LMH2180SDEVAL - National Semiconductor

August 21, 2008

LMH2180

75 MHz Dual Clock Buffer

General Description

The LMH2180 is a high speed dual clock buffer designed for

portable communications and applications requiring multiple

accurate multi-clock systems. The LMH2180 integrates two

75 MHz low noise buffers with independent shutdown pins

into a small package. The LMH2180 ensures superb system

operation between the baseband and the oscillator signal

path by eliminating crosstalk between the multiple clock sig-

nals.

Unique technology and design provides the LMH2180 with

the ability to accurately drive both large capacitive and resis-

tive loads. Low supply current combined with shutdown pins

for each channel means the LMH2180 is ideal for battery

powered applications. This part does not use an internal

ground reference, thus providing additional system flexibility.

The flexible buffers provide system designers the capacity to

manage complex clock signals in the latest wireless applica-

tions. Each buffer delivers 106 V/μs internal slew rate with

independent shutdown and duty cycle precision. Each input

is internally biased to 1V, removing the need for external re-

sistors. Both channels have rail-to-rail inputs and outputs, a

gain of one, and are AC coupled with the use of one capacitor.

Replacing a discrete buffer solution with the LMH2180 pro-

vides many benefits: simplified board layout, minimized par-

asitic components, simplified BOM, design durability across

multiple applications, simplification of clock paths, and the

ability to reduce the number of clock signal generators in the

system. The LMH2180 is produced in the tiny packages min-

imizing the required PCB space.

Features

(Typical values are: VSUPPLY = 2.7V and CL = 10 pF, unless

otherwise specified.)

■Small signal bandwidth 78 MHz

■Supply voltage range 2.4V to 5V

■Phase noise (VIN = 1 VPP,

fC = 38.4 MHz, Δf = 1 kHz)

−123 dBc/Hz

■Slew rate 106 V/μs

■Total supply current 2.3 mA

■Shutdown current 30 µA

■Rail-to-rail input and output

■Individual buffer enable pins

■Rapid Ton technology

■Crosstalk rejection circuitry

■Packages:

—8-Pin LLP, Solder Bump and no Pullback

—8-Bump micro SMD

■Temperature range −40°C to 85°C

Applications

■3G mobile applications

■WLAN–WiMAX modules

■TD_SCDMA multi-mode MP3 and camera

■GSM modules

■Oscillator modules

Typical Application

30024602

LMH2180 75 MHz Dual Clock Buffer

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltages (V+ – V−) 5.5V

ESD Tolerance

Human Body (Note 4) 2000V

Machine Model (Note 5) 200V

Charged Device Model 1000V

Storage Temperature Range −65°C to 150°C

Junction Temperature (Note 3) 150°C

Soldering Information

Infrared or Convection (35 sec.) 235°C

Operating Ratings (Note 1)

Supply Voltage (V+ – V−)2.4V to 5.0V

Temperature Range (Notes 2, 3) −40°C to 85°C

Package Thermal Resistance (Notes 2, 3)

8-Pin LLP (θJA)217°C/W

8-Bump micro SMD (θJA)90°C/W

2.7V Electrical Characteristics

Unless otherwise specified, all limits are guaranteed for TA = 25°C, VDD = 2.7V, VSS = 0V, VCM = 1V, Enable1,2 = VDD, CL = 10 pF,

RL = 30 kΩ, Load is connected to VSS, CCOUPLING = 10 nF. Boldface limits apply at temperature range extremes of operating

condition. See (Note 2)

Symbol Parameter Conditions Min

(Note 7)

Typ

(Note 6)

Max

(Note 7)

Units

Frequency Domain Response

SSBW Small Signal Bandwidth VIN = 100 mVPP; −3 dB 78 MHz

LSBW Large Signal Bandwidth VIN = 1.0 VPP; −3 dB 60 MHz

GFN Gain Flatness < 0.1 dB f > 100 kHz 4.9 MHz

Distortion and Noise Performance

φnPhase Noise VIN = 1 VPP, fC = 38.4 MHz, Δf = 1 kHz −123 dBc/Hz

VIN = 1 VPP, fC = 38.4 MHz, Δf = 10 kHz −132 dBc/Hz

enInput-Referred Voltage Noise f = 1 MHz, RSOURCE = 50Ω 13 nV/

ISOLATION Output to Input f = 1 MHz, RSOURCE = 50Ω 84 dB

CT Crosstalk Rejection f = 38.4 MHz, VIN = 1 VPP 41 dB

Time Domain Response

trRise Time 0.1 VPP Step (10-90%) 6 ns

tfFall Time 5 ns

tsSettling Time to 0.1% 1 VPP Step 120 ns

OS Overshoot 0.1 VPP Step 37 %

SR Slew Rate (Note 8) VIN = 2 VPP 106 V/µs

Static DC Performance

ISSupply Current Enable1,2 = VDD ; No Load 2.3 2.7

2.9 mA

Enable1 = VDD , Enable2 = VSS , No

Load 1.3 1.5

1.6 mA

Enable1,2 = VSS ; No Load 30 41

46 μA

PSRR Power Supply Rejection Ratio DC (3.0V to 5.0V) 65

64 68 dB

ACL Small Signal Voltage Gain VIN = 0.2 VPP 0.95 1.0 1.05 V/V

VOS Output Offset Voltage -0.5 17

18 mV

TC VOS Temperature Coefficient Output

Offset Voltage (Note 9)

2.8 µV/°C

ROUT Output Resistance f = 100 kHz 0.6 Ω

f = 38.4 MHz 166

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LMH2180

Symbol Parameter Conditions Min

(Note 7)

Typ

(Note 6)

Max

(Note 7)

Units

Miscellaneous Performance

RIN Input Resistance per Buffer Enable = VDD 137 kΩ

Enable = VSS 137

CIN Input Capacitance per Buffer Enable = VDD 1.3 pF

Enable = VSS 1.3

ZIN Input Impedance f = 38.4 MHz, Enable = VDD 4.5 kΩ

f = 38.4 MHz, Enable = VSS 4.2

VOOutput Swing Positive VIN = VDD 2.66

2.65 2.69 V

Output Swing Negative VIN = VSS 19 35

37 mV

ISC Output Short-Circuit Current

(Notes 10, 11)

Sourcing, VIN = VDD, VOUT = VSS −21

−18 −25

Sinking, VIN = VSS, VOUT = VDD 23

15 25

Ven_hmin Enable High Active Minimum Voltage 1.2

Ven_lmax Enable Low Inactive Maximum

Voltage

0.6

5V Electrical Characteristics

Unless otherwise specified, all limits are guaranteed for TA = 25°C, VDD = 5V, VSS = 0V, VCM = 1V, Enable1,2 = VDD, CL = 10 pF,

RL = 30 kΩ, Load is connected to VSS, CCOUPLING = 10 nF. Boldface limits apply at temperature range extremes of operating

condition. See (Note 2)

Symbol Parameter Conditions Min

(Note 7)

Typ

(Note 6)

Max

(Note 7)

Units

Frequency Domain Response

SSBW Small Signal Bandwidth VIN = 100 mVPP; −3 dB 87 MHz

LSBW Large Signal Bandwidth VIN = 1.0 VPP; −3 dB 68 MHz

GFN Gain Flatness < 0.1 dB f > 100 kHz 25 MHz

Distortion and Noise Performance

φnPhase Noise VIN = 1 VPP, fC = 38.4 MHz, Δf = 1 kHz −123 dBc/Hz

VIN = 1 VPP, fC = 38.4 MHz, Δf = 10 kHz −132 dBc/Hz

enInput-Referred Voltage Noise f = 1 MHz, RSOURCE = 50Ω 12 nV/

ISOLATION Output to Input f = 1 MHz, RSOURCE = 50Ω 84 dB

CT Crosstalk Rejection f = 38.4 MHz, PIN = 0 dBm 59 dB

Time Domain Response

trRise Time 0.1 VPP Step (10-90%) 6 ns

tfFall Time 6 ns

tsSettling Time to 0.1% 1 VPP Step 70 ns

OS Overshoot 0.1VPP Step 13 %

SR Slew Rate (Note 8) VIN = 2 VPP 124 V/µs

Static DC Performance

ISSupply Current Enable1,2 = VDD ; No Load 3.4 4.0

4.1 mA

Enable1 = VDD, Enable2 = VSS ; No

Load 1.8 2.2

2.3 mA

Enable1,2 = VSS ; No Load 32 43

49 μA

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LMH2180

Symbol Parameter Conditions Min

(Note 7)

Typ

(Note 6)

Max

(Note 7)

Units

PSRR Power Supply Rejection Ratio DC (3.0V to 5.0V) 65

64 68 dB

ACL Small Signal Voltage Gain VIN = 0.2 VPP 0.95 1.0 1.05 V/V

VOS Output Offset Voltage −1.4 21

22 mV

TC VOS Temperature Coefficient Output

Offset Voltage (Note 9)

2.4 µV/°C

ROUT Output Resistance f = 100 kHz 0.5 Ω

f = 38.4 MHz 126

Miscellaneous Performance

RIN Input Resistance per Buffer Enable = VDD 138 kΩ

Enable = VSS 138

CIN Input Capacitance per Buffer Enable = VDD 1.3 pF

Enable = VSS 1.3

ZIN Input Impedance f = 38.4 MHz, Enable = VDD 4.3 kΩ

f = 38.4 MHz, Enable = VSS 4.2

VOOutput Swing Positive VIN = VDD 4.96

4.95 4.99 V

Output Swing Negative VIN = VSS 10 35

50 mV

ISC Output Short-Circuit Current

(Notes 10, 11)

Sourcing, VIN = VDD, VOUT = VSS −80

−62 −90

Sinking, VIN = VSS, VOUT = VDD 60

43 65

Ven_hmin Enable High Active Minimum Voltage 1.2

Ven_lmax Enable Low Inactive Maximum

Voltage

0.6

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

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

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

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

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

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

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

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

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

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

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

characterization and are not guaranteed.

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

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

Note 9: Average Temperature Coefficient is determined by dividing the changing in a parameter at temperature extremes by the total temperature change.

Note 10: Short−Circuit test is a momentary test. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed

junction temperature of 150°C.

Note 11: Positive current corresponds to current flowing into the device.

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LMH2180

Block Diagram

30024601

Pin Descriptions

Pin No.

LLP

Pin No.

microSMD

Pin Name Description

1 A3 VDD Voltage supply connection

2 A2 IN 1 Input 1

3 A1 IN 2 Input 2

4 B1 ENABLE 2 Enable buffer 2

5 C1 VSS Ground connection

6 C2 OUT 2 Output 2

7 C3 OUT 1 Output 1

8 B3 ENABLE 1 Enable buffer 1

Connection Diagrams

8-Pin LLP

30024631

Top View

8-Bump micro SMD

30024649

Top View

Ordering Information

Package Part Number Package Marking Transport Media NSC Drawing

8-Pin LLP

Solder Bump

LMH2180YD 2180Y 1k Units Tape and Reel YDA08A

LMH2180YDX 4.5k Units Tape and Reel

8-Pin LLP

No Pullback

LMH2180SD

2180S

1k Units Tape and Reel

SDA08ALMH2180SDE 250 Units Tape and Reel

LMH2180SDX 4.5k Units Tape and Reel

8-Bump micro SMD LMH2180TM CA 250 Units Tape and Reel TMD08CCA

LMH2180TMX 3k Units Tape and Reel

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LMH2180

Typical Performance Characteristics TA = 25°C, VDD = 2.7V, VSS = 0V, Enable1,2 = VDD, CL = 10 pF,

RL = 30 kΩ and CCOUPLING = 10 nF, unless otherwise specified.

Frequency Response

30024603

Phase Response

30024604

Frequency Response Over Temperature

30024605

Frequency Response Over Temperature

30024606

Phase Response Over Temperature

30024607

Phase Response Over Temperature

30024608

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LMH2180

Large Signal Bandwidth

30024609

Gain Flatness < 0.1 dB (GFN)

30024610

Voltage Noise

30024611

Phase Noise

30024612

Isolation Output to Input vs. Frequency

30024613

Crosstalk Rejection vs. Frequency

30024614

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LMH2180

Transient Response Positive

30024615

Transient Response Negative

30024616

Small Signal Pulse Response

30024617

Small Signal Pulse Response

30024618

Large Signal Response

30024619

Large Signal Response

30024620

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LMH2180

ISUPPLY vs. VSUPPLY

30024621

ISUPPLY vs. VSUPPLY

30024622

ISUPPLY vs. VSUPPLY

30024623

PSRR vs. Frequency

30024624

VOS vs. VSUPPLY

30024625

ROUT vs. Frequency

30024626

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LMH2180

Input Impedance vs. Frequency

30024627

VOUT vs. IOUT (Sourcing)

30024628

VOUT vs. IOUT (Sourcing)

30024629

VOUT vs. IOUT (Sinking)

30024630

VOUT vs. IOUT (Sinking)

30024632

ISC Sourcing vs. VSUPPLY Over Temperature

30024633

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LMH2180

ISC Sinking vs. VSUPPLY Over Temperature

30024634

ISUPPLY vs. VENABLE

30024635

ISUPPLY vs. VENABLE

30024636

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LMH2180

Application Information

GENERAL

The LMH2180 is designed to minimize the effects of spurious

signals from the base chip to the oscillator. Also the influence

of varying load resistance and capacitance to the oscillator is

minimized, while the drive capability is increased.

The inputs of the LMH2180 are internally biased at 1V, mak-

ing AC coupling possible without external bias resistors.

To optimize current consumption, a buffer that is not in use

can be disabled by connecting it's enable pin to VSS.

The LMH2180 has no internal ground reference; therefore,

either single or split supply configurations can be used.

The LMH2180 is an easy replacement for discrete circuitry. It

simplifies board layout and minimizes the effect of layout re-

lated parasitic components.

INPUT CONFIGURATION

The internal 1V input biasing allows AC coupling of the input

signal. This biasing avoids the use of external resistors, as

depicted in Figure 1. The biasing prevents a large DC load at

the oscillators output that creates a load impedance and may

affect it's oscillating frequency. As a result of this biasing, the

maximum amplitude of the AC signal is 2VPP.

The coupling capacitance C1 should be large enough to let

the AC signal pass. This is a unity gain buffer with rail-to-rail

inputs and outputs.

30024644

FIGURE 1. Input Configuration

FREQUENCY PULLING

Frequency pulling is the frequency variation of an oscillator

caused by a varying load. In the typical application, the load

of the oscillator is a fixed capacitor (C1) in series with the input

impedance of the buffer.

To keep the input impedance as constant as possible, the in-

put is biased at 1V, even when the part is disabled. A simpli-

fied schematic of the input configuration is shown in Figure

ISOLATION AND CROSSTALK

Output to input isolation prevents the clock signal of the os-

cillator from being affected by spurious signals generated by

the digital blocks behind the output buffer. See the charac-

teristic graphic entitled Isolation Output to Input vs. Frequen-

cy.

A block diagram of the isolation is shown in Figure 2.

Crosstalk rejection between buffers prevents signals from af-

fecting each other. Figure 2 shows a Baseband IC and a

Bluetooth module as an example. See the characteristic

graphic labeled Crosstalk Rejection vs. Frequency for more

information.

30024645

FIGURE 2. Isolation Block Diagram

DRIVING CAPACITIVE LOADS

Each buffer can drive a capacitive load. Be aware that every

capacitor directly connected to the output becomes part of the

loop of the buffer. In most applications the load consists of the

capacitance of copper tracks and the input capacitance of the

application blocks. Capacitance reduces the gain/phase mar-

gin and decreases the stability. This leads to peaking in the

frequency response and in extreme situations oscillations can

occur. To drive a large capacitive load it is recommended to

include a series resistor between the buffer and the load ca-

pacitor. The best value for this isolation resistance can be

found by experimentation.

The LMH2180 datasheet reflects measurements with capac-

itive loads of 10 pF at the output of the buffers. Most common

applications will probably use a lower capacitive load, which

will result in lower peaking and significantly greater band-

width, see Figure 3.

30024646

FIGURE 3. Bandwidth and Peaking

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LMH2180

PHASE NOISE

A clock buffer adds noise to the clock signal. This noise caus-

es uncertainty in the phase of the clock signal. This uncer-

tainty is described by jitter (time domain) or phase noise

(frequency domain). Communication systems, such as Wire-

less LAN, require a low jitter/phase noise clock signal to

obtain a low Bit Error Rate. Figure 4 shows the frequency do-

main representation of a clock signal with frequency fC. With-

out Phase Noise the entire signal power would only be located

at the frequency fC. Phase Noise spreads some of the power

to adjacent frequencies. Phase Noise is usually specified in

dBc/Hz at a given frequency offset Δf from the carrier, where

dBc is the power level in dB relative to the carrier. The noise

power is measured within a 1 Hz bandwidth.

30024648

FIGURE 4. Phase Noise

Figure 5 shows the setup used to measure the LMH2180

phase noise. The clock driving the LMH2180 is a state of the

art 38.4MHz TCXO. Both the TCXO phase noise and the

phase noise at the LMH2180 output were measured. At offset

frequencies of 1 kHz and higher from the carrier, the TCXO

phase noise is sufficiently low to accurately calculate the

LMH2180 contribution to the phase noise at the output. The

LMH6559, whose phase noise contribution can be neglected,

is used to drive the 50Ω input impedance of the Signal Source

Analyzer.

LAYOUT DESIGN RECOMMENDATION

Careful consideration during circuit design and PCB layout

will eliminate problems and will optimize the performance of

the LMH2180. It is best to have the same ground plane on the

PCB for all decoupling and other ground connections.

To ensure a clean supply voltage it is best to place decoupling

capacitors close to the LMH2180, between VDD and VSS.

Another important issue is the value of the components, be-

cause this also determines the sensitivity to disturbances.

Resistor values have to be low enough to avoid a significant

noise contribution and large enough to avoid a significant in-

crease in power consumption while loading inputs or outputs

to heavily.

30024647

FIGURE 5. Measurement Setup

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LMH2180

Physical Dimensions inches (millimeters) unless otherwise noted

8-Pin LLP

NS Package Number YDA08A

8-Pin LLP

NS Package Number SDA08A

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LMH2180

8-Bump micro SMD

NS Package Number TMD08CCA

X1 = 1.215 ± 0.030 mm, X2 = 1.215 ± 0.030 mm, X3 = 0.600 ± 0.075 mm

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LMH2180

Notes

LMH2180 75 MHz Dual Clock Buffer

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