LMK00308EVM/NOPB - NATIONAL SEMICONDUCTOR

LMH1980

LMH1980 Auto-Detecting SD/HD/PC Video Sync Separator

Literature Number: SNLS263

July 2007

LMH1980

Auto-Detecting SD/HD/PC Video Sync Separator

General Description

The LMH1980 is an auto-detecting SD/HD/PC video sync

separator ideal for use in a wide range of video applications,

such as automotive LCD monitors, video capture & editing

devices, surveillance & security equipment, and machine vi-

sion and inspection systems.

The LMH1980 accepts an analog video input signal with ei-

ther bi-level or tri-level sync and automatically detects the

video format, eliminating the need for external RSET resistor

adjustment required by other sync separators (e.g.: LM1881).

The outputs provide timing signals in CMOS logic, including

Composite, Horizontal, and Vertical Syncs, Burst/Back Porch

Timing, and Odd/Even Field outputs. The HD flag output (pin

5) provides a logic low signal only when a valid HD video input

with tri-level sync is detected. The HD flag can be used to

disable an external switch-controlled SD chroma filter when

HD video is detected, or enable it when SD video is detected.

For non-standard video with bi-level sync and without vertical

serration pulses, a default vertical sync pulse will be output

and no horizontal sync pulses will be output during the vertical

sync interval.

The LMH1980 is available in a space-saving 10-ld. Mini-SO

Package (MSOP) and operates over a temperature range of

−40°C to +85°C.

Features

■Analog video sync separation for NTSC, PAL, 480I/P,

576I/P, 720P, 1080I/P/PsF, and many VESA-compatible

timing formats

■Composite Video (CVBS), S-Video (Y/C), and Component

Video (YPBPR/GBR) and PC Graphics (RGsB) interfaces

■SD/PC Bi-level sync & HD tri-level sync compatible

■Composite, Horizontal, and Vertical Sync outputs

■Burst/Back Porch Timing, Odd/Even Field, and HD Detect

Flag outputs

■Automatic video format detection

■Fixed-level sync slicing for video inputs from 0.5 to 2 VPP

■3.3V to 5V supply operation

Applications

■Consumer, Professional, Automotive & Industrial Video

■Video Capture, Editing, and Processing

■Genlock Circuits

■Surveillance & Security Video Systems

■Set-Top Boxes (STB) & Digital Video Recorders (DVR)

■LCD / Plasma Displays and Video Projectors

■Machine Vision and Inspection Systems

■Video Trigger Oscilloscopes and Waveform Monitors

Connection Diagram

10-ld. MSOP

30010301

Top View

FIGURE 1. Pinout

Pin Descriptions

Pin No. Pin Name Pin Description

1 REXT Bias Current External Resistor

2 GND Ground

3 VCC Supply Voltage

4 VIN Analog Video Input

5 HD HD Detect Flag Output

6 HSOUT Horizontal Sync Output

7 VSOUT Vertical Sync Output

8 CSOUT Composite Sync Output

9 BPOUT Burst/Back Porch Timing Output

10 OEOUT Odd/Even Field Output

Ordering Information

Package Part Number Package Marking Transport Media NSC Drawing

10-ld. MSOP LMH1980MM AL4A 1k Units Tape and Reel MUB10A

LMH1980MMX 3.5k Units Tape and Reel

LMH1980 Auto-Detecting SD/HD/PC Video Sync Separator

Absolute Maximum Ratings (Notes 1, 7)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

ESD Tolerance (Note 2)

Human Body Model 3.5 kV

Machine Model 350V

Supply Voltage, VCC 0V to 5.5V

Video Input, VIN −0.3V to VCC + 0.3V

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

Lead Temperature (soldering 10 sec.) 300°C

Junction Temperature, TJMAX(Note 3) +150°C

Thermal Resistance, θJA (no airflow) 120°C/W

Operating Ratings (Note 1)

Temperature Range (Note 3) −40°C to +85°C

VCC 3.3V −10% to 5V +10%

Input Amplitude, VIN-AMPL 140 mV to VCC–VIN-CLAMP

Electrical Characteristics (Note 4)

Unless otherwise specified, all limits are guaranteed for TA = 25°C, VCC = 3.3V, REXT = 10 kΩ 1%, RL = 10 kΩ, CL < 10 pF.

Boldface limits apply at the temperature extremes. See Figure 2 for Test Circuit.

Symbol Parameter Conditions Min

(Note 6)

Typ

(Note 5)

Max

(Note 6)

Units

ICC Supply Current No input signal VCC = 3.3V 10.5 12.5 mA

VCC = 5V 12.0 14.0

Video Input Specifications

VIN-SYNC Input Sync Amplitude Amplitude from negative sync tip to video

blanking level for SD/EDTV bi-level sync

(Notes 8, 9)

0.14 0.30 0.60

VPP

Amplitude from negative to positive sync tips

for HDTV tri-level sync

(Notes 8, 10)

0.30 0.60 1.20

VIN-CLAMP Input Sync Tip Clamp Level 0.7 V

VIN-SLICE Input Sync Slice Level Slicing level above VIN-CLAMP 70 mV

Logic Output Specifications (Note 12)

VOL Output Logic 0 See output load conditions

above

VCC = 3.3V 0.3 V

VCC = 5V 0.5

VOH Output Logic 1 See output load conditions

above

VCC = 3.3V 3.0 V

VCC = 5V 4.5

TSYNC-LOCK Sync Lock Time Time for the output signals to be correct after

the video signal settles at VIN following a

significant input change. See Start-Up Time

section for more information

2 V

periods

TVSOUT Vertical Sync Output Pulse

Width

Serration Pulses in the Vertical Interval. See

Figures 3, 4, 5, 6, 7, 8 for SDTV, EDTV &

HDTV Vertical Interval Timing

3 H periods

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

Tables.

Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. 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 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: 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.

Note 5: Typical values represent the most likely parametric norm 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 6: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using the Statistical Quality

Control (SQC) method.

Note 7: All voltages are measured with respect to GND, unless otherwise specified.

Note 8: VIN-AMPL plus VIN-CLAMP should not exceed VCC.

Note 9: Tested with 480I signal.

Note 10: Tested with 1080P signal.

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LMH1980

Note 11: Maximum voltage offset (DC bounce) between 2 consecutive input sync tips must be less than 25 mVPP; otherwise, this may cause incorrect output

signals to occur.

Note 12: Outputs are negative-polarity logic signals, except for Odd/Even Field.

LMH1980 Test Circuit

30010302

FIGURE 2. Test Circuit

The LMH1980 test circuit is shown in Figure 2. The video generator should provide a clean, low-noise video input signal with

minimal sync pulse overshoot over 75Ω coaxial cable, which should be impedance-matched with a 75Ω load termination resistor

to prevent unwanted signal distortion. The output waveforms should be monitored using a low-capacitance probe on an oscilloscope

with at least 500 MHz bandwidth. See the PCB LAYOUT CONSIDERATIONS section for more information about signal and supply

trace routing and component placement. Also, refer to the “LMH1980 Evaluation Board Instruction Manual” Application Note

(AN-1618).

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LMH1980

SDTV Vertical Interval Timing Diagrams (NTSC, PAL, 480I, 576I)

30010303

FIGURE 3. NTSC Odd Field Vertical Interval

30010304

FIGURE 4. NTSC Even Field Vertical Interval

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LMH1980

EDTV Vertical Interval Timing Diagram (480P, 576P)

30010305

FIGURE 5. 480P Vertical Interval

HDTV Vertical Interval Timing Diagram (720P, 1080P)

30010314

FIGURE 6. 720P (1080P) Vertical Interval

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LMH1980

HDTV Vertical Interval Timing Diagrams (1080I)

30010306

FIGURE 7. 1080I Field 1 Vertical Interval

30010307

FIGURE 8. 1080I Field 2 Vertical Interval

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LMH1980

SD/EDTV Horizontal Interval Timing Diagram

30010308

FIGURE 9. SD/EDTV Horizontal Interval with Bi-level Sync

SDTV Horizontal Interval Timing Characteristics

VCC = 3.3V , TA = 25°C, No Input Filter, PAL Video Input from Tek TG700 Generator with AVG7 SD video module

Symbol Parameter Conditions Typ Units

tdCSOUT Composite Sync Output Propagation Delay from Input

Sync Reference (OH)

See Figure 9 525 ns

tdHSOUT Horizontal Sync Output Propagation Delay from Input

Sync Reference (OH)

See Figure 9 530 ns

tdBPOUT Burst/Back Porch Timing Output Propagation Delay from

Input Sync Trailing Edge

See Figure 9 400 ns

THSOUT Horizontal Sync Output Pulse Width See Figure 9 2.5 µs

TBPOUT Burst/Back Porch Timing Output Pulse Width See Figure 9 3.0 µs

EDTV Horizontal Interval Timing Characteristics

VCC = 3.3V , TA = 25°C, No Input Filter, 576P Video Input from Tek TG700 Generator with AVG7 SD module

Symbol Parameter Conditions Typ Units

tdCSOUT Composite Sync Output Propagation Delay from Input

Sync Reference (OH)

See Figure 9 170 ns

tdHSOUT Horizontal Sync Output Propagation Delay from Input

Sync Reference (OH)

See Figure 9 175 ns

tdBPOUT Burst/Back Porch Timing Output Propagation Delay from

Input Sync Trailing Edge

See Figure 9 485 ns

THSOUT Horizontal Sync Output Pulse Width See Figure 9 2.3 µs

TBPOUT Burst/Back Porch Timing Output Pulse Width See Figure 9 350 ns

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LMH1980

HDTV Horizontal Interval Timing Diagram

30010309

FIGURE 10. HDTV Horizontal Interval with Tri-level Sync

HDTV Horizontal Interval Timing Characteristics

VCC = 3.3V , TA = 25°C, No Input Filter, 1080I Video Input from Tek TG700 Generator with AWVG7 HD module

Symbol Parameter Conditions Typ Units

tdCSOUT Composite Sync Output Propagation Delay from Input

Sync Leading Edge

See Figure 10 150 ns

tdHSOUT Horizontal Sync Output Propagation Delay from Input

Sync Reference (OH)

See Figure 10 60 ns

tdBPOUT Burst/Back Porch Timing Output Propagation Delay from

Input Sync Trailing Edge

See Figure 10 450 ns

THSOUT Horizontal Sync Output Pulse Width See Figure 10 525 ns

TBPOUT Burst/Back Porch Timing Output Pulse Width See Figure 10 350 ns

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LMH1980

Application Information

GENERAL DESCRIPTION

The LMH1980 is designed to extract the timing information

from various video formats with standard and non-standard

vertical serration and output the syncs and relevant timing

signals in CMOS logic. Its advanced features and easy ap-

plication make it ideal for consumer, professional, and indus-

trial video systems where sync timing needs to be extracted

from SD, HD, and PC video signals. The device can operate

from a supply voltage between 3.3V and 5V. The only re-

quired external components are bypass capacitors between

the VCC and GND pins, input coupling capacitor (CIN) from the

signal source to the VIN pin, and a fixed-value 1% precision

resistor between the REXT and GND pins. Refer to the test

circuit in Figure 2.

REXT Resistor

The precision external resistor (REXT) establishes the internal

bias current and precise reference voltage for the LMH1980.

For optimal performance, REXT should be a 10 kΩ 1% preci-

sion resistor with a low temperature coefficient to ensure

proper operation over a wide temperature range. Using a

REXT resistor with less precision may result in reduced per-

formance (like worse performance, increased propagation

delay variation, or reduced input sync amplitude range)

against temperature, supply voltage, input signal, or part-to-

part variations.

Note: The REXT resistor used with the LMH1980 serves a dif-

ferent function than the “RSET resistor” used with other previ-

ous sync separators, like the LM1881. For the LM1881, the

RSET value needed to be adjusted externally to support dif-

ferent input line rates. For the LMH1980, the REXT value is

fixed, and the device automatically detects the input line rate

to support various video formats without electrical or physical

intervention.

Automatic Format Detection and Switching

Automatic format detection eliminates the need for adjusting

an external RSET resistor or programming via a microcon-

troller. The device outputs will respond correctly to a switch

in video format after a sufficient start-up time has been satis-

fied, usually within 1 to 2 fields of video. Unlike other sync

separators, the LMH1980 does not require the power to be

cycled in order to produce correct outputs after a significant

change to the input signal. See the Start-up Time section for

more details.

Fixed-Level Sync Slicing

The LMH1980 uses fixed-level sync slicing for video inputs

with an amplitude from 0.5VPP to 2VPP, which allows for prop-

er sync separation even for improperly terminated or attenu-

ated input signals. The fixed-level sync slicing threshold is

nominally 70 mV above the clamped sync tip. This means that

for a minimum video input signal amplitude of 0.5VPP, the

slicing level is near the mid-point of the sync pulse amplitude.

This slicing level is independent of the input signal amplitude;

therefore, for a 2VPP input, the slicing level occurs at 12% of

the sync pulse amplitude.

INPUT CONSIDERATIONS

The LMH1980 supports sync separation for analog CVBS, Y

(luma) from Y/C and YPBPR, and G (sync on green) from

GBR/RGsB, as specified in the following video standards.

•Composite Video (CVBS) and S-Video (Y/C):

—SMPTE 170M (NTSC), ITU-R BT.470 (PAL)

•Component Video (YPBPR/GBR):

—SDTV: SMPTE 125M, SMPTE 267M, ITU-R BT.601

(480I, 576I)

—EDTV: ITU-R BT.1358 (480P, 576P)

—HDTV: SMPTE 296M (720P), SMPTE 274M

(1080I/P), SMPTE RP 211 (1080PsF)

•PC Graphics (RGsB):

—VESA Monitor Timing Standards and Guidelines

Version 1.0, Revision 0.8

•Non-Standard Video:

—Composite NTSC & PAL (or Component 480I & 576I)

without vertical serration & equalization pulses (i.e.:

from logical OR-ing of H & V signals)

Input Termination

The video source should be load terminated with a 75Ω re-

sistor to ensure correct video signal amplitude and minimize

signal distortion due to reflections. In extreme cases, the

LMH1980 can handle non-terminated or double-terminated

input conditions, assuming 1VPP signal amplitude for normally

terminated video.

Input Filtering

An external filter is recommended if the video signal has large

chroma amplitude that extends near the sync tip and/or has

considerable high-frequency noise, so they do not interfere

with sync separation. A simple RC low-pass chroma filter with

a series resistor (R9) and a filter capacitor (C2) to ground can

be used to sufficiently attenuate chroma such that minimum

peak of its amplitude is above the slicing level and also to

improve the overall signal-to-noise ratio. To achieve the de-

sired filter cutoff frequency, it’s advised to vary C2 and keep

R9 small (i.e.: 100Ω) to minimize sync tip clipping due to the

voltage drop across R9. Keep in mind that as the cutoff fre-

quency decreases, the LMH1980 output propagation delays

increase, which could affect the timing relationship between

the sync and video signals.

In applications where the chroma filter needs to be disabled

when HD video is input, it is possible to use a transistor switch

(Q1) controlled by the HD flag (pin 5) to open C2’s connection

to ground as shown in Figure 11. When a HD tri-level sync

input signal is applied, HD will output logic low (following a

brief delay for auto format detection) and Q1 will turn off to

disable the chroma filter, which is intended for SD composite

video only. When a SD bi-level sync signal (i.e.: NTSC/PAL)

is applied, HD will output logic high and Q1 will turn on to

enable the chroma filter.

Important: If the filter cutoff frequency (fCO) is set too low and

HD video is applied, the filter can severely roll off and atten-

uate the input's high-bandwidth tri-level sync pulses such that

the LMH1980 cannot detect a valid HD input signal. If the

LMH1980 cannot detect a valid HD input, then the HD flag will

never change from logic high to low and the switch-controlled

filter will never be disabled via Q1. In other words, fCO should

not be set too low that the filter impairs the LMH1980's ability

to detect a valid HD input. The values of R9 and C2 shown in

Figure 11 give fCO=2.79 MHz (about -4 dB at 3.58 MHz NTSC

subcarrier frequency) without impairing HD video format de-

tection.

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LMH1980

30010318

FIGURE 11. External Switch-Controlled Chroma Filter

If a PC video input with bi-level sync is to be used, C2 should

be removed to disable chroma filtering. This is necessary be-

cause HD will output logic high (like in the SD video input

case) and enable the filter. A chroma filter could severely

band-limit a high-bandwidth PC video signal, which could roll-

off and attenuate the sync pulses such that the LMH1980

cannot detect a valid input signal.

If some high-frequency noise filtering is needed for all video

inputs, a small capacitor (C1) may be optionally used in par-

allel but outside of the transistor switch. When Q1 is turned

on, then C1 and C2 will be connected in parallel (C1+C2)

Input Coupling Capacitor

The input signal should be AC coupled to the VIN (pin 4) of the

LMH1980 with a properly chosen coupling capacitor, CIN.

The primary consideration in choosing CIN is whether the

LMH1980 will interface with video sources using an AC-cou-

pled output stage. If AC-coupled video sources are expected

in the end-application , then it’s recommended to choose a

small CIN value such as 0.01 µF to avoid missing sync output

pulses due to average picture level changes. It’s important to

note that video sources with an AC-coupled output will cause

video-dependent jitter at the HSync output of the sync sepa-

rator. When only DC-coupled video sources are expected, a

larger value for CIN may be used without concern for missing

sync output pulses. A smaller CIN value can be used to in-

crease rejection of source AC hum components and also

reduce start-up time regardless of the video source's output

coupling type.

START-UP TIME

When there is a significant change to the video input signal,

such as sudden signal switching in, signal attenuation (i.e.:

load termination added via loop through) or signal gain (i.e.:

load termination removed), the quiescent operation of the

LMH1980 will be disrupted. During this dynamic input condi-

tion, the LMH1980 outputs may not be correct but will recover

to valid signals after a predictable start-up time, which con-

sists of an adjustable input settling time and a predetermined

“sync lock time”.

Input Settling Time and Coupling Capacitor Selection

Following a significant input condition, the negative sync tip

of the AC-coupled signal settles to the input clamp voltage as

the coupling capacitor, CIN, recovers a quiescent DC voltage

via the dynamic clamp current through VIN. Because CIN de-

termines the input settling time, its capacitance value is critical

when minimizing overall start-up time. A smaller CIN value

yields shorter settling time at the expense of increased line

droop voltage, whereas a larger one yields reduced line droop

but longer settling time. Settling time is proportional to the

value of CIN, so doubling CIN will also double the settling time.

Sync Lock Time

In addition to settling time, the LMH1980 has a predetermined

sync lock time, TSYNC-LOCK, before the outputs are correct.

Once the AC-coupled input has settled enough, the LMH1980

needs time to detect the valid video signal and apply fixed-

level sync slicing before the output signals are correct.

For practical values of CIN, TSYNC-LOCK is typically less than 1

or 2 video fields in duration starting from the 1st valid VSync

output pulse to the valid HSync pulses beginning thereafter.

VSync and HSync pulses are considered valid when they

align correctly with the input's vertical and horizontal sync in-

tervals.

It is recommended for the outputs to be applied to the system

after the start-up time is satisfied and outputs are valid. For

example, the oscilloscope screenshot in Figure 12 shows a

typical start-up time within 1 video field from when an NTSC

signal is just applied to when the LMH1980 outputs are valid.

30010317

FIGURE 12. Typical Start-Up Time for NTSC Input to

LMH1980 (CIN = 0.1 µF)

LOGIC OUTPUTS

In the absence of a video input signal, the LMH1980 outputs

are logic high except for the odd/even field, which is undefined

and depends on its previous state, and the composite sync

output.

Horizontal Sync Output

HSOUT (pin 6) produces a negative-polarity horizontal sync

signal, or HSync, extracted from the input signal. For bi-level

and tri-level sync signals, HSync's negative-going leading

edge is derived from the input's sync reference, OH, with a

propagation delay.

Important: The HSync output has good performance on its

negative-going leading edge, so it should be used as the ref-

erence to a negative-edge triggered PLL input. If HSync is

used as the reference to a positive-edge triggered PLL input,

like in some FPGAs, the signal must be inverted first to pro-

duce a positive-polarity HSync signal (i.e.: positive-going

leading edge) before the PLL input. HSync's trailing edge

should not be used as the reference to a PLL because for a

NTSC/PAL input, the input's half-width pulses (½TSYNC) in the

vertical interval cause the trailing edges of the HSync output

to occur earlier than for the normal-width sync pulses

(TSYNC). This bi-modal timing variation on HSync's trailing

edge, as shown in Figure 13, could affect the performance of

the PLL. The bi-modal trailing edge timing also occurs on the

CSync output as well.

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LMH1980

30010313

FIGURE 13. Bi-modal Timing on HSync's Trailing Edge

for Half-Width Pulses for NTSC

Vertical Sync Output

VSOUT (pin 7) produces a negative-polarity vertical sync sig-

nal, or VSync. VSync's negative-going leading edge is de-

rived from the first vertical serration pulse with a propagation

delay, and its output pulse width, TVSOUT, spans approximate-

ly three horizontal periods (3H). When there is no vertical

serration pulses (i.e.: non-standard video signal), the

LMH1980 will output a default VSync pulse derived from the

input's vertical sync leading edge with a propagation delay.

Composite Sync Output

CSOUT (pin 8) simply reproduces the video input sync pulses

below the video blanking level. This is obtained by clamping

the video signal sync tip to the internal clamp voltage at VIN

and extracting the resultant composite sync signal, or CSync.

For both bi-level and tri-level syncs, CSync's negative-going

leading edge is derived from the input's negative-going lead-

ing edge with a propagation delay.

Burst/Back Porch Timing Output

BPOUT (pin 9) provides a negative-polarity burst/back porch

signal, which is pulsed low for a fixed width during the back

porch interval following the input's sync pulse. The burst/back

porch timing pulse is useful as a burst gate signal for NTSC/

PAL color burst synchronization and as a clamp signal for

black level clamping (DC restoration) and sync stripping ap-

plications.

For SDTV formats, the back porch pulse's negative-going

leading edge is derived from the input's positive-going sync

edge with a propagation delay, and the pulse width spans an

appropriate duration of the color burst envelope for NTSC/

PAL. For EDTV formats, the back porch pulse behaves similar

to the SDTV case except with a narrow pulse width. For HDTV

formats, the pulse's leading edge is derived from the input's

negative-going trailing sync edge with a propagation delay,

and the pulse width is narrow to correspond with the short

back porch durations. During the vertical sync period, the

back porch output will be muted (no pulses) and remain logic

high.

Odd/Even Field Output

OEOUT (pin 10) provides an odd/even field output signal,

which facilitates identification of odd and even fields for inter-

laced or segmented frame (sF) formats. For interlaced or

segmented frame formats, the odd/even output is logic high

during an odd field (field 1) and logic low during an even field

(field 2). The odd/even output edge transitions align with

VSync's leading edge to designate the start of odd and even

fields. For progressive (non-interlaced) video formats, the

output is held constantly at logic high.

HD Detect Flag Output

HD (pin 5) is an active-low flag output that only outputs a logic

low signal when a valid HD video input (i.e.: 720P, 1080I and

1080P) with tri-level sync is detected; otherwise, it will output

logic high. Note that there is a processing delay (within 1 to 2

video fields) from when an HD video signal is applied to when

the outputs are correct and the HD flag changes from logic

high (default) to logic low, to indicate a valid HD input has

been detected.

The HD flag can be used to disable an external switch-con-

trolled SD chroma filter when HD video is detected and con-

versely, enable it when SD video is detected. This is important

because a non-switched chroma filter attenuates signal com-

ponents above 500 kHz to 3 MHz, which could roll-off and/or

attenuate the high bandwidth HD tri-level sync signal prior to

the LMH1980 and may increase output propagation delay and

jitter. See the Input Filtering section for more information.

ADDITIONAL CONSIDERATIONS

Using an AC-Coupled Video Source into the LMH1980

An AC coupled video source typically has a 100 µF or larger

output coupling capacitor (COUT) for protection and to remove

the DC bias of the amplifier output from the video signal.

When the video source is load terminated, the average value

of the video signal will shift dynamically as the video duty cycle

varies due to the averaging effect of the COUT and termination

resistors. The average picture level or APL of the video con-

tent is closely related to the duty cycle.

For example, a significant decrease in APL such as a white-

to-black field transition will cause a positive-going shift in the

sync tips characterized by the source’s RC time constant, tRC-

OUT (150Ω*COUT). The LMH1980’s input clamp circuitry may

have difficulty stabilizing the input signal under this type of

shifting; consequently, the unstable signal at VIN may cause

missing sync output pulses to result, unless a proper value

for CIN is chosen.

To avoid this potential problem when interfacing AC-coupled

sources to the LMH1980, it’s necessary to introduce a voltage

droop component via CIN to compensate for video signal shift-

ing related to changes in the APL. This can be accomplished

by selecting CIN such that the effective time constant of the

LMH1980’s input circuit, tRC-IN, is less than tRC-OUT.

The effective time constant of the input circuit can be approx-

imated as: tRC-IN = (RS+RI)*CIN*TLINE/TCLAMP, where

RS = 150Ω, RI = 1 kΩ (input resistance when clamping),

TLINE ∼ 64 μs for NTSC, and TCLAMP = 250 ns (internal clamp

duration). A white-to-black field transition in NTSC video

through COUT will exhibit the maximum sync tip shifting due to

its long line period (TLINE). By setting tRC-IN < tRC-OUT, the max-

imum value of CIN can be calculated to ensure proper opera-

tion under this worst-case condition.

For instance, tRC-OUT is about 33 ms for COUT = 220 µF. To

ensure tRC-IN < 33 ms, CIN must be about 100 nF or less. By

choosing CIN = 47 nF, the LMH1980 will function properly with

AC-coupled video sources using COUT ≥ 220 μF.

PCB LAYOUT CONSIDERATIONS

Please refer to the “LMH1980 Evaluation Board Instruction

Manual” Application Note (AN-1618) for a good PCB layout

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LMH1980

example, which adheres to the following suggestions for com-

ponent placement and signal routing.

LMH1980 IC Placement

The LMH1980 should be placed such that critical signal paths

are short and direct to minimize PCB parasitics from degrad-

ing the video input and logic output signals.

Ground Plane

A two-layer, FR-4 PCB is sufficient for this device. One of the

PCB layers should be dedicated to a single, solid ground

plane that connects to the GND pin of the device and connects

to other components, serving as the common ground refer-

ence. It also helps to reduce trace inductances and minimize

ground loops. Routing supply and signal traces on another

layer can help to maintain as much ground plane continuity

as possible.

Power Supply Routing

The power supply pin should be connected using short traces

with minimal inductance. When routing the supply traces, try

not to disrupt the solid ground plane.

For high frequency bypassing, place 0.1 µF and 0.01 µF SMD

ceramic bypass capacitors with very short connections to

VCC and GND pins. Place a 4.7 or 10 µF SMD tantalum by-

pass capacitor nearby the VCC for low frequency supply by-

passing.

REXT Resistor

The REXT resistor should be a 10 kΩ 1% SMD precision re-

sistor. Place REXT as close as possible to the device and

connect to pin 1 and the ground plane using the shortest pos-

sible connections. All input and output signals should be kept

as far as possible from this pin to prevent unwanted signals

from coupling into this bias reference pin.

Video Input

The input signal path should be routed using short, direct

traces between video source and input pin. Use a 75Ω load

termination on the board, if not on the cable. If applicable, the

chroma filter components should be connected using short

traces and the filter capacitor should be connected to the

ground plane. There should be a sufficient return path from

the LMH1980 back to the input source via the ground plane.

Output Routing

The output signal paths should be routed using short, direct

traces to minimize parasitic effects that may degrade these

high-speed logic signals. The logic outputs do not have high

output drive capability. Each output should have a resistive

load of about 10kΩ or more and capacitive load of about 10pF

including parasitic capacitances for optimal signal quality.

Each output can be protected against brief short-circuit

events using a small series resistor, like 100Ω, to limit output

current.

www.national.com 12

LMH1980

Physical Dimensions inches (millimeters) unless otherwise noted

10-Pin MSOP

NS Package Number MUB10A

13 www.national.com

LMH1980

Notes

LMH1980 Auto-Detecting SD/HD/PC Video Sync Separator

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