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

On Sep 20, 7:00 pm, "David L. Jones" <altz...@xxxxxxxxx> wrote:
On Sep 21, 2:07 am, a7yvm109gf...@xxxxxxxxxxx wrote:



On Sep 20, 8:18 am, NoSp <no-sp...@xxxxxxxxx> wrote:

1) there are pins for digital and analog power supplies. Do I really
need two different power supplies for this, or can I simply connect
pin 4 (analog +5V) and pin 14 (digital +5V) together to a +5V supply,
then pin 2 (analog GND) and pin 13 (digital GND) together to GND of
the power supply?

Depends. How noisy is your 5V supply? Sometimes all you need is a big
LC filter to "clean up" your supply for the analog side.

2) I don't quite understand the oscillator on pin 4. I need a PAL
signal, but I'm confused about the "OSC" as it looks like a simplified
symbol rather than a component.
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?

It is an entire circuit in itself. You need to have a clock that is
within a known error of the clock that generated the RGB signal.
A free-running oscillator will not work properly since it will have no
known phase or frequency relationship with the RGB's clock.
Unless you have access to the clock signal that is driving the RGB
part of your project, you will need something like a 4FSC genlock
chip.http://www.intersil.com/cda/deviceinfo/0,0,EL4584.html

The AD725 does not need this, it will happily work with a free-running
oscillator, as shown in the app circuit in the data***.

Dave.

How is this possible? The display will not be very good then.

.