AD9883/PCB AD [Analog Devices], AD9883/PCB Datasheet - Page 22

• Manufacturer Part Number: AD9883/PCB
• Description: 110 MSPS Analog Interface for Flat Panel Displays
• Manufacturer: AD [Analog Devices]

Mux
Nos.
1 and 2
4
Sync Slicer
The purpose of the sync slicer is to extract the sync signal from
the green graphics channel. A sync signal is not present on all
graphics systems, only those with “sync-on-green”. The sync
signal is extracted from the green channel in a two step process.
First, the SOG input is clamped to its negative peak, (typically
0.3 V below the black level). Next, the signal goes to a com-
parator with a variable trigger level, nominally 0.15 V above the
clamped level. The “sliced” sync is typically a composite sync
signal containing both Hsync and Vsync.
Sync Separator
A sync separator extracts the Vsync signal from a composite sync
signal. It does this through a low-pass filter-like or integrator-like
operation. It works on the idea that the Vsync signal stays active
for a much longer time than the Hsync signal, so it rejects any
signal shorter than a threshold value, which is somewhere between
an Hsync pulsewidth and a Vsync pulsewidth.
The sync separator on the AD9883 is simply an 8-bit digital
counter with a 5 MHz clock. It works independently of the
polarity of the composite sync signal. (Polarities are determined
elsewhere on the chip.) The basic idea is that the counter counts
up when Hsync pulses are present. But since Hsync pulses are
relatively short in width, the counter only reaches a value of N
before the pulse ends. It then starts counting down eventually
reaching 0 before the next Hsync pulse arrives. The specific
value of N will vary for different video modes, but will always be
less than 255. For example with a 1 µs width Hsync, the counter
will only reach 5 (1 µs/200 ns = 5). Now, when Vsync is present
on the composite sync the counter will also count up. However,
since the Vsync signal is much longer, it will count to a higher
number M. For most video modes, M will be at least 255. So,
Vsync can be detected on the composite sync signal by detecting
when the counter counts to higher than N. The specific count
that triggers detection (T) can be programmed through the
serial register (0fh).
Once Vsync has been detected, there is a similar process to detect
when it goes inactive. At detection, the counter first resets to 0,
then starts counting up when Vsync goes away. Similar to the
previous case, it will detect the absence of Vsync when the
counter reaches the threshold count (T). In this way, it will
reject noise and/or serration pulses. Once Vsync is detected to
be absent, the counter resets to 0 and begins the cycle again.
AD9883
Table XXXIV. Control of the Sync Block Muxes via the
Serial Register
Serial Bus
Control Bit
0EH: Bit 3
0EH: Bit 0
Control
Bit
State
0
1
0
1
Result
Pass Hsync
Pass Sync-on-Green
Pass Vsync
Pass Sync Separator Signal
PCB LAYOUT RECOMMENDATIONS
The AD9883 is a high-precision, high-speed analog device. As
such, to get the maximum performance out of the part it is
important to have a well laid-out board. The following is a guide
for designing a board using the AD9883.
Analog Interface Inputs
Using the following layout techniques on the graphics inputs is
extremely important.
Minimize the trace length running into the graphics inputs. This
is accomplished by placing the AD9883 as close as possible to
the graphics VGA connector. Long input trace lengths are unde-
sirable because they will pick up more noise from the board and
other external sources.
Place the 75 Ω termination resistors (see Figure 1) as close to the
AD9883 chip as possible. Any additional trace length between the
termination resistors and the input of the AD9883 increases the
magnitude of reflections, which will corrupt the graphics signal.
Use 75 Ω matched impedance traces. Trace impedances other
than 75 Ω will also increase the chance of reflections.
The AD9883 has very high input bandwidth, (500 MHz). While
this is desirable for acquiring a high resolution PC graphics
signal with fast edges, it means that it will also capture any high
frequency noise present. Therefore, it is important to reduce the
amount of noise that gets coupled to the inputs. Avoid running
any digital traces near the analog inputs.
Due to the high bandwidth of the AD9883, sometimes low-pass
filtering the analog inputs can help to reduce noise. (For many
applications, filtering is unnecessary.) Experiments have shown
that placing a series ferrite bead prior to the 75 Ω termination
resistor is helpful in filtering out excess noise. Specifically, the
part used was the # 2508051217Z0 from Fair-Rite, but each
application may work best with a different bead value. Alternately,
placing a 100 Ω to 120 Ω ohm resistor between the 75 Ω termi-
nation resistor and the input coupling capacitor can also benefit.
Power Supply Bypassing
It is recommended to bypass each power supply pin with a
0.1 µF capacitor. The exception is in the case where two or
more supply pins are adjacent to each other. For these group-
ings of powers/grounds, it is only necessary to have one bypass
capacitor. The fundamental idea is to have a bypass capacitor
within about 0.5 cm of each power pin. Also, avoid placing the
capacitor on the opposite side of the PC board from the AD9883,
as that interposes resistive vias in the path.
The bypass capacitors should be physically located between the
power plane and the power pin. Current should flow from the
power plane => capacitor => power pin. Do not make the power
connection between the capacitor and the power pin. Placing a
via underneath the capacitor pads, down to the power plane, is
generally the best approach.
It is particularly important to maintain low noise and good sta-
bility of PV
PV
phase and frequency. This can be avoided by careful attention to
regulation, filtering, and bypassing. It is highly desirable to
provide separate regulated supplies for each of the analog cir-
cuitry groups (V
D
can result in similarly abrupt changes in sampling clock
D
(the clock generator supply). Abrupt changes in
D
and PVD).