Re: RGB to S-video/composite using AD-725

On Sep 21, 1:57 am, "David L. Jones" <altz...@xxxxxxxxx> wrote:
On Sep 20, 10:18 pm, NoSp <no-sp...@xxxxxxxxx> wrote:

I need to convert an RGB signal to S-video and composite. Apparently
the Analog Devices AD-725 (http://www.analog.com/UploadedFiles/
Data_Sheets/AD725.pdf) is ideal for this.

There's a schematic in that data*** which I've extracted as an image
file here:http://img530.imageshack.us/img530/8/ad725vh7.png

but as a novice there are a few things I don't quite understand:

1) there are pins for digital and analog power supplies. Do I really
need two different power supplies for this,

Not really. You should at least LC filter the analog +5 supply.

I don't know exactly what kind of supply I'll be using, but since the
circuit is going to be used with a computer, installed inside the
enclosure I assume I'll be getting some sort of switched mode power
supply (the existing computer supply is too small for all the add-ons
I'll be putting inside the rack-enclosure. It's an Atari ST computer
by the way, in case you were wondering ;-)
Will a normal computer power supply do for a joint +5V and GND?


Do I simply get hold of a 17.734475 MHz oscillator as in one of those
metallic square thingies with 4 pins, or is the schematic referring to
a more complex circuit you have to figure out yourself?

You need an oscillator module (yes, one of those metallic square
thingies).

That's what I was hoping for :-)
I've finished my schematic now, which is taken from the example
schematic in the AD-725 data***, but made complete and for PAL
systems. Does it look OK?
I've uploaded it here: http://img219.imageshack.us/img219/9256/ad72510jv0.png


5) my application will "tap" the RGB and sync signals several places.

Yes, but you have to be careful of the signal levels. The RGB monitor
is 75ohm terminated, so if you don't connect it you'll get double your
volts into the AD725.

Hmm... I'm a little uncertain about how to approach this as I have
another schematic I'm working on which will turn the Atari-specific
video output (demanding Atari colour or monochrome monitors) into
selectable SCART and VGA in addition to the existing connector for
Atari monitors, and now of course S-video and composite.
Here's that schematic, but please disregard the AD-725 circuitry
(which of course isn't complete) and one of the multiplexers (U1)
which I'm still having trouble to figure out:
http://img248.imageshack.us/my.php?image=atarivga41xm0.png

I assume having the signals go through the AD8073 circuitry will
distort it a little bit. If that's the case I would want the best
possible signal to go directly to the (new) Atari ST video out (13-pin
DIN) connector and the VGA output as those are the outputs I'll be
using on a daily basis.
I still want a good signal for those other ports, but I'll be using
the computer mostly in "monochrome mode" as opposed to "colour mode"
which is where the SCART, S-video and Composite outputs are used for
TVs etc.
Finally, there's already a composite output available directly from
the Atari ST. I haven't yet tried it as I've heard discussions about
it being just composite sync, or a complete composite video signal.
If it's a complete video signal I'm not sure if I should use that for
the composite output or use the AD-725 for composite and S-video.
How do you suggest I approach all of this?

.

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