Analog Devices AN731 Application Notes

AN-731

APPLICATION NOTE

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AD9883A to AD9985 Design Conversion Considerations

by Del Jones

OVERVIEW

The AD9985 is a pin-for-pin replacement of the AD9883A
that includes an automatic offset calibration feature. By
monitoring the output of each ADC during the back porch
of the input signals, the AD9985 can self-adjust to elimi­nate any offset errors in its own ADC channels as well
as any offset errors present on the incoming graphics or
video signals.

Register Bit Bit Name Bit Description

0x16 – 0x18 7:0 Test Registers These registers are reserved for future use.

0x19 7:0 Red Target Code Sets the Red (Pr) channel target code when Auto Offset is enabled
(0x1D:7 = 1).

0x1A 7:0 Green Target Code Sets the Green (Y) channel target code when Auto Offset is enabled
(0x1D:7 = 1).

0x1B 7:0 Blue Target Code Sets the Blue (Pb) channel target code when Auto Offset is enabled
(0x1D:7 = 1).

0x1C 7:0 Test Bits Must be set to default (11\h) for proper operation.

0x1D 7 Auto Offset Enable 0 = Auto Offset disabled, 1 = Auto Offset enabled.
6 Hold Auto Offset 0 = Update Auto Offset circuit according to bits 1:0 of 0x1D.
1 = Hold the current Auto Offset circuit value.
5:2 Test Bits Must be set to default (**10 01**) for proper operation.
1:0 Update Mode Determines how often the Auto Offset circuit is updated:
00 = every clamp
01 = every 16 clamps
10 = every 64 clamps
11 = not valid

0x1E 7:0 Test Register Must be set to default (0x00) for proper operation.

REGISTER DIFFERENCES

Registers 0x00–0x15 on the AD9985 are identical to
the AD9883A. Table 1 shows the new registers on the
AD9985 that control the Auto Offset function.

In addition to these new registers, the functionality of
the offset registers (0x0B – 0x0D) is slightly different
when the Auto Offset function is enabled, as shown in
Table 2.

Table 1.

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Table 2.

Register Normal Function Auto Offset Function

0x0B Red Channel Offset Red channel offset from target code (0x19)
(in binary notation) (value in twos compliment notation).

0x0C Green Channel Offset Green channel offset from target code (0x1A)
(in binary notation) (value in twos compliment notation).

0x0D Blue Channel Offset Blue channel offset from target code (0x1B)
(in binary notation) (value in twos compliment notation).

AN-731

USING AUTO OFFSET

To activate the Auto Offset mode, set Register 0x1D, Bit 7,
to 1. Next, the target code registers (0x19 through 0x1B)
must be programmed. The values programmed into the
target code registers should be the output code desired
from the AD9985 during the back porch reference time.
For example, for RGB signals, all three registers would
normally be programmed to code 1, while for YPbPr
signals, the green (Y) channel would normally be pro­grammed to code 1, and the blue and red channels
(Pb and Pr) would normally be set to 128. Any target
code value between 1 and 254 can be set, although the
AD9985’s offset range may not be able to reach every
value. Intended target code values range from (but aren’t
limited to) 1 to 40 when ground clamping, and 90 to 170
when mid scale clamping.

The ability to program a target code for each channel
gives users a large degree of freedom and exibilit y.
While in most cases all channels will be set to either
1 or 128, the exibility to select other values allows for
the possibility of inserting intentional skews between
channels. It also allows for the ADC range to be skewed

so that voltages outside of the normal range can be digi­tized. (For example, setting the target code to 40 would
allow the sync tip, which is normally below black level,
to be digitized and evaluated.)

Lastly, when in Auto Of fset mode, the manual offset
registers (0x0B to 0x0D) have new functionalit y. The
values in these registers are digitally added to the value
of the ADC output. The purpose of doing this is to match
a benet 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. The digital adder is only
used when in Auto Offset mode. Although it cannot be
disabled, setting the offset registers to all 0s will effec ­tively disable it by always adding 0.

PCB CONSIDERATIONS FOR COMPATIBILITY

Since the pin definitions for the AD9883A and the
AD9985 are identical, there are no changes necessary
in the PCB layout.

AN04886–0–7/04(0)

© 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.

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