AD AD725ARZ Low cost rgb to ntsc/pal encoder with luma trap port Datasheet

a
Low Cost RGB to NTSC/PAL Encoder
with Luma Trap Port
AD725
FEATURES
Composite Video Output: Both NTSC and PAL
Chrominance and Luminance (S-Video) Outputs
Luma Trap Port to Eliminate Cross Color Artifacts
TTL Logic Levels
Integrated Delay Line and Auto-Tuned Filters
Drives 75 V Reverse-Terminated Loads
Low Power +5 V Operation
Power-Down to <1 mA
Very Low Cost
PRODUCT DESCRIPTION
APPLICATIONS
RGB/VGA to NTSC/PAL Encoding
Personal Computers/Network Computers
Video Games
Video Conference Cameras
Digital Still Cameras
The AD725 features a luminance trap (YTRAP) pin that provides a means of reducing cross color generated by subcarrier
frequency components found in the luminance signal. For portable or other power-sensitive applications, the device can be
powered down to less than 1 µA of current consumption. All
logic levels are TTL compatible thus supporting the logic requirements of 3 V CMOS systems.
The AD725 is a very low cost general purpose RGB to NTSC/
PAL encoder that converts red, green and blue color component signals into their corresponding luminance (baseband
amplitude) and chrominance (subcarrier amplitude and phase)
signals in accordance with either NTSC or PAL standards.
These two outputs are also combined on-chip to provide a
composite video output. All three outputs are available separately at voltages of twice the standard signal levels as required for driving 75 Ω, reverse-terminated cables.
The AD725 is packaged in a low cost 16-lead SOIC and operates from a +5 V supply.
FUNCTIONAL BLOCK DIAGRAM
NTSC/PAL
HSYNC
VSYNC
4FSC CLOCK
XNOR
CSYNC
BURST
4FSC
4FSC
RED
CSYNC
SYNC
SEPARATOR
QUADRATURE
+4
DECODER
FSC 908C
FSC 08C
Y
DC
CLAMP
NTSC/PAL
FSC 908C/2708C
61808C
(PAL ONLY)
CSYNC
CLOCK
AT 8FSC
3-POLE
LP PREFILTER
SAMPLEDDATA
DELAY LINE
2-POLE
LP POSTFILTER
X2
LUMINANCE
OUTPUT
LUMINANCE
TRAP
GREEN
BLUE
DC
CLAMP
DC
CLAMP
RGB-TO-YUV
ENCODING
MATRIX
U
4-POLE
LPF
NTSC/PAL
U
CLAMP
BALANCED
MODULATORS
V
4-POLE
LPF
4-POLE
LPF
S
X2
COMPOSITE
OUTPUT
X2
CHROMINANCE
OUTPUT
V
CLAMP
BURST
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1997
(Unless otherwise noted, VS = +5, TA = +258C, using 4FSC synchronous clock. All loads are
AD725–SPECIFICATIONS 150 V 6 5% at the IC pins. Outputs are measured at the 75 V reverse terminated load.)
Parameter
Conditions
SIGNAL INPUTS (RIN, GIN, BIN)
Input Amplitude
Black Level1
Input Resistance2
Input Capacitance
Min
Typ
Full Scale
Max
Units
714
mV p-p
V
MΩ
pF
1
V
V
µA
µA
0.8
RIN, GIN, BIN
1
5
LOGIC INPUTS (HSYNC, VSYNC, 4FSC, CE, STND)
Logic Low Input Voltage
Logic High Input Voltage
Logic Low Input Current (DC)
Logic High Input Current (DC)
TTL Logic Levels
2
1
1
VIDEO OUTPUTS3
Luminance (LUMA)
Bandwidth, –3 dB
NTSC
PAL
Gain Error
Nonlinearity
Sync Level
–7
max p-p
NTSC
PAL
DC Black Level
Luminance Trap (YTRAP)
Output Resistance
DC Black Level
Chrominance (CRMA)
Bandwidth, –3 dB
NTSC
PAL
NTSC
PAL
NTSC
PAL
Color Burst Amplitude
Color Burst Width
Chroma Level Error4
Chroma Phase Error5
DC Black Level
Chroma Feedthrough
Composite (COMP)
Absolute Gain Error
Differential Gain
Differential Phase
DC Black Level
Chroma/Luma Time Alignment
252
264
206
221
R, G, B = 0
With Respect to Luma
With Respect to Chroma
With Respect to Chroma
–5
S-Video
POWER SUPPLIES
Recommended Supply Range
Quiescent Current—Encode Mode
Quiescent Current—Power Down
Single Supply
4.4
5.2
–2
0.3
279
291
1.3
+7
310
325
MHz
MHz
%
%
mV
mV
V
1.0
1.0
kΩ
V
1.2
1.5
255
291
2.51
2.28
–4
±3
2.0
15
MHz
MHz
mV p-p
mV p-p
µs
µs
%
Degrees
V
mV p-p
–1
0.5
1.5
1.4
20
+4.75
30
<1
305
362
40
+3
%
%
Degrees
V
ns
+5.25 V
36
mA
µA
NOTES
1
R, G, and B signals are inputted via an external ac coupling capacitor.
2
Except during dc restore period (back porch clamp).
3
All outputs measured at a 75 Ω reverse-terminated load; ac voltages at the IC output pins are twice those specified here.
4
Difference between ideal and actual color bar subcarrier amplitudes.
5
Difference between ideal and actual color bar subcarrier phases.
Specifications are subject to change without notice.
–2–
REV. 0
AD725
ABSOLUTE MAXIMUM RATINGS*
PIN CONFIGURATION
16-Lead Wide Body (SOIC)
(R-16)
Supply Voltage, APOS to AGND . . . . . . . . . . . . . . . . . . +6 V
Supply Voltage, DPOS to DGND . . . . . . . . . . . . . . . . . . +6 V
AGND to DGND . . . . . . . . . . . . . . . . . . . . . –0.3 V to +0.3 V
Inputs . . . . . . . . . . . . . . . . . . . DGND – 0.3 to DPOS + 0.3 V
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . . . 800 mW
Operating Temperature Range . . . . . . . . . . . . –40°C to +85°C
Storage Temperature Range . . . . . . . . . . . . . –65°C to +125°C
Lead Temperature Range (Soldering 30 sec) . . . . . . . . +230°C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
Thermal Characteristics: 16-Pin SOIC Package: θJA = 100°C/W.
STND 1
16 HSYNC
AGND
2
15 VSYNC
4FSC
3
APOS
4
CE
14 DPOS
AD725
13 DGND
TOP VIEW
5 (Not to Scale) 12 YTRAP
RIN
6
11 LUMA
GIN
7
10 COMP
BIN
8
9 CRMA
ORDERING GUIDE
Model
Temperature
Range
AD725AR
–40°C to +85°C
AD725AR-Reel –40°C to +85°C
AD725AR-Reel7 –40°C to +85°C
AD725-EB
Package
Description
Package
Option
16-Lead SOIC
16-Lead SOIC
16-Lead SOIC
Evaluation Board
R-16
R-16
R-16
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD725 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
REV. 0
–3–
WARNING!
ESD SENSITIVE DEVICE
AD725
PIN DESCRIPTIONS
Pin
Mnemonic
Description
Equivalent Circuit
1
STND
Circuit A
2
3
AGND
4FSC
4
5
APOS
CE
6
RIN
7
GIN
8
BIN
9
CRMA
10
COMP
11
LUMA
12
13
14
15
16
YTRAP
DGND
DPOS
VSYNC
HSYNC
Encoding Standard Pin. A Logic HIGH input selects NTSC encoding.
A Logic LOW input selects PAL encoding.
TTL Logic Levels.
Analog Ground Connection.
4FSC Clock Input.
For NTSC: 14.318 180 MHz.
For PAL: 17.734 475 MHz.
TTL Logic Levels.
Analog Positive Supply (+5 V ± 5%).
Chip Enable. A Logic HIGH input enables the encode function.
A Logic LOW input powers down chip when not in use.
TTL Logic Levels.
Red Component Video Input.
0 mV to 714 mV AC-Coupled.
Green Component Video Input.
0 mV to 714 mV AC-Coupled.
Blue Component Video Input.
0 mV to 714 mV AC-Coupled.
Chrominance Output.*
Approximately 1.8 V peak-to-peak for both NTSC and PAL.
Composite Video Output.*
Approximately 2.5 V peak-to-peak for both NTSC and PAL.
Luminance plus CSYNC Output.*
Approximately 2 V peak-to-peak for both NTSC and PAL.
Luminance Trap Filter Tap. Attach L-C resonant network to reduce cross-color artifacts.
Digital Ground Connection.
Digital Positive Supply (+5 V ± 5%).
Vertical Sync Signal (if using external CSYNC set at > +2 V). TTL Logic Levels.
Horizontal Sync Signal (or CSYNC signal). TTL Logic Levels.
Circuit A
Circuit A
Circuit B
Circuit B
Circuit B
Circuit C
Circuit C
Circuit C
Circuit D
Circuit A
Circuit A
*The Luminance, Chrominance and Composite Outputs are at twice normal levels for driving 75 Ω reverse-terminated lines.
APOS
DPOS
APOS
DPOS
DPOS
DPOS
1kV
12
1
6
9
3
7
10
5
8
11
15
DGND
AGND
DGND
DGND
AGND
DGND
VCLAMP
16
Circuit A
Circuit B
Circuit C
Circuit D
Figure 1. Equivalent Circuits
–4–
REV. 0
Typical Characteristics–AD725
+5V
COMPOSITE
SYNC
TEKTRONIX
TG2000
SIGNAL
GENERATION
PLATFORM
COMPOSITE
VIDEO
AD725
RGB TO
NTSC/PAL
ENCODER
RGB
SONY
MONITOR
MODEL
PVM-1354Q
3
75V
4FSC
GENLOCK
FSC
(3.579545MHz
OR
4.433618MHz)
OSCILLATOR
75V
FSC
HP3314A
3 4 PLL
TEKTRONIX
VM700A
WAVEFORM
MONITOR
Figure 2. Evaluation Setup
1.0
1.0
APL = 50.8%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V @ 6.63ms
APL = 50.6%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V @ 6.72ms
100
0.5
0.5
VOLTS
IRE
VOLTS
50
0.0
0
0.0
–50
–0.5
0
10
20
30
µs
40
50
–0.5
0
60
10
20
30
ms
40
50
60
Figure 3. 100% Color Bars, NTSC
Figure 5. 100% Color Bars, PAL
Figure 4. 100% Color Bars on Vector Scope, NTSC
Figure 6. 100% Color Bars on Vector Scope, PAL
REV. 0
–5–
AD725–Typical Characteristics
1.0
1.0
APL = 46.6%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V
@ 6.63ms
APL = 33.5%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V
@ 6.72 ms
100
0.5
0.5
VOLTS
IRE
VOLTS
50
0.0
0.0
0
–50
–0.5
0
10
20
30
ms
40
50
–0.5
60
Figure 7. Modulated Pulse and Bar, NTSC
200mV
0
10
20
30
ms
40
50
60
Figure 9. Modulated Pulse and Bar, PAL
1ms
200mV
1ms
Figure 10. Zoom on Modulated Pulse, PAL
Figure 8. Zoom on Modulated Pulse, NTSC
–6–
REV. 0
AD725
1.0
1.0
APL = 48.2%
525 LINE NTSC NO FILTERING
SLOW CLAMP TO 0.00V @ 6.63ms
APL = 48.2%
625 LINE PAL NO FILTERING
SLOW CLAMP TO 0.00V @ 6.72ms
100
100
0.5
0.5
0.5MHz 1MHz
2MHz
3MHz
4MHz
50
IRE
VOLTS
IRE
VOLTS
50
1MHz
5MHz
0.0
2MHz
3MHz
4MHz
5MHz
6MHz
0.0
0
0
–50
–50
–0.5
–0.5
0
10
20
30
ms
40
50
60
0
10
30
ms
40
50
60
Figure 14. Multiburst, PAL
Figure 11. Multiburst, NTSC
H TIMING (PAL)
H TIMING MEASUREMENT RS–170A (NTSC)
FIELD = 1 LINE = 22
LINE = 25
9.35ms
5.67ms
9.0
CYCLES
5.57ms
20
2.29ms
4.90ms
4.80ms
273.4mV
36.1 IRE
70ns
85ns
89ns
38.0 IRE
287.7mV
89ns
AVERAGE $ 256
AVERAGE $ 256
Figure 12. Horizontal Timing, NTSC
DG DP (NTSC)
DIFFERENTIAL GAIN (%)
0.00
0.07
0.5
0.4
0.3
0.2
0.1
0.0
–0.1
–0.2
DIFFERENTIAL PHASE (deg)
–0.33
0.00
2.0
MIN = –0.05
–0.05
Figure 15. Horizontal Timing, PAL
Wfm —> MOD 5 STEP
MAX = 0.39
pk–pk/MAX = 0.44
0.20
0.22
0.39
DG DP (PAL)
DIFFERENTIAL GAIN (%)
0.00
–0.06
0.5
0.4
0.3
0.2
0.1
0.0
–0.1
–0.2
DIFFERENTIAL PHASE (deg)
–0.44
0.00
2.0
MIN = –0.33
MAX = 1.17
pk–pk = 1.50
0.10
0.70
1.05
1.17
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
–0.5
–0.5
–1.0
Wfm —> MOD 5 STEP
MIN = –0.06
MAX = 0.43
pk–pk = 0.49
0.15
0.23
0.43
0.38
MIN = –0.44
MAX = 1.34
–0.02
0.70
1.17
pk–pk = 1.79
1.34
–1.0
1ST
2ND
3RD
4TH
5TH
6TH
1ST
Figure 13. Composite Output Differential Phase
and Gain, NTSC
REV. 0
2ND
3RD
4TH
5TH
6TH
Figure 16. Composite Output Differential Phase
and Gain, PAL
–7–
AD725
Following the dc clamps, the RGB inputs are buffered and split
into two signal paths for constructing the luminance and
chrominance outputs.
THEORY OF OPERATION
The AD725 is a predominantly analog design, with digital logic
control of timing. This timing logic is driven by a external frequency reference at four times the color subcarrier frequency,
input into the 4FSC pin of the AD725. This frequency should
be 14.318 180 MHz for NTSC encoding, and 17.734 475 MHz
for PAL encoding. The 4FSC input accepts standard TTL logic
levels. The duty cycle of this input clock is not critical, but a fastedged clock should be used to prevent excessive jitter in the timing.
Luminance Signal Path
The luminance path begins with the luma (Y) matrix. This
matrix combines the RGB inputs to form the brightness information in the output video. The inputs are combined by the
standard transformation
Y = 0.299 × R + 0.587 × G + 0.114 × B
The AD725 accepts two common sync standards, composite
sync or separate horizontal and vertical syncs. To use an external composite sync, a logic high signal is input to the VSYNC
pin and the composite sync is input to the HSYNC pin. If separate horizontal and vertical syncs are available, the horizontal
sync can be input to the HSYNC pin and vertical sync to the
VSYNC pin. Internally, the device XNORs the two sync inputs
to combine them into one negative-going composite sync.
This equation describes the sensitivity of the human eye to the
individual component colors, combining them into one value of
brightness. The equation is balanced so that full-scale RGB
inputs give a full-scale Y output.
Following the luma matrix, the composite sync is added. The
user-supplied sync (from the HSYNC and VSYNC inputs) is
latched into the AD725 at half the master clock rate, gating a
sync pulse into the luminance signal. With the exception of
transitioning on the clock edges, the output sync timing will be
in the same format as the input sync timing. The output sync
level will depend on the encoding standard, 286 mV (40 IRE)
for NTSC and 300 mV for PAL (voltages at the pin will be
twice these levels).
The AD725 detects the falling sync pulse edges, and times their
width. A sync pulse of standard horizontal width will cause the
insertion of a colorburst vector into the chroma modulators at
the proper time. A sync pulse outside the detection range will
cause suppression of the color burst, and the device will enter its
vertical blanking mode. During this mode, the on-chip RC time
constants are verified using the input frequency reference, and
the filter cutoff frequencies are retuned as needed.
In order to be time-aligned with the filtered chrominance signal
path, the luma signal must be delayed before it is output. The
AD725 uses a sampled delay line to achieve this delay.
The component color inputs, RIN, GIN and BIN, receive analog signals specifying the desired active video output. The fullscale range of the inputs is 0.714 mV (for either NTSC or PAL
operation). External black level is not important as these inputs
are terminated externally, and then ac coupled to the AD725.
Following the luma matrix and prior to this delay line, a prefilter
removes higher frequencies from the luma signal to prevent aliasing
by the sampled delay line. This three-pole Bessel low-pass filter has
a –3 dB frequency of 4.85 MHz for NTSC, 6 MHz for PAL.
The AD725 contains on-chip RGB input clamps to restore the
dc level on-chip to match its single supply signal path. This dc
restore timing is coincident with the burst flag, starting approximately 5.5 µs after the falling sync edge and lasting for 2.5 µs.
During this time, the device should be driven with a black input.
After the luma prefilter, the bandlimited luma signal is sampled
onto a set of capacitors at twice the master reference clock rate.
After an appropriate delay, the data is read off the delay line,
reconstructing the luma signal. The 8FSC oversampling of this
delay line limits the amount of jitter in the reconstructed sync
output. The clocks driving the delay line are reset once per
video line during the burst flag. The output of the luma path
will remain unchanged during this period and will not respond
to changing RGB inputs.
NTSC/PAL
HSYNC
VSYNC
4FSC CLOCK
POWER AND GROUNDS
XNOR
BURST
4FSC
4FSC
RED
CSYNC
CSYNC
SYNC
SEPARATOR
QUADRATURE
+4
DECODER
FSC 908C
FSC 08C
Y
DC
CLAMP
NTSC/PAL
61808C
(PAL ONLY)
FSC 908C/2708C
CSYNC
CLOCK
AT 8FSC
3-POLE
LP PREFILTER
SAMPLEDDATA
DELAY LINE
+5V
+5V
AGND
DGND
LOGIC
ANALOG
ANALOG
LOGIC
NOTE:
THE LUMINANCE, COMPOSITE AND CHROMINANCE
OUTPUTS ARE AT TWICE NORMAL LEVELS FOR
DRIVING 75V REVERSE-TERMINATED LINES.
2-POLE
LP POSTFILTER
X2
LUMINANCE
OUTPUT
LUMINANCE
TRAP
GREEN
BLUE
DC
CLAMP
DC
CLAMP
RGB-TO-YUV
ENCODING
MATRIX
U
4-POLE
LPF
NTSC/PAL
U
CLAMP
BALANCED
MODULATORS
V
4-POLE
LPF
4-POLE
LPF
S
X2
COMPOSITE
OUTPUT
X2
CHROMINANCE
OUTPUT
V
CLAMP
BURST
Figure 17. Functional Block Diagram
–8–
REV. 0
AD725
The reconstructed luma signal is then smoothed with a two pole
Bessel low-pass filter. This filter has a –3 dB bandwidth of
5.25 MHz for NTSC, 6.5 MHz for PAL. A final buffer provides current drive for the LUMA output pin.
Chrominance Signal Path
The chrominance path begins with the U and V color-difference
matrices. The AD725 uses U and V modulation vectors for
NTSC and PAL (+U being defined as 0 degrees phase), simplifying the design compared to I and Q designs. The U and V matrices combine the RGB inputs by the standard transformations:
U = 0.493 × (B – Y)
V = 0.877 × (R – Y)
The Y signal in these transformations is provided by the luminance matrix.
Before modulation, the U and V signals are prefiltered to prevent aliasing. These four-pole modified Bessel low-pass filters
have a –3 dB bandwidth of 1.2 MHz for NTSC and 1.5 MHz
for PAL.
Between the prefilters and the modulators, the colorburst vectors are added to the U and V signals. The colorburst levels are
defined according to the encoding standard. For NTSC, the
colorburst is in the –U direction (with no V component) with a
resultant amplitude of 286 mV (40 IRE) at 180 degrees phase.
For PAL, the colorburst has equal parts of –U and ± V vectors
(changing V phase every line) for a resultant amplitude of
300 mV alternating between 135 and 225 degrees phase (voltages at the pin will be twice these levels).
The burst gate timing is generated by waiting for a certain number of reference clock cycles following the falling sync edge. If
the sync pulse width is measured to be outside the standard
horizontal width, it is assumed that the device is in an h/2 period
(vertical blanking interval) and the burst is suppressed.
The U and V signals are used to modulate a pair of quadrature
clocks (sine and cosine) at one-fourth the reference frequency
input (3.579 545 MHz for NTSC, 4.433618 MHz for PAL).
For PAL operation, the phase of the cosine (V) clock is changed
after each falling sync edge is detected. This will change the
V-vector phase in PAL mode every horizontal line. By driving
the AD725 with an odd number of sync edges per field, any
individual line will flip phase each field as required by the standard.
In order to suppress the carriers in the chrominance signal, the
U and V modulators are balanced. Once per horizontal line the
offsets in the modulators are cancelled in order to minimize
residual subcarrier when the RGB inputs are equal. This offset
cancellation also provides a dc restore for the U and V signal
paths, so it is important that the RGB inputs be held at black
REV. 0
level during this time. The offset cancellation occurs after each
falling sync edge, approximately 350 ns after the falling sync
edge, lasting for a period of 140 ns. If the inputs are unbalanced
during this time (for example, if a sync-on-green RGB input
were used), there will be an offset in this chrominance response
of the inputs during the remainder of the horizontal line, including the colorburst.
The U signal is sampled by the sine clock and the V signal is
sampled by the cosine clock in the modulators, after which they
are summed to form the chrominance (C) signal.
The chrominance signal then passes through a final four-pole
modified Bessel low-pass filter to remove the harmonics of the
switching modulation. This filter has a –3 dB frequency of
4.4 MHz for NTSC and 5.9 MHz for PAL. A final buffer provides current drive for the CRMA output pin.
Composite Output
To provide a composite video output, the separate (S-Video)
luminance and chrominance signal paths are summed. Prior to
summing, however, a filter tap for removing cross-color artifacts
in the receiver is provided.
The luminance path contains a resistor, output pin (YTRAP),
and buffer prior to entering the composite summer. By connecting
an inductor and capacitor on this pin, an R-L-C series-resonant
circuit can be tuned to null out the luminance frequency
response at the chrominance subcarrier frequency (3.579 545 MHz
for NTSC, 4.433 618 MHz for PAL). The center frequency (fC)
of this filter will be determined by the external inductor and
capacitor by the equation:
fC =
1
2 π LC
It can be seen from this equation that the center frequency of
the trap is entirely dependent on external components.
The ratio of center frequency to bandwidth of the notch (Q =
fC /BW) can be described by the equation:
Q=
1
1000
L
C
When choosing the Q of the filter, it should be kept in mind that
the sharper the notch, the more critical the tolerance of the
components must be in order to target the subcarrier frequency.
Additionally, higher Q notches will exhibit a transient response
with more ringing after a luminance step. The magnitude of this
ringing can be large enough to cause visible shadowing for Q
values much greater than 1.5.
–9–
AD725
HSYNC/VSYNC
(USER INPUTS)
tSW
RIN/GIN BIN
(USER INPUTS)
tSB
tSM
MODULATOR
RESTORE
tMW
INPUT
CLAMPS
tSR
tRW
BURST FLAG/
DELAY LINE RESET
tSD
tDW
LUMA
tSS
tBY
tBC
tSC
CRMA
Figure 18. Timing Diagram (Not to Scale)
Table I. Timing Description (See Figure 18)
NTSC1
Symbol
Name
Description
tSW
Sync Width
tSB
Sync to Blanking
End
Sync to Modulator
Restore
Modulator Restore
Width
Sync to RGB DC
Restore
DC Restore Width
Input valid sync width for burst
insertion (user-controlled).
Minimum sync to color delay
(user-controlled).
Delay to modulator clamp start.
tSM
tMW
tSR
tRW
tSD
tDW
tSS
tBY
tSC
tBC
Sync to Delay Line
Reset
Delay Line Reset
Width
Sync Input to Luma
Sync Output
Blanking End to
LUMA Start
Sync to Colorburst
Blanking End to
CRMA Start
Min
Max
2.8 µs
5.3 µs
Min
Max
3.3 µs
5.4 µs
Min
8.2 µs
Min
8.1 µs
Length of modulator offset clamp
(no chroma during this period).
Delay to input clamping start.
Length of input clamp (no RGB
response during this period).
Delay to start of delay line
clock reset.
Length of delay line clock reset
(no luma response during this
period), also burst gate.
Delay from sync input assertion
to sync in LUMA output.
Delay from RGB input assertion
to LUMA output response.
Delay from valid horizontal sync
start to CRMA colorburst output.
Delay from RGB input assertion
to CRMA output response.
PAL2
392 ns
298 ns
140 ns
113 ns
5.4 µs
5.6 µs
2.5 µs
2.3 µs
5.7 µs
5.8 µs
2.5 µs
2.3 µs
typ
310 ns
typ
265 ns
typ
340 ns
typ
280 ns
typ
5.8 µs
typ
5.9 µs
typ
360 ns
typ
300 ns
NOTES
1
Input clock = 14.318180 MHz, STND pin = logic high.
2
Input cock = 17.734475 MHz, STND pin = logic low.
–10–
REV. 0
AD725
APPLYING THE AD725
Inputs
The AD725 will operate with subcarrier frequencies that deviate
quite far from those specified by the TV standards. However,
the monitor will in general not be quite so forgiving. Most monitors can tolerate a subcarrier frequency that deviates several hundred Hz from the nominal standard without any degradation in
picture quality. These conditions imply that the subcarrier frequency accuracy is a system specification and not a specification
of the AD725 itself.
RIN, BIN, GIN are analog inputs that should be terminated to
ground with 75 Ω in close proximity to the IC. When properly
terminated the peak-to-peak voltage for a maximum input level
should be 714 mV p-p. The horizontal blanking interval should
be the most negative part of each signal.
The inputs should be held at the input signal’s black level during the horizontal blanking interval. The internal dc clamps will
clamp this level during color burst to a reference that is used
internally as the black level. Any noise present on the RIN,
GIN, BIN or AGND pins during this interval will be sampled
onto the input capacitors. This can result in varying dc levels
from line to line in all outputs, or if imbalanced, subcarrier
feedthrough in the COMP and CRMA outputs.
The STND pin is used to select between NTSC and PAL operation. Various blocks inside the AD725 use this input to program
their operation. Most of the more common variants of NTSC and
PAL are supported. There are, however, two known specific standards which are not supported by the standard AD725. These are
NTSC 4.43 and M-PAL.
For increased noise rejection, larger input capacitors are desired.
A capacitor of 0.1 µF is usually adequate.
Similarly, the U and V clamps balance the modulators during an
interval shortly after the falling CSYNC input. Noise present
during this interval will be sampled in the modulators, resulting
in residual subcarrier in the COMP and CRMA outputs.
HSYNC and VSYNC are two logic level inputs that are combined internally to produce a composite sync signal. If a composite sync signal is to be used, it can be input to HSYNC while
VSYNC is pulled to logic HI (> +2 V).
The form of the input sync signal(s) will determine the form of
the composite sync on the composite video (COMP) and luminance (LUMA) outputs. If no equalization or serration pulses
are included in the HSYNC input there won’t be any in the
outputs. Although sync signals without equalization and serration pulses do not technically meet the video standards’ specifications, many monitors do not require these pulses in order to
display good pictures. The decision whether to include these
signals is a system trade-off between cost and complexity and
adhering strictly to the video standards.
The HSYNC and VSYNC logic inputs have a small amount of
built-in hysteresis to avoid interpreting noisy input edges as
multiple sync edges. This is critical to proper device operation, as
the sync pulses are timed for vertical blanking interval detection.
The logic inputs have been designed for VIL < 1.0 V and VIH
> 2.0 V for the entire temperature and supply range of operation. This allows the AD725 to directly interface to TTL or 3 V
CMOS compatible outputs, as well as 5 V CMOS outputs
where VOL is less than 1.0 V.
The NTSC specification calls for a frequency accuracy of ±10 Hz
from the nominal subcarrier frequency of 3.579545 MHz. While
maintaining this accuracy in a broadcast studio might not be a
severe hardship, it can be quite expensive in a low cost consumer application.
REV. 0
Basically these two standards use most of the features of the
standard that their names imply, but use the subcarrier that is
equal to or approximately equal to the frequency of the other
standard. Because of the automatic programming of the filters in
the chrominance path and other timing considerations, a factoryprogrammed special version of the AD725 is necessary to support these standards.
Layout Considerations
The AD725 is an all CMOS mixed signal part. It has separate
pins for the analog and digital +5 V and ground power supplies.
Both the analog and digital ground pins should be tied to the
ground plane by a short, low inductance path. Each power
supply pin should be bypassed to ground by a low inductance
0.1 µF capacitor and a larger tantalum capacitor of about 10 µF.
The three analog inputs (RIN, GIN, BIN) should be terminated
with 75 Ω to ground close to the respective pins. However, as
these are high impedance inputs, they can be in a loop-through
configuration. This technique is used to drive two or more
devices with high frequency signals that are separated by some
distance. A connection is made to the AD725 with no local
termination, and the signals are run to another distant device
where the termination for these signals is provided.
The output amplitudes of the AD725 are double that required
by the devices that it drives. This compensates for the halving of
the signal levels by the required terminations. A 75 Ω series
resistor is required close to each AD725 output, while 75 Ω to
ground should terminate the far end of each line.
The outputs have a dc bias and must be ac coupled for proper
operation. The COMP and LUMA outputs have information
down to 30 Hz for NTSC (25 MHz for PAL) that must be transmitted. Each output requires a 220 µF series capacitor to work
with the 75 Ω resistance to pass these low frequencies. The CRMA
signal has information mostly up at the chroma frequency and
can use a smaller capacitor if desired, but 220 µF can be used to
minimize the number of different components used in the design.
–11–
AD725
system, the internal 4FSC (14.318 180 MHz) clock that drives
the VGA controller can be used for 4FSC on the AD725. This
signal is not directly accessible from outside the computer, but it
does appear on the VGA card. (A 1FSC-input encoder, the
AD724, is also available.)
Displaying VGA Output on a TV
The AD725 can be used to convert the analog RGB output from a
personal computer’s VGA card to the NTSC or PAL television
standards. To accomplish this it is important to understand that
the AD725 requires interlaced RGB video and clock rates that
are consistent with those required by the television standards.
In most computers the default output is a noninterlaced RGB
signal at a frame rate higher than used by either NTSC or PAL.
If a separate RGB monitor is also to be used, it is not possible to
simply connect it to the R, G and B signals. The monitor provides a termination that would double terminate these signals.
The R, G, and B signals should be buffered by three amplifiers
with high input impedances. These should be configured for a
gain of two, which is normalized by the divide by two termination scheme used for the RGB monitor.
Most VGA controllers support a wide variety of output modes
that are controlled by altering the contents of internal registers.
It is best to consult with the VGA controller manufacturer to
determine the exact configuration required to provide an interlaced output at 60 Hz (50 Hz for PAL).
The AD8073 is a low cost triple video amplifier that can provide the buffering required in this application. However, since
the R, G and B signals go all the way to ground during horizontal sync, the AD8073 will require a –5 V supply to handle these
signals. To be able to buffer the R, G and B signals using a
single supply, a rail-to-rail amplifier is required. In this application, the AD8051 (single) and AD8052 (dual) can be used to
provide the three required channels. These can be operated on a
single supply of 3 V to 5 V.
Figure 19 shows a circuit for connection to the VGA port of a
PC. The RGB outputs are ac coupled to the respective inputs of
the AD725. These signals should each be terminated to ground
with 75 Ω.
The standard 15-pin VGA connector has HSYNC on Pin 13
and VSYNC on Pin 14. These signals also connect directly to
the same name signals on the AD725. For a synchronous NTSC
+5V
10mF
0.1mF
POWER DOWN
0.1mF
5
CE
6
RIN
7
GIN
0.1mF
0.1mF
+5V
4
14
APOS
DPOS
0.1mF
10mF
75V
COMPOSITE
VIDEO
CMPS 10
220mF
AD725
8
BIN
16
HSYNC
15
VSYNC
75V
75V
75V
75V
+5V
4FSC CLOCK
14.318180MHz (NTSC)
OR
17.734475MHz (PAL)
220mF
OSC
4FSC
C
CRMA 9
220mF
9pF
STND
68mH
S-VIDEO
(Y/C VIDEO)
YTRAP
NTSC/PAL
+5V (VAA)
18pF
1N4148
AGND
DGND
2
13
0.1mF
75V
1/3
AD8073
1kV
75V
0.1mF
47kV
VGA OUTPUT
CONNECTOR
Y
LUMA 11
1kV
VSYNC
75V
1/3
AD8073
HSYNC
FROM VGA PORT
B
75V
1kV
1kV
G
75V
75V
1/3
AD8073
R
75V
0.1mF
–5V
1kV
1kV
RGB MONITOR
Figure 19. Interfacing the AD725 to the (Interlaced) VGA Port of a PC
–12–
REV. 0
AD725
Low Cost Crystal Oscillator
A low cost oscillator can be made that provides a CW clock that
can be used to drive both the AD725 4FSC and other devices in
the system that require a clock at this frequency. Figure 20 shows a
circuit that uses one inverter of a 74HC04 package to create a
crystal oscillator and another inverter to buffer the oscillator
and drive other loads. The logic family must be a CMOS type
that can support the frequency of operation, and it must NOT
be a Schmitt trigger type of inverter. Resistor R1 from input to
output of U1A linearizes the inverter’s gain such that it provides
useful gain and a 180 degree phase shift to drive the oscillator.
R1
1MV
U1A
U1B
HC04
Y1
C3
~15pF
(OPT)
C1
47pF
TO PIN 3
OF AD725
HC04
R2
200V
TO OTHER
DEVICE CLOCKS
C2
60pF
Figure 20. Low Cost Crystal Oscillator
The crystal should be a parallel resonant type at the appropriate
frequency (NTSC/PAL, 4FSC). The series combination of C1
and C2 should approximately equal to the crystal manufacturer’s
specification for the parallel capacitance required for the crystal
to operate at its specified frequency. C1 will usually want to be
a somewhat smaller value because of the input parasitic capacitance of the inverter. If it is desired to tune the frequency to
greater accuracy, C1 can be made still smaller and a parallel
adjustable capacitor can be used to adjust the frequency to the
desired accuracy.
Resistor R2 serves to provide the additional phase shift
required by the circuit to sustain oscillation. It can be sized by
R2 = 1/(2 × π × f × C2). Other functions of R2 are to provide a
low pass filter that suppresses oscillations at harmonics of the
fundamental of the crystal and to isolate the output of the inverter from the resonant load that the crystal network presents.
The basic oscillator described above is buffered by U1B to drive
the AD725 4FSC pin and other devices in the system. For a
system that requires both an NTSC and PAL oscillator, the
circuit can be duplicated by using a different pair of inverters
from the same package.
Dot Crawl
There are numerous distortions that are apparent in the presentation of composite signals on TV monitors. These effects will
vary in degree depending on the circuitry used by the monitor
to process the signal and on the nature of the image being displayed. It is generally not possible to produce pictures on a
composite monitor that are as high quality as those produced by
standard quality RGB, VGA monitors.
One well known distortion of composite video images is called
dot crawl. It shows up as a moving dot pattern at the interface
between two areas of different color. It is caused by the inability
of the monitor circuitry to adequately separate the luminance
and chrominance signals.
One way to prevent dot crawl is to use a video signal that has
separate luminance and chrominance. Such a signal is referred
REV. 0
to as S-video or Y/C video. Since the luminance and chrominance are already separated, the monitor does not have to perform this function. The S-video outputs of the AD725 can be
used to create higher quality pictures when there is an S-video
input available on the monitor.
Flicker
In a VGA conversion application, where the software controlled
registers are correctly set, there are two techniques that are
commonly used by VGA controller manufacturers to generate
the interlaced signal. Each of these techniques introduces a
unique characteristic into the display created by the AD725.
The artifacts described below are not due to the encoder or its
encoding algorithm as all encoders will generate the same display when presented with these inputs. They are due to the
method used by the controller display chip to convert a noninterlaced output to an interlaced signal.
The first interlacing technique outputs a true interlaced signal
with odd and even fields (one each to a frame Figure 21a). This
provides the best picture quality when displaying photography,
CD video and animation (games, etc.). However, it will introduce a defect commonly referred to as flicker into the display.
Flicker is a fundamental defect of all interlaced displays and is
caused by the alternating field characteristic of the interlace
technique. Consider a one pixel high black line which extends
horizontally across a white screen. This line will exist in only
one field and will be refreshed at a rate of 30 Hz (25 Hz for
PAL). During the time that the other field is being displayed the
line will not be displayed. The human eye is capable of detecting this, and the display will be perceived to have a pulsating or
flickering black line. This effect is highly content sensitive and
is most pronounced in applications in which text and thin
horizontal lines are present. In applications such as CD video,
photography and animation, portions of objects naturally
occur in both odd and even fields and the effect of flicker is
imperceptible.
The second commonly used technique is to output an odd and
even field that are identical (Figure 21b). This ignores the data
that naturally occurs in one of the fields. In this case the same
one pixel high line mentioned above would either appear as a
two pixel high line, (one pixel high in both the odd and even field)
or not appear at all if it is in the data that is ignored by the controller. Which of these cases occurs is dependent on the placement
of the line on the screen. This technique provides a stable (i.e.,
nonflickering) display for all applications, but small text can be
difficult to read and lines in drawings (or spreadsheets) can
disappear. As above, graphics and animation are not particularly
affected although some resolution is lost.
There are methods to dramatically reduce the effect of flicker and
maintain high resolution. The most common is to ensure that
display data never exists solely in a single line. This can be accomplished by averaging/weighting the contents of successive/multiple
noninterlaced lines prior to creating a true interlaced output (Figure 21c). In a sense, this provides an output that will lie between
the two extremes described above. The weight or percentage of
one line that appears in another, and the number of lines used,
are variables that must be considered in developing a system of
this type. If this type of signal processing is performed, it must
be completed prior to the data being presented to the AD725
for encoding.
–13–
AD725
Vertical Scaling
NONINTERLACED
In addition to converting the computer generated image from
noninterlaced to interlaced format, it is also necessary to scale
the image down to fit into NTSC or PAL format. The most
common vertical lines/screen for VGA display are 480 and 600
lines. NTSC can only accommodate approximately 400 visible
lines/frame (200 per field), PAL can accommodate 576 lines/
frame (288 per field). If scaling is not performed, portions of
the original image will not appear in the television display.
1
2
3
4
5
6
7
ODD FIELD
EVEN FIELD
1
2
3
=
+
4
5
6
7
a. Conversion of Noninterlace to Interlace
NONINTERLACED
This line reduction can be performed by merely eliminating
every Nth (6th line in converting 480 lines to NSTC or every
25th line in converting 600 lines to PAL). This risks generation
of jagged edges and jerky movement. It is best to combine the
scaling with the interpolation/averaging technique discussed
above to ensure that valuable data is not arbitrarily discarded in
the scaling process. Like the flicker reduction technique mentioned above, the line reduction must be accomplished prior to
the AD725 encoding operation.
1
2
3
4
5
6
7
ODD FIELD
EVEN FIELD
1
2
3
=
+
4
5
6
7
b. Line Doubled Conversion Technique
NONINTERLACED
There is a new generation of VGA controllers on the market
specifically designed to utilize these techniques to provide a
crisp and stable display for both text and graphics oriented
applications. In addition these chips rescale the output from the
computer to fit correctly on the screen of a television. A list of
known devices is available through Analog Devices’ Applications group, but the most complete and current information will
be available from the manufacturers of graphics controller ICs.
1
2
3
4
5
6
7
ODD FIELD
EVEN FIELD
1
2
3
=
+
4
5
6
7
c. Line Averaging Technique
Figure 21.
Synchronous vs. Asynchronous Operation
The source of RGB video and synchronization used as an input
to the AD725 in some systems is derived from the same clock
signal as used for the AD725 subcarrier input (4FSC). These
systems are said to be operating synchronously. In systems
where two different clock sources are used for these signals, the
operation is called asynchronous.
The AD725 supports both synchronous and asynchronous
operation, but some minor differences might be noticed between them. These can be caused by some details of the internal circuitry of the AD725.
There is an attempt to process all of the video and synchronization signals totally asynchronous with respect to the subcarrier
signal. This was achieved everywhere except for the sampled
delay line used in the luminance channel to time align the luminance and chrominance. This delay line uses a signal at eight
times the subcarrier frequency as its clock.
The phasing between the delay line clock and the luminance
signal (with inserted composite sync) will be constant during
synchronous operation, while the phasing will demonstrate a
periodic variation during asynchronous operation. The jitter of
the asynchronous video output will be slightly greater due to
these periodic phase variations.
LUMA TRAP-THEORY
The composite video output of the AD725 can be improved for
some types of images by incorporating a luma trap (or Y-Trap)
in the encoder circuit. The basic configuration for such a circuit
is a notch or band elimination filter that is centered at the
subcarrier frequency. The luma trap is only functional for the
composite video output of the AD725; it has no influence on
the S-Video (or Y/C-Video) output.
The need for a luma trap arises from the method used by composite video to encode the color part (chrominance or chroma)
of the video signal. This is performed by amplitude and phase
modulation of a subcarrier. The saturation (or lack of dilution of
a color with white) is represented in the subcarrier’s amplitude
modulation, while the hue (or color as thought of as the sections
of a rainbow) information is contained in the subcarrier’s phase
modulation. The modulated subcarrier occupies a bandwidth
somewhat greater than 1 MHz depending on the video standard.
For a composite signal, the chroma is linearly added to the
luminance (luma or brightness) plus sync signal to form a single
composite signal with all of the picture information. Once this
addition is performed, it is no longer possible to ascertain which
component contributed which part of the composite signal.
At the receiver, this single composite signal must be separated
into its various parts to be properly processed. In particular, the
chroma must be separated and then demodulated into its orthogonal components, U and V. Then, along with the luma
signal, the U and V signals generate the RGB signals that control the three video guns in the monitor.
A basic problem arises when the luma signal (which contains no
color information) contains frequency components that fall
–14–
REV. 0
AD725
within the chroma band. All signals in this band are processed
as chroma information since the chroma processing circuit has
no knowledge as to where these signals originated. Therefore,
the color that results from the luma signals in the chroma band
is a false color. This effect is referred to as cross chrominance.
quality. S-video will not just eliminate cross chrominance, but
will also not have this notch in the luma response.
Implementing a Luma Trap
The AD725 implementation of a luma trap uses an on-chip
resistor along with an off-chip inductor and capacitor to create
an RLC notch filter. The filter must be tuned to the center
frequency of the video standard being output by the AD725,
3.58 MHz for NTSC or 4.43 MHz for PAL.
The cross chrominance effect is sometimes evident in white text
on a black background as a moving rainbow pattern around the
characters. The sharp transitions from black to white (and vice
versa) that comprise the text dots contain frequency components across the whole video band, and those in the chroma
band create cross chrominance. This is especially pronounced
when the dot clock used to generate the characters is an integer
multiple of the chroma subcarrier frequency.
The circuit is shown in Figure 22. The 1 kΩ series resistor in
the composite video luma path on the AD725 works against the
impedance of the off-chip series LC to form a notch filter. The
frequency of the filter is given by:
f =
Another common contributor to cross chrominance effects is
certain striped clothing patterns that are televised. At a specific
amount of zoom, the spatial frequency of vertical stripe patterns
will generate luma frequencies in the chroma band. These frequency components will ultimately get turned into color by the
video monitor. Since the phase of these signals is not coherent
with the subcarrier, the effect shows up as random colors. If the
zoom of a TV camera is modified or there is motion of the
striped pattern, the false colors can vary quite radically and
produce a quite objectionable “moving rainbow” effect. Most
TV-savvy people have learned to adapt by just not wearing
certain patterns when appearing on TV.
14.318180MHz
B
A/B
220mF 75V
COMP
220mF 75V
LUMA
220mF 75V
1.0kV
LUMA
STND
47kV
YTRAP
C2
9pF
L
68mH
NTSC/PAL
D1
1N4148
C1
18pF
Figure 22. Luma Trap Circuit for NTSC and PAL Video
To ensure compatibility with the input capabilities of the majority of TVs in existence, composite video must be supplied.
Many more TVs have a composite baseband video input port
than have an S-video port to connect cameras and VCRs.
However, still the only common denominator for virtually all
TVs is an RF input. This requires modulating the baseband
video onto an RF carrier that is usually tuned to either Channel
3 or 4 (for NTSC). Most video games that can afford only a
single output use an RF interface because of its universality.
Sound can also be carried on this channel.
Since it is not practical to rely exclusively on S-video to improve
the picture quality by eliminating cross chrominance, a luma
trap can be used to minimize this effect for systems that use
composite video. The luma trap notches out or “traps” the
offending frequencies from the luma signal before it is added to
the chroma. The cross chrominance that would be generated by
these frequencies is thereby significantly attenuated.
REV. 0
AD725
CRMA
4FSC
17.734475MHz
An excellent way to eliminate virtually all cross chrominance
effects is to use S-video. Since the luma and chroma are carried
on two separate circuits, there is no confusion as to which circuit should process which signals. Unfortunately, not all TVs
that exist today, and probably still not even half of those being
sold, have a provision for S-video input.
The only sacrifice that results is that the luma response has a
“hole” in it at the chroma frequency. This will lower the luminance resolution of details whose spatial frequency causes
frequency components in the chroma band. However, the
attenuation of cross chrominance outweighs this in the picture
A
1
2π LC
Dual-Standard Luma Trap
For a filter that will work for both PAL and NTSC a means is
required to switch the tuning of the filter between the two
subcarrier frequencies. The PAL standard requires a higher
frequency than NTSC. A basic filter can be made that is tuned
to the PAL subcarrier and a simple diode circuit can then be
used to switch in an extra parallel capacitor that will lower the
filter’s frequency for NTSC operation.
Figure 22 shows how the logic signal that drives STND (Pin 1)
can also be used to drive the circuit that selects the tuning of the
luma trap circuit. When the signal applied to STND (Pin 1) is
low (ground), the PAL mode is selected. This results in a bias of
0 V across D1, which is an off condition. As a result, C2 is out
of the filter circuit and only C1 tunes the notch filter to the PAL
subcarrier frequency, 4.43 MHz.
On the other hand, when STND is high (+5 V), NTSC is selected and there is a forward bias across D1. This turns the
diode on and adds C2 in parallel with C1. The notch filter is
now tuned to the NTSC subcarrier frequency, 3.58 MHz.
–15–
AD725
Measuring the Luma Trap Frequency Response
The frequency response of the luma trap can be measured in
two different ways. The first involves using an RGB frequency
sweep input pattern into the AD725 and observing the composite output on a TV monitor, a TV waveform monitor or on an
oscilloscope.
On a TV monitor, the composite video display will look like
vertical black and white lines that are coarsely spaced (low frequency) on the left side and progress to tightly spaced (high
frequency) on the right side. Somewhere to the right of center,
there will not be discernible stripes, but rather only a gray vertical area. This is the effect of the luma trap, which filters out
luminance detail at a band of frequencies.
At the bottom of the display are markings at each megahertz
that establish a scale of frequency vs. horizontal position. The
location of the center of the gray area along the frequency
marker scale indicates the range of frequencies that are being
filtered out. The gray area should be about halfway between the
3 MHz and 4 MHz markers for NTSC, and about halfway
between the 4 MHz and 5 MHz markers for PAL.
tarily apply an HSYNC signal to reset the timing and perform
the dc restore. Because the inputs are high-impedance, the
droop during testing will be minimal. It is not desirable to apply
a steady pulse train of HSYNC inputs because the spectrum of
these pulses will show up in the output response.
A more stable, low noise method is shown in Figure 23. The
RGB inputs are biased using a power supply and the source port
bias input of the network analyzer. A momentary sync input is
still applied to the device to reset its internal timing, but droop
during testing will no longer be an issue.
The signal source is applied to the GIN input for largest output
response. This input should be terminated through the appropriate termination resistor (matching the output impedance of
the network analyzer). If necessary, calibration inaccuracies can
be flattened out by reading back the input reference using a
FET probe.
NETWORK ANALYZER
REF
When a horizontal line is viewed on an oscilloscope or video
waveform monitor, the notch in the response will be apparent.
The frequency will have to be interpolated from the location of
the notch position along the H-line.
FET
PROBE
SOURCE
SOURCE
BIAS
MEASURE
1V
OUT
15V
15V
IN
1
0.1mF
10mF
1.0
4
7
14
APOS DPOS
GIN
75V
100
COMP
0.5
VOLTS
50
1
10mF
0.1mF
6
RIN
8
BIN
15
VSYNC
220mF 75V
1
AD725
IRE
1V
0.0
15V
CRMA 9 NC
15V
0
MOMENTARY
LUMA 11 NC
10kV
CSYNC
16
HSYNC
ENCD 5
–50
–0.5
0
10
20
30
ms
40
50
STND
60
47kV
9pF
The second method involves using a network analyzer to measure the frequency response of the composite signal. In order to
perform this successfully, the AD725 must be given the appropriate signals so that it will pass video signals through it. Figure
24 illustrates the setup used for these measurements.
The second requirement is that the RGB inputs are properly
biased for linear operation, and the timing logic is properly
reset. It is acceptable to ac-couple the RGB inputs and momen-
YTRAP
NTSC/PAL
Figure 23. Luminance Sweep with Trap, COMP Pin
The first requirement is that the part must receive a subcarrier
clock. This will provide clocking to the internal delay line and
enable it to pass the video signal. The subcarrier clock should be
at the 4FSC frequency for either NTSC or PAL.
15V
4FSC
OSC
68mH
AGND
DGND
2
13
NC = NO CONNECT
1N4148
18pF
Figure 24. Measurement Setup for Determining Luma
Trap Frequency
The composite output is reverse terminated with a 50 Ω or 75 Ω
resistor and input to the measuring channel of a network analyzer.
Since only the green input is driven, this method does not yield
an absolute measurement of composite signal levels, but the
notch in the composite output will be readily discernible. The
frequency measuring functions of the network analyzer can then
be use to accurately measure the frequency of the luma notch
filter (luma trap).
–16–
REV. 0
AD725
6
signal should occur at the output of an on-chip XNOR gate on
the AD725 whose two inputs are HSYNC (Pin 16) and VSYNC
(Pin 15). There are several options for meeting these conditions.
3
0
LUMA PIN
The first is to have separate signals for HSYNC and VSYNC.
Each should be mostly low and then high going during their
respective time of assertion. This is the convention used by
RGB monitors for most PCs. The proper composite sync signal
will be produced by the on-chip XNOR gate when using these
inputs.
GAIN – dB
–3
COMP PIN
–6
–9
–12
–15
If a composite sync signal is already available, it can be input
into HSYNC (Pin 16), while VSYNC (Pin 15) can be used to
change the polarity. (In actuality, HSYNC and VSYNC are
interchangeable since they are symmetric inputs to a two-input
gate).
–18
–21
–24
0.1
1.0
FREQUENCY – MHz
10.0
Figure 25. Luminance Frequency Response with NTSC Trap
SYNCHRONIZING SIGNALS
The AD725 requires explicit horizontal and vertical synchronizing signals for proper operation. This information cannot and
should not be incorporated in any of the RGB signals. However,
the synchronizing information can be provided as either separate
horizontal (HSYNC) and vertical (VSYNC) signals or as a
single composite sync (CSYNC) signal.
Internally the AD725 requires a composite sync logic signal that
is mostly high and goes low during horizontal sync time. The
vertical interval will have an inverted duty cycle from this. This
If the composite sync input is mostly high and then low going
for active HSYNC time (and inverted duty cycle during VSYNC),
then it is already of the proper polarity. Pulling VSYNC high,
while inputting the composite sync signal to HSYNC will pass
this signal though the XNOR gate without inversion.
On the other hand, if the composite sync signal is the opposite
polarity as described above, pulling VSYNC low will cause the
XNOR gate to invert the signal. This will make it the proper
polarity for use inside the AD725. These logic conditions are
illustrated in Figure 26.
HSYNC
VSYNC
CSYNC
Figure 26. Sync Logic Levels (Equalization and Serration Pulses Not Shown)
REV. 0
–17–
AD725
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Lead Wide Body SOIC
(R-16)
9
1
8
PIN 1
0.0118 (0.30)
0.0040 (0.10)
0.0500
(1.27)
BSC
0.4193 (10.65)
0.3937 (10.00)
16
0.2992 (7.60)
0.2914 (7.40)
0.4133 (10.50)
0.3977 (10.00)
0.1043 (2.65)
0.0926 (2.35)
0.0291 (0.74)
x 45°
0.0098 (0.25)
8° 0.0500 (1.27)
0.0192 (0.49)
SEATING 0.0125 (0.32) 0° 0.0157 (0.40)
0.0138 (0.35) PLANE
0.0091 (0.23)
–18–
REV. 0
–19–
–20–
PRINTED IN U.S.A.
C3199–8–10/97
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