LMH6552
LMH6552 1.5 GHz Fully Differential Amplifier
Literature Number: SNOSAX9G
LMH6552
July 19, 2011
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.
© 2011 National Semiconductor Corporation 300035 www.national.com
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)
Maximum Junction Temperature 150°C
Soldering Information
For soldering specifications
see product folder at www.national.com and
www.national.com/ms/MS/MS-SOLDERING.pdf
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
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LMH6552
Symbol Parameter Conditions Min
(Note 8)
Typ
(Note 7)
Max
(Note 8)Units
CMRR Common Mode Rejection Ratio
(Note 7)
DC, VCM = 0V, VID = 0V 80 dBc
RIN Input Resistance Differential 15 Ω
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, (V+ - |V-|) = ±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
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LMH6552
±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
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
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LMH6552
Symbol Parameter Conditions Min
(Note 8)
Typ
(Note 7)
Max
(Note 8)Units
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
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
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LMH6552
Connection Diagrams
8-Pin SOIC
30003508
Top View
8-Pin LLP
30003564
Top View
Pin Descriptions
Pin No. Pin Name Description
1 -IN Negative Input
2 VCM Output Common Mode Control
3 V+ Positive Supply
4 +OUT Positive Output
5 -OUT Negative Output
6 V- Negative Supply
7 EN Enable
8 +IN Positive Input
DAP DAP Die Attach Pad (See Thermal Performance section for more information)
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. Typi-
cally RS=50Ω while RM=RS||RT.
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
17 www.national.com
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 TJMAXof 150°C 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,
www.national.com 18
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.
THERMAL PERFORMANCE
The LLP package is designed for enhanced thermal perfor-
mance and features an exposed die attach pad (DAP) at the
bottom center of the package that creates a direct path to the
PCB for maximum power dissipation. The DAP is floating and
is not electrically connected to internal circuitry. Compared to
the traditional leaded packages where the die attach pad is
embedded inside the molding compound, the LLP reduces
one layer in the thermal path.
The thermal advantage of the LLP package is fully realized
only when the exposed die attach pad is soldered down to a
thermal land on the PCB board with thermal vias planted un-
derneath the thermal land. The thermal land can be connect-
ed to any power or ground plane within the allowable supply
voltage range of the device. Based on thermal analysis of the
LLP package, the junction-to-ambient thermal resistance
(θJA) can be improved by a factor of two when the die attach
pad of the LLP package is soldered directly onto the PCB with
thermal land and thermal vias are 1.27 mm and 0.33 mm re-
spectively. Typical copper via barrel plating is 1 oz, although
thicker copper may be used to further improve thermal per-
formance.
For more information on board layout techniques, refer to Ap-
plication Note 1187 “Leadless Lead Frame Package (LLP).”
This application note also discusses package handling, sol-
der stencil and the assembly process.
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
See the LMH6552 Product Folder on www.national.com for
evaluation board availability and ordering information.
19 www.national.com
LMH6552
Physical Dimensions inches (millimeters) unless otherwise noted
8-Pin SOIC
NS Package Number M08A
8-Pin LLP
NS Package Number SDA08C
www.national.com 20
LMH6552
Notes
21 www.national.com
LMH6552
Notes
LMH6552 1.5 GHz Fully Differential Amplifier
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