AD9882 Analog Devices, AD9882 Datasheet - Page 15

• Manufacturer Part Number: AD9882
• Description: Dual Interface for Flat Panel Displays
• Manufacturer: Analog Devices

Sync-on-Green (SOG)
The Sync-on-Green input operates in two steps. First, it sets
a baseline clamp level off of the incoming video signal with a
negative peak detector. Second, it sets the Sync trigger level
(nominally 150 mV above the negative peak). The exact trigger
level is variable and can be programmed via Register 0FH,
Bits 7–3. The Sync-on-Green input must be ac-coupled to the
green analog input through its own capacitor as shown in Figure 3.
The value of the capacitor must be 1 nF ± 20%. If Sync-on-Green
is not used, this connection is not required and SOGIN should
be left unconnected. (Note: The Sync-on-Green signal is always
negative polarity.) Please refer to the Sync Processing section
for further information.
Clock Generation
A Phase Locked Loop (PLL) is employed to generate the pixel
clock. The Hsync input provides a reference frequency for the
PLL. A Voltage Controlled Oscillator (VCO) generates a much
higher pixel clock frequency. This pixel clock is divided by the
PLL divide value (Registers 01H and 02H) and phase compared
with the Hsync input. Any error is used to shift the VCO frequency
and maintain lock between the two signals.
REV. A
Figure 3. Typical Clamp Configuration
Figure 2. Gain and Offset Control
1.0 V
0.5 V
0.0 V
00H
47nF
47nF
47nF
1nF
SOGIN
OFFSET = 3FH
R
B
G
GAIN
AIN
AIN
AIN
FFH
–15–
The stability of this clock is a very important element in provid-
ing the clearest and most stable image. During each pixel time,
there is a period during which the signal is slewing from the old
pixel amplitude and settling at its new value. Then there is a
time when the input voltage is stable, before the signal must
slew to a new value (Figure 4). The ratio of the slewing time to
the stable time is a function of the bandwidth of the graphics
DAC and the bandwidth of the transmission system (cable and
termination). It is also a function of the overall pixel rate. Clearly,
if the dynamic characteristics of the system remain fixed, then
the slewing and settling time is likewise fixed. This time must be
subtracted from the total pixel period, leaving the stable period.
At higher pixel frequencies, the total cycle time is shorter, and
the stable pixel time becomes shorter as well.
Any jitter in the clock reduces the precision with which the
sampling time can be determined and must also be subtracted
from the stable pixel time.
Considerable care has been taken in the design of the AD9882’s
clock generation circuit to minimize jitter. As indicated in Fig-
ure 5, the clock jitter of the AD9882 is less than 6% of the total
pixel time in all operating modes, making the reduction in the
valid sampling time due to jitter negligible.
10
8
6
4
2
0
25.1 31.5 36.0 40.0
Figure 5. Pixel Clock Jitter vs. Frequency
PIXEL CLOCK
Figure 4. Pixel Sampling Times
PIXEL CLOCK FREQUENCY – MHz
50.0 56.2 65.0 75.0 78.7
INVALID SAMPLE TIMES
85.5
94.5 108.0
AD9882
135.0