Q
I have a question regarding the ADV7123 Video DAC. I connected the chip asdescribed 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?
A
The Rset value should be 530 Ohm, we don't recommend operating below thisvalue.
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.