Question
I have a question regarding the ADV7123 Video DAC. I connected the chip as
described in the datasheet with a 562 Ohm resistor on the RSET pin and SYNCN
(Sync on Green) to GND via a pull-down resistor. My test design is a rectangle
(should be gray) with a dark gray border and a thin white line (one pixel) on
the edge of the screen.
I can see what I would like to show but the colors are not correct since the
green signal (all three outputs with a 75 Ohm resistor to GND) has a larger
peak-to-peak signal than blue and red, although they are the same level. Any
Ideas? Could my voltage reference cause that? What happens if that voltage
reference drops below 1.25V?
Answer
The Rset value should be 530 Ohm, we don't recommend operating below this
value.
We have seen that when customers are operating the ADV7123 at high speed along
with rapid black/white transitions, an external reference such as the AD1580
should be used.
The internal ref at 5V is given in the datasheet.
We don't have a min and max spec for the internal ref at 3.3V, however we would
expect similar performance as the 5V spec's.
* To summatise, the max value for Rset should not exceed 530ohms for specified
performance.
* Using the part at high speeds and rapid black/white changes, places a high
current demand on the ADV7123 which affects the accuracy of the internal
reference. We have found that using a larger cap on Vref (i.e 4.4Uf)may allow
you to use the internal ref in this application, however we recommend using an
external ref.
* In order to achieve tight fullscale matching between parts, we recommend the
use of an external reference.