Question
My question concerns DAC ADV7123.
On our board we use EP9315 (Cirrus Logic) as a processor. It contains video
interface connected to DAC. Digital level is 0 to 3,3V. DAC is powered from 5V,
also has external reference voltage AD589, pulled by 3.9k resistor from Vref to
5V. Rset = 430 ohm, COMP capacitor = 100nF. After few months some PCB's back to
us with ADV7123 damage. Picture is gray but it can be see on it an mosaic of
proper picture. What is the reason of this kind of damage? I have also a few
questions.
1. How ADV7123 detects working with external reverence voltage?
2. We use only 8 bit for each channel. Does 2 MSB's each channel should be
tied to GND ?
3. ADV7123 works without power supply - it is possible that the power is
supplies by digital inputs of RGB ?
Answer
1. You can use an external Voltage reference or as we have seen in some low
cost instances a voltage divider network with approximately 10Kohm values. For
low clock speed operation the internal voltage reference should be adequate.
2. Yes (connect to ground) , or use the ADV7125 pin compatible 8-bit version of
the IC.
3. It is advised to run the part within the specified operating limits.
Non standard supplies may switch on the protection diodes and although the part
may operate damage can result.
When you have no power supply (VAA=0 volts) , the voltage applied to the
digital pins needs to be within the range -0.5 to +0.5 volts.
Otherwise you will forward bias the built in ESD protection diodes.
See the Absolute maximum ratings on page 8 of the ADV7123 datasheet.