AD9887A Analog Devices, Inc., AD9887A Datasheet - Page 19

• Manufacturer Part Number: AD9887A
• Description: Dual Interface For Flat Panel Displays
• Manufacturer: Analog Devices, Inc.

SYNC-ON-GREEN INPUT
The sync-on-green input operates in two steps. First, with the
aid of a negative peak detector, it sets a baseline clamp level
from the incoming video signal. 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 0x11.
The sync-on-green input must be ac-coupled to the green analog
input through its own capacitor, as shown in Figure 8.
1.0
0.5
0x00
0
IN
Figure 5. ADC Block Diagram (Single-Channel Output)
Figure 7. Relationship of Offset Range to Input Range
CLAMP
OFFSET
V
(128 CODES)
DAC
7
OFF
Figure 6. Gain and Offset Control
0.5V
0V
V
OFF
x1.2
GAIN
OFFSET = 0x7F
V
(128 CODES)
GAIN
DAC
8
ADC
OFF
OFFSET = 0x3F
OFFSET = 0x00
OFFSET = 0x7F
OFFSET = 0x3F
OFFSET = 0x00
1V
0V
8
REF
0xFF
Rev. B | Page 19 of 52
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 that the sync-on-green signal is always
negative polarity.) See the Theory of Operation—Sync Processing
section for more information.
CLOCK GENERATION
A phase-locked loop (PLL) is used 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 is divided by the PLL divide value
(MSBs in Register 0x01 and LSBs in Register 0x02) and phase
compared with the HSYNC input. Any error is used to shift the
VCO frequency and maintain lock between the two signals.
The stability of this clock is important for providing the clearest,
most stable image. During each pixel time, there is a period when
the signal slews from the old pixel amplitude and settles at its
new value. Then, the input voltage is stable until the signal slews
to a new value (see Figure 9). The ratio of the slewing time to the
stable time is a function of the bandwidth of the graphics DAC,
the bandwidth of the transmission system (cable and termination),
and the overall pixel rate. Clearly, if the dynamic characteristics
of the system remain fixed, the slewing and settling times are
likewise fixed. Subtract these times from the total pixel period to
determine the stable period. At higher pixel frequencies, both the
total cycle time and stable pixel time are shorter.
Figure 8. Typical Clamp Configuration for RGB and YUV Applications
PIXEL CLOCK
Figure 9. Pixel Sampling Times
47nF
47nF
47nF
1nF
INVALID SAMPLE TIMES
R
B
G
SOGIN
AIN
AIN
AIN
AD9887A