Datasheet LMH6644MTX, LMH6644MT, LMH6644MAX, LMH6643MWC, LMH6643MDC Datasheet (NSC)

LMH6642/6643/6644
3V, Low Power, 130MHz, 75mA Rail-to-Rail Output
Amplifiers

General Description

The LMH664X family true single supply voltage feedback
amplifiers offer high speed (130MHz), low distortion
(−62dBc), and exceptionally high output current (approxi­mately 75mA) at low cost and with reduced power consump­tion when compared 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
especially 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
under all operating voltages and modes. The result is a very
well behaved frequency response characteristic (0.1dB gain
flatness up the 12MHz under 150Ω load and A

V

= +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
applications as well as other applications.

This device family offers professional quality video perfor­mance with low DG (0.01%) and DP (0.01˚) characteristics.
Differential Gain and Differential Phase characteristics are
also 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
unless specified).

n −3dB BW (A

V

= +1) 130MHz

n Supply voltage range 3V to 12.8V
n Slew rate (Note 8), (A

V

= −1) 130V/µs

n Supply current (no load) 2.7mA/amp
n Output short circuit current +115mA/−145mA
n Linear output current

±

75mA

n Input common mode volt. 0.5V beyond V

−

, 1V from V

+

n Output voltage swing 40mV from rails
n Input voltage noise (100kHz) 17nV/
n Input current noise (100kHz) 0.9pA/
n THD (5MHz, RL=2kΩ,VO=2VPP,AV= +2) −62dBc
n Settling time 68ns
n Fully characterized for 3V, 5V, and

±

5V

n Overdrive recovery 100ns
n Output short circuit protected (Note 11)
n No output phase reversal with CMVR exceeded

Applications

n Active filters
n CD/DVD ROM
n ADC buffer amp
n Portable video
n Current sense buffer

Closed Loop Gain vs. Frequency for Various Gain Large Signal Frequency Response

20018535

20018547

May 2003

LMH6642/6643/6644 3V, Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

© 2003 National Semiconductor Corporation DS200185 www.national.com

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 2KV (Note 2)

200V (Note 9)

V

IN

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−

) 3V to 12.8V

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

Package Thermal Resistance (Note 4) (θ

JA

)

SOT23-5 265˚C/W

SOIC-8 190˚C/W

MSOP-8 235˚C/W

SOIC-14 145˚C/W

TSSOP-14 155˚C/W

3V Electrical Characteristics

Unless otherwise specified, all limits guaranteed for at TJ= 25˚C, V+= 3V, V−= 0V, VCM=VO=V+/2, 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 A

V

= +1, V

OUT

= 200mV

PP

80 115

MHz

A

V

= +2, −1, V

OUT

= 200mV

PP

46

BW

0.1dB

0.1dB Gain Flatness AV= +2, RL= 150Ω to V+/2,
R

L

= 402Ω,V

OUT

= 200mV

PP

19 MHz

PBW Full Power Bandwidth A

V

= +1, −1dB, V

OUT

=1V

PP

40 MHz

e

n

Input-Referred Voltage Noise f = 100kHz 17

nV/

f = 1kHz 48

i

n

Input-Referred Current Noise f = 100kHz 0.90

pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, V

O

=2VPP,AV= −1,

R

L

= 100Ω to V+/2

−48
dBc

DG Differential Gain V

CM

= 1V, NTSC, AV=+2

R

L

=150Ω to V+/2

0.17
%

R

L

=1kΩ to V+/2 0.03

DP Differential Phase V

CM

= 1V, NTSC, AV=+2

R

L

=150Ω to V+/2

0.05

deg

R

L

=1kΩ to V+/2 0.03

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

R

f=Rg

= 510Ω,AV=+2

47 dB

T

S

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

V

S

=5V

68 ns

SR Slew Rate (Note 8) A

V

= −1, VI=2V

PP

90 120 V/µs

V

OS

Input Offset Voltage

±

1

±

5

±

7

mV

TC V

OS

Input Offset Average Drift (Note 12)

±

5 µV/˚C

I

B

Input Bias Current (Note 7) −1.50 −2.60

−3.25

µA

I

OS

Input Offset Current 20 800

1000

nA

R

IN

Common Mode Input
Resistance

3MΩ

C

IN

Common Mode Input
Capacitance

2pF

LMH6642/6643/6644

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3V Electrical Characteristics (Continued)

Unless otherwise specified, all limits guaranteed for at TJ= 25˚C, V+= 3V, V−= 0V, VCM=VO=V+/2, 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

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

V

CM

Stepped from 0V to 1.5V 72 95 dB

A

VOL

Large Signal Voltage Gain VO= 0.5V to 2.5V

R

L

=2kΩ to V+/2

80

75

96

dB

V

O

= 0.5V to 2.5V

R

L

= 150Ω to V+/2

74

70

82

V

O

Output Swing
High

RL=2kΩ to V+/2, VID= 200mV 2.90 2.98

V

R

L

= 150Ω to V+/2, VID= 200mV 2.80 2.93

Output Swing
Low

R

L

=2kΩ to V+/2, VID= −200mV 25 75

mV

R

L

= 150Ω to V+/2, VID= −200mV 75 150

I

SC

Output Short Circuit Current Sourcing to V+/2

V

ID

= 200mV (Note 10)

50

35

95

mA

Sinking to V

+

/2

V

ID

= −200mV (Note 10)

55

40

110

I

OUT

Output Current V

OUT

= 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

I

S

Supply 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, 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 A

V

= +1, V

OUT

= 200mV

PP

90 120

MHz

A

V

= +2, −1, V

OUT

= 200mV

PP

46

BW

0.1dB

0.1dB Gain Flatness AV= +2, RL= 150Ω to V+/2,
R

f

= 402Ω,V

OUT

= 200mV

PP

15 MHz

PBW Full Power Bandwidth A

V

= +1, −1dB, V

OUT

=2V

PP

22 MHz

e

n

Input-Referred Voltage Noise f = 100kHz 17

nV/

f = 1kHz 48

i

n

Input-Referred Current Noise f = 100kHz 0.90

pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, V

O

=2VPP,AV= +2 −60 dBc

DG Differential Gain NTSC, A

V

=+2

R

L

=150Ω to V+/2

0.16
%

R

L

=1kΩ to V+/2 0.05

DP Differential Phase NTSC, A

V

=+2

R

L

=150Ω to V+/2

0.05

deg

R

L

=1kΩ to V+/2 0.01

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

R

f=Rg

= 510Ω,AV=+2

47 dB

T

S

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

LMH6642/6643/6644

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5V Electrical Characteristics (Continued)

Unless otherwise specified, all limits guaranteed for at TJ= 25˚C, V+= 5V, V−= 0V, VCM=VO=V+/2, 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

SR Slew Rate (Note 8) A

V

= −1, VI=2V

PP

95 125 V/µs

V

OS

Input Offset Voltage

±

1

±

5

±

7

mV

TC V

OS

Input Offset Average Drift (Note 12)

±

5 µV/˚C

I

B

Input Bias Current (Note 7)

−1.70

−2.60

−3.25

µA

I

OS

Input Offset Current

20

800

1000

nA

R

IN

Common Mode Input
Resistance

3

MΩ

C

IN

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

V

CM

Stepped from 0V to 3.5V

72 95

dB

A

VOL

Large Signal Voltage Gain VO= 0.5V to 4.50V

R

L

=2kΩ to V+/2

86

82

98

dB

V

O

= 0.5V to 4.25V

R

L

= 150Ω to V+/2

76

72

82

V

O

Output Swing
High

RL=2kΩ to V+/2, VID= 200mV 4.90 4.98

V

R

L

= 150Ω to V+/2, VID= 200mV 4.65 4.90

Output Swing
Low

R

L

=2kΩ to V+/2, VID= −200mV 25 100

mV

R

L

= 150Ω to V+/2, VID= −200mV 100 150

I

SC

Output Short Circuit Current Sourcing to V+/2

V

ID

= 200mV (Note 10)

55

40

115

mA

Sinking to V

+

/2

V

ID

= −200mV (Note 10)

70

55

140

I

OUT

Output Current VO= 0.5V from either supply

±

70 mA

+PSRR Positive Power Supply

Rejection Ratio

V

+

= 4.0V to 6V

79 90

dB

I

S

Supply 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 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 A

V

= +1, V

OUT

= 200mV

PP

95 130

MHz

A

V

= +2, −1, V

OUT

= 200mV

PP

46

BW

0.1dB

0.1dB Gain Flatness AV= +2, RL= 150Ω to V+/2,
R

f

= 806Ω,V

OUT

= 200mV

PP

12 MHz

PBW Full Power Bandwidth A

V

= +1, −1dB, V

OUT

=2V

PP

24 MHz

e

n

Input-Referred Voltage Noise f = 100kHz 17

nV/

f = 1kHz 48

LMH6642/6643/6644

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±

5V Electrical Characteristics (Continued)

Unless otherwise specified, all limits guaranteed for at TJ= 25˚C, V+= 5V, V−= −5V, VCM=VO= 0V 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

i

n

Input-Referred Current Noise f = 100kHz 0.90

pA/

f = 1kHz 3.3

THD Total Harmonic Distortion f = 5MHz, V

O

=2VPP,AV= +2 −62 dBc

DG Differential Gain NTSC, A

V

=+2

R

L

=150Ω to V+/2

0.15
%

R

L

=1kΩ to V+/2 0.01

DP Differential Phase NTSC, A

V

=+2

R

L

=150Ω to V+/2

0.04

deg

R

L

=1kΩ to V+/2 0.01

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

Rf=Rg= 510Ω,AV=+2

47 dB

T

S

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

V

S

=5V

68

ns

SR Slew Rate (Note 8) A

V

= −1, VI=2V

PP

100 135 V/µs

V

OS

Input Offset Voltage

±

1

±

5

±

7

mV

TC V

OS

Input Offset Average Drift (Note 12)

±

5 µV/˚C

I

B

Input Bias Current (Note 7)

−1.60

−2.60

−3.25

µA

I

OS

Input Offset Current

20

800

1000

nA

R

IN

Common Mode Input
Resistance

3

MΩ

C

IN

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

V

CM

Stepped from −5V to 3.5V

74 95

dB

A

VOL

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

R

L

=2kΩ

88

84

96

dB

V

O

= −4.0V to 4.0V,

R

L

= 150Ω

78

74

82

V

O

Output Swing
High

RL=2kΩ,VID= 200mV 4.90 4.96

V

R

L

= 150Ω,VID= 200mV 4.65 4.80

Output Swing
Low

R

L

=2kΩ,VID= −200mV −4.96 −4.90

V

R

L

= 150Ω,VID= −200mV −4.80 −4.65

I

SC

Output Short Circuit Current Sourcing to Ground

V

ID

= 200mV (Note 10)

60

35

115

mA

Sinking to Ground
V

ID

= −200mV (Note 10)

85

65

145

I

OUT

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

I

S

Supply Current (per channel) No Load

2.70

4.50

5.50

mA

LMH6642/6643/6644

www.national.com5

±

5V Electrical Characteristics (Continued)

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 T

J(MAX)

, θJA, and TA. The maximum allowable power dissipation at any ambient temperature is

P

D

=(T

J(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 V

S

<

6V at room temperature and below. For V

S

>

6V, allowable short circuit duration is 1.5ms.

Note 12: Offset voltage average drift determined by dividing the change in V

OS

at temperature extremes by the total temperature change.

Connection Diagrams

SOT23-5 (LMH6642) SOIC-8 (LMH6642)

SOIC-8 and MSOP-8

(LMH6643)

20018561

Top View

20018562

Top View

20018563

Top View

SOIC-14 and TSSOP-14

(LMH6644)

20018568

Top View

LMH6642/6643/6644

www.national.com 6

Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified.

Closed Loop Frequency Response for Various Supplies Closed Loop Gain vs. Frequency for Various Gain

20018557

20018551

Closed Loop Gain vs. Frequency for Various Gain

Closed Loop Frequency Response for Various

Temperature

20018535

20018550

Closed Loop Gain vs. Frequency for Various Supplies

Closed Loop Frequency Response for Various

Temperature

20018548

20018534

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

Large Signal Frequency Response

Closed Loop Small Signal Frequency Response for

Various Supplies

20018547

20018546

Closed Loop Frequency Response for Various Supplies

±

0.1dB Gain Flatness for Various Supplies

20018544

20018545

V

OUT(VPP

) for THD<0.5% V

OUT(VPP

) for THD<0.5%

20018509 20018508

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

V

OUT(VPP

) for THD<0.5% Open Loop Gain/Phase for Various Temperature

20018510

20018532

Open Loop Gain/Phase for Various Temperature HD2 (dBc) vs. Output Swing

20018533

20018514

HD3 (dBc) vs. Output Swing HD2 vs. Output Swing

20018515

20018504

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

HD3 vs. Output Swing THD (dBc) vs. Output Swing

20018505 20018506

Settling Time vs. Input Step Amplitude

(Output Slew and Settle Time) Input Noise vs. Frequency

20018513

20018512

V

OUT

from V+vs. I

SOURCE

V

OUT

from V−vs. I

SINK

20018518 20018519

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

V

OUT

from V+vs. I

SOURCE

V

OUT

from V−vs. I

SINK

20018516 20018517

Swing vs. V

S

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

S

20018529 20018531

Output Sinking Saturation Voltage vs. I

OUT

Output Sourcing Saturation Voltage vs. I

OUT

20018520

20018501

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

Closed Loop Output Impedance vs. Frequency A

V

= +1 PSRR vs. Frequency

20018502 20018503

CMRR vs. Frequency

Crosstalk Rejection vs. Frequency

(Output to Output)

20018507

20018511

VOSvs. V

OUT

(Typical Unit) VOSvs. VCM(Typical Unit)

20018530

20018527

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

V

OS

vs. VS(for 3 Representative Units) VOSvs. VS(for 3 Representative Units)

20018522 20018523

VOSvs. VS(for 3 Representative Units) IBvs. V

S

20018524

20018525

IOSvs. V

S

ISvs. V

CM

20018526

20018528

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

I

S

vs. V

S

Small Signal Step Response

20018521

20018553

Large Signal Step Response Large Signal Step Response

20018541 20018539

Small Signal Step Response Small Signal Step Response

20018556 20018536

LMH6642/6643/6644

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Typical Performance Characteristics At T

J

= 25˚C, V+= +5, V−= −5V, RF=RL=2kΩ. Unless

otherwise specified. (Continued)

Small Signal Step Response Small Signal Step Response

20018552

20018538

Large Signal Step Response Large Signal Step Response

20018537 20018554

Large Signal Step Response

20018560

LMH6642/6643/6644

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Application Notes

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 f

t

(∼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 milli­volts of either supply rail.

•

Consistent performance from any supply voltage (3V­10V) with little variation with supply voltage for the most
important specifications (e.g. BW, SR, I

OUT

, 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 addi­tion, 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. How­ever, 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 response 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
output load range, adding to the LMH664X’s versatility.

Because of the LMH664X’s high output current capability
attention 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
reversal. 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:

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

+

is
exceeded by more than a diode drop, the internal ESD
protection 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:

20018542

FIGURE 1. Input and Output Shown with CMVR

Exceeded

20018543

FIGURE 2. Overload Recovery Waveform

LMH6642/6643/6644

www.national.com 16

Application Notes (Continued)

SINGLE SUPPLY, LOW POWER PHOTODIODE
AMPLIFIER:

The circuit shown in Figure 3 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 transim­pedance 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 transimped­ance gain and capable of handling up to 1mA

pp

from the
photodiode. Q1, in a common base configuration, isolates
the high capacitance of the photodiode (C

d

) 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 V

CC

. Note
that Q1 collector does not have any voltage swing and the
Miller effect is minimized. 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 R

f

is selected, there is a need for Cfin

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

Amp input capacitance and Q1 equivalent collector capaci­tance together (C

IN

) will cause additional phase shift to the

signal fed back to the inverting node. C

f

will function as a

zero in the feedback path counter-acting the effect of the C

IN

and acting to stabilized the circuit. By proper selection of C

f

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, f

p

, at:

(2)
With Op Amp input capacitance of 3pF and an estimate for

Q1 output capacitance of about 3pF as well, C

IN

= 6pF. From
the typical performance plots, LMH6642/6643 family GBWP
is approximately 57MHz. Therefore, with R

f

= 1k, from Equa-

tion 1 and 2 above.
C

f

= ∼4.1pF, and fp= 39MHz

20018564

FIGURE 3. Single Supply Photodiode I-V Converter

LMH6642/6643/6644

www.national.com17

Application Notes (Continued)

For this example, optimum C

f

was empirically determined to
be around 5pF. This time domain response is shown in
Figure 4 below showing about 9ns rise/fall times, corre­sponding to about 39MHz for f

p

. The overall supply current

from the +5V supply is around 5mA with no load.

Printed Circuit Board Layout and Component Values
Sections:

Generally, a good high frequency layout will keep power
supply 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
information). National Semiconductor suggests the following
evaluation boards as a guide for high frequency layout and
as an aid in device testing and characterization:

Device Package Evaluation

Board PN

LMH6642MF SOT23-5 CLC730068

LMH6642MA 8-Pin SOIC CLC730027

LMH6643MA 8-Pin SOIC CLC730036

LMH6643MM 8-Pin MSOP CLC730123

LMH6644MA 14-Pin SOIC CLC730031

LMH6644MT 14-Pin TSSOP CLC730131

These free evaluation boards are shipped when a device
sample request is placed with National Semiconductor.

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
interaction of these resistors with parasitic capacitances.
These capacitors could be inherent to the device or a by­product 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

SOIC-8 LMH6642MA LMH6642MA Rails M08A

LMH6642MAX 2.5k Units Tape and Reel

LMH6643MA LMH6643MA Rails

LMH6643MAX 2.5k Units Tape and Reel

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

LMH6643MMX 3.5k Units Tape and Reel

SOIC-14 LMH6644MA LMH6644MA Rails M14A

LNH6644MAX 2.5k Units Tape and Reel

TSSOP-14 LMH6644MT LMH6644MT Rails MTC14

LMH6644MTX 2.5k Units Tape and Reel

20018565

FIGURE 4. Converter Step Response (1VPP, 20 ns/DIV)

LMH6642/6643/6644

www.national.com 18

Physical Dimensions inches (millimeters) unless otherwise noted

5-Pin SOT23

NS Package Number MF05A

8-Pin SOIC

NS Package Number M08A

LMH6642/6643/6644

www.national.com19

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

8-Pin MSOP

NS Package Number MUA08A

LMH6642/6643/6644

www.national.com 20

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

14-Pin SOIC

NS Package Number M14A

LMH6642/6643/6644

www.national.com21

Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

14-Pin TSSOP

NS Package Number MTC14

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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

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Support Center

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www.national.com

LMH6642/6643/6644 3V, Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.