LMH6642MAX - Texas Instruments High-Performance Analog

LMH6642/LMH6643/LMH6644

January 9, 2012

Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

General Description

The LMH664X family true single supply voltage feedback am-

plifiers offer high speed (130MHz), low distortion (−62dBc),

and exceptionally high output current (approximately 75mA)

at low cost and with reduced power consumption when com-

pared against existing devices with similar performance.

Input common mode voltage range extends to 0.5V below V

− and 1V from V+. Output voltage range extends to within

40mV of either supply rail, allowing wide dynamic range es-

pecially desirable in low voltage applications. The output

stage is capable of approximately 75mA in order to drive

heavy loads. Fast output Slew Rate (130V/µs) ensures large

peak-to-peak output swings can be maintained even at higher

speeds, resulting in exceptional full power bandwidth of

40MHz with a 3V supply. These characteristics, along with

low cost, are ideal features for a multitude of industrial and

commercial applications.

Careful attention has been paid to ensure device stability un-

der all operating voltages and modes. The result is a very well

behaved frequency response characteristic (0.1dB gain flat-

ness up the 12MHz under 150Ω load and AV = +2) with

minimal peaking (typically 2dB maximum) for any gain setting

and under both heavy and light loads. This along with fast

settling time (68ns) and low distortion allows the device to

operate well in ADC buffer, and high frequency filter applica-

tions as well as other applications.

This device family offers professional quality video perfor-

mance with low DG (0.01%) and DP (0.01°) characteristics.

Differential Gain and Differential Phase characteristics are al-

so well maintained under heavy loads (150Ω) and throughout

the output voltage range. The LMH664X family is offered in

single (LMH6642), dual (LMH6643), and quad (LMH6644)

options. See ordering information for packages offered.

Features

(VS = ±5V, TA = 25°C, RL = 2kΩ, AV = +1. Typical values un-

less specified).

■−3dB BW (AV = +1) 130MHz

■Supply voltage range 2.7V to 12.8V

■Slew rate (Note 8), (AV = −1) 130V/µs

■Supply current (no load) 2.7mA/amp

■Output short circuit current +115mA/−145mA

■Linear output current ±75mA

■Input common mode volt. 0.5V beyond V−, 1V from V+

■Output voltage swing 40mV from rails

■Input voltage noise (100kHz) 17nV/

■Input current noise (100kHz) 0.9pA/

■THD (5MHz, RL = 2kΩ, VO = 2VPP, AV = +2) −62dBc

■Settling time 68ns

■Fully characterized for 3V, 5V, and ±5V

■Overdrive recovery 100ns

■Output short circuit protected (Note 11)

■No output phase reversal with CMVR exceeded

Applications

■Active filters

■CD/DVD ROM

■ADC buffer amp

■Portable video

■Current sense buffer

Closed Loop Gain vs. Frequency for Various Gain

20018535

Large Signal Frequency Response

20018547

LMH6642/LMH6643/LMH6644 Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the Texas Instruments Sales Office/

Distributors for availability and specifications.

ESD Tolerance 2KV (Note 2)

200V (Note 9)

1000V (Note 13)

VIN Differential ±2.5V

Output Short Circuit Duration (Note 3), (Note 11)

Supply Voltage (V+ - V−)13.5V

Voltage at Input/Output pins V+ +0.8V, V− −0.8V

Input Current ±10mA

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

Junction Temperature (Note 4) +150°C

Soldering Information

Infrared or Convection Reflow(20 sec) 235°C

Wave Soldering Lead Temp.(10 sec) 260°C

Operating Ratings (Note 1)

Supply Voltage (V+ – V−)2.7V to 12.8V

Junction Temperature Range (Note 4) −40°C to +85°C

Package Thermal Resistance (Note 4) (θJA)

5-Pin SOT-23 265°C/W

8-Pin SOIC 190°C/W

8-Pin MSOP 235°C/W

14-Pin SOIC 145°C/W

14- Pin TSSOP 155°C/W

3V Electrical Characteristics

Unless otherwise specified, all limits guaranteed for at TJ = 25°C, V+ = 3V, V− = 0V, VCM = VO = V+/2, VID (input differential voltage)

as noted (where applicable) and RL = 2kΩ to V+/2. Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

BW −3dB BW AV = +1, VOUT = 200mVPP 80 115 MHz

AV = +2, −1, VOUT = 200mVPP 46

BW0.1dB 0.1dB Gain Flatness AV = +2, RL = 150Ω to V+/2,

RL = 402Ω, VOUT = 200mVPP

19 MHz

PBW Full Power Bandwidth AV = +1, −1dB, VOUT = 1VPP 40 MHz

enInput-Referred Voltage Noise f = 100kHz 17 nV/

f = 1kHz 48

inInput-Referred Current Noise f = 100kHz 0.90 pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, VO = 2VPP, AV = −1,

RL = 100Ω to V+/2

−48 dBc

DG Differential Gain VCM = 1V, NTSC, AV = +2

RL =150Ω to V+/2

0.17

RL =1kΩ to V+/2 0.03

DP Differential Phase VCM = 1V, NTSC, AV = +2

RL =150Ω to V+/2

0.05

deg

RL =1kΩ to V+/2 0.03

CT Rej. Cross-Talk Rejection f = 5MHz, Receiver:

Rf = Rg = 510Ω, AV = +2

47 dB

TSSettling Time VO = 2VPP, ±0.1%, 8pF Load,

VS = 5V

68 ns

SR Slew Rate (Note 8) AV = −1, VI = 2VPP 90 120 V/µs

VOS Input Offset Voltage For LMH6642 and LMH6644 ±1 ±5

±7 mV

For LMH6643 ±1 ±3.4

±7

TC VOS Input Offset Average Drift (Note 12) ±5 µV/°C

IBInput Bias Current (Note 7) −1.50 −2.60

−3.25 µA

IOS Input Offset Current 20 800

1000 nA

RIN Common Mode Input Resistance 3 MΩ

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LMH6642/LMH6643/LMH6644

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

CIN Common Mode Input

Capacitance

2 pF

CMVR Input Common-Mode Voltage

Range

CMRR ≥ 50dB −0.5 −0.2

−0.1 V

1.8

1.6

2.0

CMRR Common Mode Rejection Ratio VCM Stepped from 0V to 1.5V 72 95 dB

AVOL Large Signal Voltage Gain VO = 0.5V to 2.5V

RL = 2kΩ to V+/2

VO = 0.5V to 2.5V

RL = 150Ω to V+/2

VOOutput Swing

High

RL = 2kΩ to V+/2, VID = 200mV 2.90 2.98

RL = 150Ω to V+/2, VID = 200mV 2.80 2.93

Output Swing

Low

RL = 2kΩ to V+/2, VID = −200mV 25 75

RL = 150Ω to V+/2, VID = −200mV 75 150

ISC Output Short Circuit Current Sourcing to V+/2

VID = 200mV (Note 10)

Sinking to V+/2

VID = −200mV (Note 10)

110

IOUT Output Current VOUT = 0.5V from either supply ±65 mA

+PSRR Positive Power Supply Rejection

Ratio

V+ = 3.0V to 3.5V, VCM = 1.5V 75 85 dB

ISSupply Current (per channel) No Load 2.70 4.00

4.50 mA

5V Electrical Characteristics

Unless otherwise specified, all limits guaranteed for at TJ = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2, VID (input differential voltage)

as noted (where applicable) and RL = 2kΩ to V+/2. Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

BW −3dB BW AV = +1, VOUT = 200mVPP 90 120 MHz

AV = +2, −1, VOUT = 200mVPP 46

BW0.1dB 0.1dB Gain Flatness AV = +2, RL = 150Ω to V+/2,

Rf = 402Ω, VOUT = 200mVPP

15 MHz

PBW Full Power Bandwidth AV = +1, −1dB, VOUT = 2VPP 22 MHz

enInput-Referred Voltage Noise f = 100kHz 17 nV/

f = 1kHz 48

inInput-Referred Current Noise f = 100kHz 0.90 pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, VO = 2VPP, AV = +2 −60 dBc

DG Differential Gain NTSC, AV = +2

RL =150Ω to V+/2

0.16

RL = 1kΩ to V+/2 0.05

DP Differential Phase NTSC, AV = +2

RL = 150Ω to V+/2

0.05

deg

RL = 1kΩ to V+/2 0.01

CT Rej. Cross-Talk Rejection f = 5MHz, Receiver:

Rf = Rg = 510Ω, AV = +2

47 dB

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LMH6642/LMH6643/LMH6644

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

TSSettling Time VO = 2VPP, ±0.1%, 8pF Load 68 ns

SR Slew Rate (Note 8) AV = −1, VI = 2VPP 95 125 V/µs

VOS Input Offset Voltage For LMH6642 and LMH6644 ±1 ±5

±7 mV

For LMH6643 ±1 ±3.4

±7

TC VOS Input Offset Average Drift (Note 12) ±5 µV/°C

IBInput Bias Current (Note 7) −1.70 −2.60

−3.25 µA

IOS Input Offset Current 20 800

1000 nA

RIN Common Mode Input Resistance 3 MΩ

CIN Common Mode Input

Capacitance

2 pF

CMVR Input Common-Mode Voltage

Range

CMRR ≥ 50dB −0.5 −0.2

−0.1 V

3.8

3.6

4.0

CMRR Common Mode Rejection Ratio VCM Stepped from 0V to 3.5V 72 95 dB

AVOL Large Signal Voltage Gain VO = 0.5V to 4.50V

RL = 2kΩ to V+/2

VO = 0.5V to 4.25V

RL = 150Ω to V+/2

VOOutput Swing

High

RL = 2kΩ to V+/2, VID = 200mV 4.90 4.98

RL = 150Ω to V+/2, VID = 200mV 4.65 4.90

Output Swing

Low

RL = 2kΩ to V+/2, VID = −200mV 25 100

RL = 150Ω to V+/2, VID = −200mV 100 150

ISC Output Short Circuit Current Sourcing to V+/2

VID = 200mV (Note 10)

115

Sinking to V+/2

VID = −200mV (Note 10)

140

IOUT Output Current VO = 0.5V from either supply ±70 mA

+PSRR Positive Power Supply Rejection

Ratio

V+ = 4.0V to 6V 79 90 dB

ISSupply Current (per channel) No Load 2.70 4.25

5.00 mA

±5V Electrical Characteristics

Unless otherwise specified, all limits guaranteed for at TJ = 25°C, V+ = 5V, V− = −5V, VCM = VO = 0V, VID (input differential voltage)

as noted (where applicable) and RL = 2kΩ to ground. Boldface limits apply at the temperature extremes.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

BW −3dB BW AV = +1, VOUT = 200mVPP 95 130 MHz

AV = +2, −1, VOUT = 200mVPP 46

BW0.1dB 0.1dB Gain Flatness AV = +2, RL = 150Ω to V+/2,

Rf = 806Ω, VOUT = 200mVPP

12 MHz

PBW Full Power Bandwidth AV = +1, −1dB, VOUT = 2VPP 24 MHz

enInput-Referred Voltage Noise f = 100kHz 17 nV/

f = 1kHz 48

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LMH6642/LMH6643/LMH6644

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

inInput-Referred Current Noise f = 100kHz 0.90 pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, VO = 2VPP, AV = +2 −62 dBc

DG Differential Gain NTSC, AV = +2

RL = 150Ω to V+/2

0.15 %

RL = 1kΩ to V+/2 0.01

DP Differential Phase NTSC, AV = +2

RL = 150Ω to V+/2

0.04 deg

RL = 1kΩ to V+/2 0.01

CT Rej. Cross-Talk Rejection f = 5MHz, Receiver:

Rf = Rg = 510Ω, AV = +2

47 dB

TSSettling Time VO = 2VPP, ±0.1%, 8pF Load,

VS = 5V

68 ns

SR Slew Rate (Note 8) AV = −1, VI = 2VPP 100 135 V/µs

VOS Input Offset Voltage For LMH6642 and LMH6644 ±1 ±5

±7 mV

For LMH6643 ±1 ±3.4

±7

TC VOS Input Offset Average Drift (Note 12) ±5 µV/°C

IBInput Bias Current (Note 7) −1.60 −2.60

−3.25 µA

IOS Input Offset Current 20 800

1000 nA

RIN Common Mode Input Resistance 3 MΩ

CIN Common Mode Input

Capacitance

2 pF

CMVR Input Common-Mode Voltage

Range

CMRR ≥ 50dB −5.5 −5.2

−5.1 V

3.8

3.6

4.0

CMRR Common Mode Rejection Ratio VCM Stepped from −5V to 3.5V 74 95 dB

AVOL Large Signal Voltage Gain VO = −4.5V to 4.5V,

RL = 2kΩ

VO = −4.0V to 4.0V,

RL = 150Ω

VOOutput Swing

High

RL = 2kΩ, VID = 200mV 4.90 4.96 V

RL = 150Ω, VID = 200mV 4.65 4.80

Output Swing

Low

RL = 2kΩ, VID = −200mV −4.96 −4.90

RL = 150Ω, VID = −200mV −4.80 −4.65

ISC Output Short Circuit Current Sourcing to Ground

VID = 200mV (Note 10)

115

Sinking to Ground

VID = −200mV (Note 10)

145

IOUT Output Current VO = 0.5V from either supply ±75 mA

PSRR Power Supply Rejection Ratio (V+, V−) = (4.5V, −4.5V) to (5.5V,

−5.5V)

78 90 dB

ISSupply Current (per channel) No Load 2.70 4.50

5.50 mA

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LMH6642/LMH6643/LMH6644

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.

Note 2: Human body model, 1.5kΩ in series with 100pF.

Note 3: Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the

maximum allowed junction temperature of 150°C.

Note 4: 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 5: Typical values represent the most likely parametric norm.

Note 6: All limits are guaranteed by testing or statistical analysis.

Note 7: Positive current corresponds to current flowing into the device.

Note 8: Slew rate is the average of the rising and falling slew rates.

Note 9: Machine Model, 0Ω in series with 200pF.

Note 10: Short circuit test is a momentary test. See Note 11.

Note 11: Output short circuit duration is infinite for VS < 6V at room temperature and below. For VS > 6V, allowable short circuit duration is 1.5ms.

Note 12: Offset voltage average drift determined by dividing the change in VOS at temperature extremes by the total temperature change.

Note 13: CDM: Charge Device Model

Connection Diagrams

5-Pin SOT23 (LMH6642)

20018561

Top View

8-Pin SOIC (LMH6642)

20018562

Top View

8-Pin SOIC and 8-Pin MSOP

(LMH6643)

20018563

Top View

14-Pin SOIC and 14-Pin TSSOP

(LMH6644)

20018568

Top View

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LMH6642/LMH6643/LMH6644

Typical Performance Characteristics At TJ = 25°C, V+ = +5, V− = −5V, RF = RL = 2kΩ. Unless otherwise

specified.

Closed Loop Frequency Response for Various Supplies

20018557

Closed Loop Gain vs. Frequency for Various Gain

20018551

Closed Loop Gain vs. Frequency for Various Gain

20018535

Closed Loop Frequency Response for Various Temperature

20018550

Closed Loop Gain vs. Frequency for Various Supplies

20018548

Closed Loop Frequency Response for Various Temperature

20018534

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LMH6642/LMH6643/LMH6644

Large Signal Frequency Response

20018547

Closed Loop Small Signal Frequency Response for Various

Supplies

20018546

Closed Loop Frequency Response for Various Supplies

20018544

±0.1dB Gain Flatness for Various Supplies

20018545

VOUT (VPP) for THD < 0.5%

20018509

VOUT (VPP) for THD < 0.5%

20018508

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LMH6642/LMH6643/LMH6644

VOUT (VPP) for THD < 0.5%

20018510

Open Loop Gain/Phase for Various Temperature

20018532

Open Loop Gain/Phase for Various Temperature

20018533

HD2 (dBc) vs. Output Swing

20018514

HD3 (dBc) vs. Output Swing

20018515

HD2 vs. Output Swing

20018504

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LMH6642/LMH6643/LMH6644

HD3 vs. Output Swing

20018505

THD (dBc) vs. Output Swing

20018506

Settling Time vs. Input Step Amplitude

(Output Slew and Settle Time)

20018513

Input Noise vs. Frequency

20018512

VOUT from V+ vs. ISOURCE

20018518

VOUT from V− vs. ISINK

20018519

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LMH6642/LMH6643/LMH6644

VOUT from V+ vs. ISOURCE

20018516

VOUT from V− vs. ISINK

20018517

Swing vs. VS

20018529

Short Circuit Current (to VS/2) vs. VS

20018531

Output Sinking Saturation Voltage vs. IOUT

20018520

Output Sourcing Saturation Voltage vs. IOUT

20018501

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Closed Loop Output Impedance vs. Frequency AV = +1

20018502

PSRR vs. Frequency

20018503

CMRR vs. Frequency

20018507

Crosstalk Rejection vs. Frequency

(Output to Output)

20018511

VOS vs. VOUT (Typical Unit)

20018530

VOS vs. VCM (Typical Unit)

20018527

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LMH6642/LMH6643/LMH6644

VOS vs. VS (for 3 Representative Units)

20018522

VOS vs. VS (for 3 Representative Units)

20018523

VOS vs. VS (for 3 Representative Units)

20018524

IB vs. VS

20018525

IOS vs. VS

20018526

IS vs. VCM

20018528

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LMH6642/LMH6643/LMH6644

IS vs. VS

20018521

Small Signal Step Response

20018553

Large Signal Step Response

20018541

Large Signal Step Response

20018539

Small Signal Step Response

20018556

Small Signal Step Response

20018536

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LMH6642/LMH6643/LMH6644

Small Signal Step Response

20018552

Small Signal Step Response

20018538

Large Signal Step Response

20018537

Large Signal Step Response

20018554

Large Signal Step Response

20018560

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LMH6642/LMH6643/LMH6644

Application Information

CIRCUIT DESCRIPTION

The LMH664X family is based on National Semiconductor’s

proprietary VIP10 dielectrically isolated bipolar process.

This device family architecture features the following:

•Complimentary bipolar devices with exceptionally high ft

(∼8GHz) even under low supply voltage (2.7V) and low

bias current.

•A class A-B “turn-around” stage with improved noise,

offset, and reduced power dissipation compared to similar

speed devices (patent pending).

•Common Emitter push-push output stage capable of

75mA output current (at 0.5V from the supply rails) while

consuming only 2.7mA of total supply current per channel.

This architecture allows output to reach within milli-volts of

either supply rail.

•Consistent performance over the entire operating supply

voltage range with little variation for the most important

specifications (e.g. BW, SR, IOUT, etc.)

•Significant power saving (∼40%) compared to competitive

devices on the market with similar performance.

Application Hints

This Op Amp family is a drop-in replacement for the AD805X

family of high speed Op Amps in most applications. In addi-

tion, the LMH664X will typically save about 40% on power

dissipation, due to lower supply current, when compared to

competition. All AD805X family’s guaranteed parameters are

included in the list of LMH664X guaranteed specifications in

order to ensure equal or better level of performance. Howev-

er, as in most high performance parts, due to subtleties of

applications, it is strongly recommended that the performance

of the part to be evaluated is tested under actual operating

conditions to ensure full compliance to all specifications.

With 3V supplies and a common mode input voltage range

that extends 0.5V below V−, the LMH664X find applications

in low voltage/low power applications. Even with 3V supplies,

the −3dB BW (@ AV = +1) is typically 115MHz with a tested

limit of 80MHz. Production testing guarantees that process

variations with not compromise speed. High frequency re-

sponse is exceptionally stable confining the typical −3dB BW

over the industrial temperature range to ±2.5%.

As can be seen from the typical performance plots, the

LMH664X output current capability (∼75mA) is enhanced

compared to AD805X. This enhancement, increases the out-

put load range, adding to the LMH664X’s versatility.

Because of the LMH664X’s high output current capability at-

tention should be given to device junction temperature in

order not to exceed the Absolute Maximum Rating.

This device family was designed to avoid output phase re-

versal. With input overdrive, the output is kept near supply rail

(or as closed to it as mandated by the closed loop gain setting

and the input voltage). See Figure 1:

20018542

FIGURE 1. Input and Output Shown with CMVR Exceeded

However, if the input voltage range of −0.5V to 1V from V+ is

exceeded by more than a diode drop, the internal ESD pro-

tection diodes will start to conduct. The current in the diodes

should be kept at or below 10mA.

Output overdrive recovery time is less than 100ns as can be

seen from Figure 2 plot:

20018543

FIGURE 2. Overload Recovery Waveform

INPUT AND OUTPUT TOPOLOGY

All input / output pins are protected against excessive volt-

ages by ESD diodes connected to V+ and V- rails (see Figure

3). These diodes start conducting when the input / output pin

voltage approaches 1Vbe beyond V+ or V- to protect against

over voltage. These diodes are normally reverse biased. Fur-

ther protection of the inputs is provided by the two resistors

(R in Figure 3), in conjunction with the string of anti-parallel

diodes connected between both bases of the input stage. The

combination of these resistors and diodes reduces excessive

differential input voltages approaching 2Vbe. The most com-

mon situation when this occurs is when the device is used as

a comparator (or with little or no feedback) and the device

inputs no longer follow each other. In such a case, the diodes

may conduct. As a consequence, input current increases and

the differential input voltage is clamped. It is important to

make sure that the subsequent current flow through the de-

vice input pins does not violate the Absolute Maximum Rat-

ings of the device. To limit the current through this protection

circuit, extra series resistors can be placed. Together with the

built-in series resistors of several hundred ohms, these ex-

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LMH6642/LMH6643/LMH6644

ternal resistors can limit the input current to a safe number

(i.e. < 10mA). Be aware that these input series resistors may

impact the switching speed of the device and could slow down

the device.

20018569

FIGURE 3. Input Equivalent Circuit

SINGLE SUPPLY, LOW POWER PHOTODIODE

AMPLIFIER

The circuit shown in Figure 4 is used to amplify the current

from a photo-diode into a voltage output. In this circuit, the

emphasis is on achieving high bandwidth and the tran-

simpedance gain setting is kept relatively low. Because of its

high slew rate limit and high speed, the LMH664X family lends

itself well to such an application.

This circuit achieves approximately 1V/mA of tran-

simpedance gain and capable of handling up to 1mApp from

the photodiode. Q1, in a common base configuration, isolates

the high capacitance of the photodiode (Cd) from the Op Amp

input in order to maximize speed. Input is AC coupled through

C1 to ease biasing and allow single supply operation. With 5V

single supply, the device input/output is shifted to near half

supply using a voltage divider from VCC. Note that Q1 collector

does not have any voltage swing and the Miller effect is min-

imized. D1, tied to Q1 base, is for temperature compensation

of Q1’s bias point. Q1 collector current was set to be large

enough to handle the peak-to-peak photodiode excitation and

not too large to shift the U1 output too far from mid-supply.

No matter how low an Rf is selected, there is a need for Cf in

order to stabilize the circuit. The reason for this is that the Op

Amp input capacitance and Q1 equivalent collector capaci-

tance together (CIN) will cause additional phase shift to the

signal fed back to the inverting node. Cf will function as a zero

in the feedback path counter-acting the effect of the CIN and

acting to stabilized the circuit. By proper selection of Cf such

that the Op Amp open loop gain is equal to the inverse of the

feedback factor at that frequency, the response is optimized

with a theoretical 45° phase margin.

(1)

where GBWP is the Gain Bandwidth Product of the Op Amp

Optimized as such, the I-V converter will have a theoretical

pole, fp, at:

(2)

With Op Amp input capacitance of 3pF and an estimate for

Q1 output capacitance of about 3pF as well, CIN = 6pF. From

the typical performance plots, LMH6642/6643 family GBWP

is approximately 57MHz. Therefore, with Rf = 1k, from Equa-

tion 1 and 2 above.

Cf = ∼4.1pF, and fp = 39MHz

20018564

FIGURE 4. Single Supply Photodiode I-V Converter

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LMH6642/LMH6643/LMH6644

For this example, optimum Cf was empirically determined to

be around 5pF. This time domain response is shown in Figure

5 below showing about 9ns rise/fall times, corresponding to

about 39MHz for fp. The overall supply current from the +5V

supply is around 5mA with no load.

20018565

FIGURE 5. Converter Step Response (1VPP, 20 ns/DIV)

PRINTED CIRCUIT BOARD LAYOUT AND COMPONENT

VALUES SECTION

Generally, a good high frequency layout will keep power sup-

ply and ground traces away from the inverting input and

output pins. Parasitic capacitances on these nodes to ground

will cause frequency response peaking and possible circuit

oscillations (see Application Note OA-15 for more informa-

tion). National Semiconductor suggests the following evalua-

tion boards as a guide for high frequency layout and as an aid

in device testing and characterization:

Device Package Evaluation Board

LMH6642MF 5-Pin SOT-23 LMH730216

LMH6642MA 8-Pin SOIC LMH730227

LMH6643MA 8-Pin SOIC LMH730036

LMH6643MM 8-Pin MSOP LMH730123

LMH6644MA 14-Pin SOIC LMH730231

LMH6644MT 14-Pin TSSOP LMH730131

Another important parameter in working with high speed/high

performance amplifiers, is the component values selection.

Choosing external resistors that are large in value will effect

the closed loop behavior of the stage because of the interac-

tion of these resistors with parasitic capacitances. These

capacitors could be inherent to the device or a by-product of

the board layout and component placement. Either way,

keeping the resistor values lower, will diminish this interaction

to a large extent. On the other hand, choosing very low value

resistors could load down nodes and will contribute to higher

overall power dissipation.

Ordering Information

Package Part Number Package Marking Transport Media NSC Drawing

5-Pin SOT-23 LMH6642MF A64A 1k Units Tape and Reel MF05A

LMH6642MFX 3k Units Tape and Reel

8-Pin SOIC

LMH6642MA LMH6642MA 95 Units/Rail

M08A

LMH6642MAX 2.5k Units Tape and Reel

LMH6643MA LMH6643MA 95 Units/Rail

LMH6643MAX 2.5k Units Tape and Reel

8-Pin MSOP LMH6643MM A65A 1k Units Tape and Reel MUA08A

LMH6643MMX 3.5k Units Tape and Reel

14-Pin SOIC LMH6644MA LMH6644MA 55 units/Rail M14A

LMH6644MAX 2.5k Units Tape and Reel

14-Pin TSSO LMH6644MT LMH6644MT 94 Units/Rail MTC14

LMH6644MTX 2.5k Units Tape and Reel

www.ti.com 18

LMH6642/LMH6643/LMH6644

Physical Dimensions inches (millimeters) unless otherwise noted

5-Pin SOT23

NS Package Number MF05A

8-Pin SOIC

NS Package Number M08A

19 www.ti.com

LMH6642/LMH6643/LMH6644

8-Pin MSOP

NS Package Number MUA08A

14-Pin SOIC

NS Package Number M14A

www.ti.com 20

LMH6642/LMH6643/LMH6644

14-Pin TSSOP

NS Package Number MTC14

21 www.ti.com

LMH6642/LMH6643/LMH6644

Notes

LMH6642/LMH6643/LMH6644 Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

www.ti.com

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