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AD9985A
The power-up default value of PLLDIV is 1693
CLAMP TIMING
(PLLDIVM = 0x69, PLLDIVL = 0xDx). The
AD9985A updates the full divide ratio only when this
register is written to.
CLOCK GENERATOR CONTROL
05
7–0 Clamp Placement
An 8-bit register that sets the position of the internally
generated clamp.
When clamp function (Register 0x0F, Bit 7) = 0, a
03
7–6 VCO Range Select
Two bits that establish the operating range of the clock
generator.
VCORNGE must be set to correspond with the
desired operating frequency (incoming pixel rate).
The PLL gives the best jitter performance at high
frequencies. For this reason, to output low pixel rates
and still get good jitter performance, the PLL actually
operates at a higher frequency but then divides down
the clock rate afterwards. Table 11 shows the pixel
rates for each VCO range setting. The PLL output
clamp signal is generated internally, at a position
established by the clamp placement and for a duration
set by the clamp duration. Clamping is started (clamp
placement) pixel periods after the trailing edge of
Hsync. The clamp placement can be programmed to
any value between 1 and 255.
The clamp should be placed during a time that the
input signal presents a stable black-level reference,
usually the back porch period between Hsync and the
image.
When clamp function = 1, this register is ignored.
divisor is automatically selected with the VCO range
setting.
Table 11. VCO Ranges
Pixel Clock Range (MHz)
06
7–0 Clamp Duration
An 8-bit register that sets the duration of the
internally generated clamp.
PV1
0
0
1
1
PV0
0
1
0
1
AD9985AKSTZ
12 to 32
32 to 64 (power-up
default)
64 to 110
110 to 140
AD9985ABSTZ
12 to 30
30 to 60 (power-up
default)
60 to 110
For the best results, the clamp duration should be set
to include the majority of the black reference signal
time that follows the Hsync signal trailing edge.
Insufficient clamping time can produce brightness
changes at the top of the screen, and a slow recovery
from large changes in the average picture level (APL),
03
5–3 Current Charge Pump Current
or brightness.
Three bits that establish the current driving the loop
filter in the clock generator.
Current must be set to correspond with the desired
operating frequency (incoming pixel rate).
Table 12. Charge Pump Currents
When clamp function = 1, this register is ignored.
HSYNC PULSE WIDTH
07 7–0 Hsync Output Pulse Width
An 8-bit register that sets the duration of the Hsync
output pulse.
CURRENT
Current (μA)
000
001
010
011
100
101
110
50
100 (power-up default)
150
250
350
500
750
The leading edge of the Hsync output is triggered by
the internally generated, phase-adjusted PLL feedback
clock. The AD9985A then counts a number of pixel
clocks equal to the value in this register. This triggers
the trailing edge of the Hsync output, which is also
phase adjusted.
INPUT GAIN
111
04
1500
7–3 Clock Phase Adjust
A 5-bit value that adjusts the sampling phase in 32
steps across one pixel time. Each step represents an
11.25° shift in sampling phase.
08
7–0 Red Channel Gain Adjust (REDGAIN)
An 8-bit word that sets the gain of the red channel.
The AD9985A can accommodate input signals with a
full-scale range of between 0.5 V and 1.0 V p-p.
Setting REDGAIN to 255 corresponds to a 1.0 V input
range. A REDGAIN of 0 establishes a 0.5 V input
The power-up default value is 16.
Rev. 0 | Page 20 of 32
range. Increasing REDGAIN results in the picture
having less contrast (the input signal uses fewer of the
available converter codes). See Figure 4.
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