Question Could you please send us the ADV7123 Evaluatiopn Board schematic Answer Please refer to the attached

ADV7123: Analog O/P values

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.

ADV7123: Output with no power applied

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.

ADV7123 - Vref minimum and maximum specification is missing in the datasheet

Question In our application we use the ADV7123 JST240 with internal Reference voltage @3,3V operating voltage. Referred to the datasheet (Rev D, page 4) the internal reference voltage is specified at 1,235V (typ) and no min/max value is stated there. Because we’ve detected that the voltage is varied from the typical value I ask you what the variation of the internal reference voltage actually is. Answer There is a mistake in the datasheet. I have a printout of the original datasheet and this shows the following for both 3.3V and 5V: min typ max Vref Internal: 1.12 1.235 1.35 Vref External: 1.14 1.235 1.26 So internal tolerances show the min and max that can be generated, whereas external tolerances show min and max that should be applied. External Vref is strongly recommended for better performance especially at higher speeds. When using the internal reference the VREF pin needs to be tied to VAA via a 0.1uF capacitor.

ADV7123: Driving terminated video lines

Question To be able to drive a regular monitor cable from the chip to the monitor, do I need to use output buffers on the RGB outputs or can I really drive it directly out of the chip as stated in the datasheet? Answer The ADV7123 has been designed to drive doubly terminated 75 Ohm video lines (as shown in figures 4a and 4b in the datasheet, RevB pg 14) and explained further in the attached application note. All the curves in the datasheet are generated with such a double terminated load. On page 15 of the datasheet there is a section describing the conditions where an external buffer amplifier is required "Video Output Buffers". Broadly speaking, this applies to cable lengths greater than 10m where attenuation and variable group delay for high frequencies can be a problem. Since the line is terminated, it appears as a pure resistance, the capacitive element disappears. If there is stray parasitic capacitance at the output this will certainly degrade performance, cause ringing etc.