and precision resistors (0.1%). The output match-

ing resistors should be placed within 3 or 4 mm of the amplifier

as should the supply bypass capacitors. Refer to the section

titled Power Supply Bypassing for recommendations on by-

pass circuit layout. Evaluation boards are available free of

charge through the product folder on National’s web site.

By design, the LMH6552 is relatively insensitive to parasitic

capacitance at its inputs. Nonetheless, ground and power

plane metal should be removed from beneath the amplifier

and from beneath RF and RG for best performance at high

frequency.

With any differential signal path, symmetry is very important.

Even small amounts of asymmetry can contribute to distortion

and balance errors.

EVALUATION BOARD

National Semiconductor suggests the following evaluation

boards to be used with the LMH6552:

Device Package Evaluation Board

Ordering ID

LMH6552MA SOIC LMH730154

LMH6552SD LLP LMH730168

These evaluation boards can be shipped when a device sam-

ple request is placed with National Semiconductor.

www.national.com 18

LMH6552

Physical Dimensions inches (millimeters) unless otherwise noted

8-Pin SOIC

NS Package Number M08A

8-Pin LLP

NS Package Number SDA08C

19 www.national.com

LMH6552

Notes

LMH6552 1.5 GHz Fully Differential Amplifier

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