AD9985SLASHPCB datasheet(20/32 Pages) AD

20 / 32 page

AD9985

Rev. 0 | Page 20 of 32

CLAMP TIMING

7–0

Clamp Placement

An 8-bit register that sets the position of the internally

generated clamp.

When Clamp Function (Register 0FH, Bit 7) = 0, a

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 may 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.

7–0

Clamp Duration

An 8-bit register that sets the duration of the

internally generated clamp.

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),

or brightness.

When Clamp Function = 1, this register is ignored.

HSYNC PULSEWIDTH

7–0

Hsync Output Pulsewidth

An 8-bit register that sets the duration of the Hsync

output pulse.

The leading edge of the Hsync output is triggered by

the internally generated, phase-adjusted PLL feedback

clock. The AD9985 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

7–0

Red Channel Gain Adjust

An 8-bit word that sets the gain of the Red channel.

The AD9985 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

range. Note that increasing REDGAIN results in the

picture having less contrast (the input signal uses

fewer of the available converter codes). See Figure 4.

7–0

Green Channel Gain Adjust

An 8-bit word that sets the gain of the Green channel.

See REDGAIN (08).

7–0

Blue Channel Gain Adjust

An 8-bit word that sets the gain of the Blue channel.

See REDGAIN (08).

INPUT OFFSET

7–1

Red Channel Offset Adjust

This and the following two offset registers have two

modes of operation. One mode is when the auto offset

function is turned off (manual mode) and the other is

when auto offset is turned on.

When in manual offset adjustment mode (auto offset

turned off) this register behaves exactly like the

AD9883A. It is a 7-bit offset binary word that sets the

dc offset of the Red channel. One LSB of offset

adjustment equals approximately one LSB change in

the ADC offset. Therefore, the absolute magnitude of

the offset adjustment scales as the gain of the channel

is changed. A nominal setting of 63 results in the

channel nominally clamping the back porch (during

the clamping interval) to Code 00. An offset setting of

127 results in the channel clamping to Code 64 of the

ADC. An offset setting of 0 clamps to Code –63 (off

the bottom of the range). Increasing the value of Red

Offset decreases the brightness of the channel.

When in auto offset mode, the value in this register is

digitally added to the red channel ADC output. The

purpose of doing this is to match a benefit that is

present with manual offset adjustment. Adjusting

these registers is an easy way to make brightness

adjustments. Although some signal range is lost with

this method, it has proven to be a very popular

function. In order to be able to increase and decrease

brightness, the values in these registers in this mode

are signed twos complement (as opposed to manual

mode where the values in this register are binary). The

digital adder is used only when in auto offset mode.

Although it cannot be disabled, setting this register to

all 0’s will effectively disable it by always adding 0.

7–1

Green Channel Offset Adjust

This register works exactly like the Red Channel

Offset Adjust register (0Bh), except it is for the Green

Channel.

7–1

Blue Channel Offset Adjust

This register works exactly like the Red Channel

Offset Adjust register (0Bh), except it is for the Blue

Channel.

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