AD725: Loss of voltage level

Document created by analog-archivist Employee on Feb 23, 2016
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Customer described a measured loss of voltage level: 59 IRE(421mV) instead of
100 IRE(714mV) within the data information
of the pattern.
Customer refered to the datasheet and remarks on page 5, figure 3.
Could you please clarify the obvious discrepancy in level?

 

The AD725 will out standard NTSC levels (White = 100IRE) provided the source
RGB source outputs NTSC level signals at 60 Hz interlaced and the Sync signal
meet the AD725 timing requirements.

Here are a few things to check in order to determine why the output is lower
than expected.

1) Your RGB source is not outputting NTSC type signals. There is a wide variety
of scan options available on modern graphics cards. Most Graphics Controllers
still support "legacy" output scanning. These are the scanning modes that were
used in the earliest days of PCs. It requires a trivial amount of extra
circuitry to support this and I know that most products as recently as a few
years ago still provided legacy support.

The AD725 combines the RGB signals according to form the luminance(Y) according
to the standard transformation:
Y = 0.299 R + 0.587 G + 0.144 B

The RGB signal source must have appropriate output scaling in order to produce
standard NTSC levels.

2) your HSYNC / VSYNC signal does not meet the Required timing. The SYNC signal
must be between 2.8us and 5.3us for NTSC. The RGB source must output black
level during this time and continue to output black level tSB (8.2us) in order
for the clamp circuitry to correctly set the black level. If the black level is
set too high or too low, internal circuits will saturate.

A simple way to check this is to use Greyscale bars instead of colour bars and
measure the black level at the output of the AD725 ( pin10 prior to the 220uF
cap). The output black level should be approx 1.3V. Any significant deviation
from this level would indicate incorrect clamping.

3) Ensure the RGB source is properly terminated with 75ohms,ensure you have the
correct value of AC coupling capacitors (as fig 19). Ensure the S-Video and
comp video signals are properly terminated with 75ohm resistors. The output of
the AD725 is gained up by 2 which assumes that the output will be divided by
two at the monitor side.

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