AD723 데이터 시트보기 (PDF) - Analog Devices
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AD723 Datasheet PDF : 20 Pages
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AD723
Implementing a Luma Trap
The AD723 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 AD723,
3.58 MHz for NTSC or 4.43 MHz for PAL.
The circuit is shown in Figure 11. The 1.4 kΩ series resistor in
the composite video luma path on the AD723 works against the
impedance of the off-chip series LC to form a notch filter. The
frequency of the filter is given by:
fC = 1
2 π LC
14.318180MHz A
17.734475MHz
B
A/B
CHROMA
4FSC
AD723
C
CSET
CV
CVSET
NTSC/PAL
LUMA
STND
1.4k⍀
Y
YSET
YTRAP
C2 L
47k⍀ 9pF 68H
D1
1N4148
C1
18pF
375⍀
300⍀
375⍀
300⍀
375⍀
300⍀
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 AD723 and observing the compos-
ite 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 fre-
quency) 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 verti-
cal 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.
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.
1.0
100
0.5
50
Figure 11. Luma Trap Circuit for NTSC and PAL Video
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 11 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 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 (3 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.
0.0
0
–50
–0.5
0
10
20
30
40
50
60
s
Figure 12. Luminance Sweep with Trap, CV Pin
6
3
0
Y (LUMA)
–3
–6
C (COMP)
–9
–12
–15
–18
–21
–24
0.1
1.0
10.0
FREQUENCY – MHz
Figure 13. Luminance Frequency Response with NTSC Trap
–18–
REV. 0
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