Post Go back to editing

[ADV7480] Input Common Mode Voltage measurement results

Category: Hardware

Hi,

We are evaluating the peripheral circuits of the ADV7480.

It is unclear whether the interpretation of the HDMI standard is correct, so please let me check.

We measured the Vicm1 characteristics in the HDMI standard,

but the measurement results differed from the standard values.

If it is according to the standard, I think it will be 3.0V to 3.2625V,

but the measurement result is 2.55V.

Does this result have any impact on the ADV7480?

Best regards.

  • Hi Tatsuya San,

           Generally, the common-mode level of the signal is determined by the coupling of the transmission line (AC-coupled vs. DC coupled) connecting the ADV7480 Rx to the upstream transmitter, the signal amplitude, and the termination supply voltages of the Transmitters/Receivers.

           I am not sure, what type of impact this 2.55 Voltage will create on ADV7480 part.

           Also please ensure your layout according to the HDMI guidelines.

           Refer this thread Intermittent Video on ADV7623 - Q&A - Video - EngineerZone (analog.com) Here customer is reported similar problem about about the common mode voltage.

    Please refer the below FAQ, We created this FAQ recently for custom board based customers to rule out some Hardware/Software issues.

    Also we suggested some Video parts with feature details.

    Queries and Primary Suggestions for our Video Customers! - Documents - Video - EngineerZone (analog.com)

    Kindly check and provide your feedback on this.

    Thanks,

    Poornima

  • Hi,

    Thank you for your reply.

    Could you tell me how the ADV7480 is designed?

    ・Avcc in Figure 4-23 below is TVDD (=3.3V) for ADV7480?

        Or is the power supply that is internally divided into Avcc?

    ・Is the termination resistance Rt inside the ADV7480 50Ω±10%?

    ・If 2.55V is caused by the HDMI Source device,

        what kind of circuit does the HDMI Source device generate 2.55V?

    Looking at the figure below, the HDMI Transmitter (Source) side is current driven,

    and the polarity signal that is not driven is the voltage value of the pull-up power supply AVcc.

    Therefore, I presume that circuitry inside the ADV7480 drives Vicm1 to 2.55V.

    Excerpt from HDMI1.4a standard

    Best regards.

  • Hi Tatsuya San,

      Apologies for the delayed response, I was on leave.

            Please find the below comments,

    Avcc in Figure 4-23 below is TVDD (=3.3V) for ADV7480 ?

    Or is the power supply that is internally divided into Avcc ?

       As per the data sheet TVDD = 3.14 V to 3.46 V.

       The device is specified for a 3.14 to 3.46 V range. Anything outside this range has not been tested and we cannot guarantee performance when out of the specified range. However, it might function correctly depending on the format. You would have to assume full testing and certifications at the different voltages.
    DVDDIO supplies the digital interfaces, TVDD supplies the analog receiver and 50 Ohm pull up resistors on the TMDS lines.
    As per expert comment, Recommend generating 3.3V with a simple boost circuit to get within specifications.

    Note: Please look at the HDMI specification to see how HDMI TMDS are terminated. 

     HDMI receivers must provide proper termination on the TMDS lines. Normally this is done internally so you can turn off the termination to save power and indicate to the transmitter the receiver is not set up yet. 

     Usually, transmitters will not transmit until they see the correct termination.  Transmitters monitor the dc voltage on the TMDS clock line for something greater then 2V (All TMDS lines when operating properly should be at ~3V with a 300mVpp signal on them) and if it see this then it assumes it is properly terminated and allows you to turn on the transmitter drivers.  Of course, you can override this and force it to transmit. 

    As per expert comment the optimal solution would be,

         1) Design for a 50 Ohm board (55 +- 10% can be done easily at most board fabricators)

         2) Use 22-47 series resistors in the control and pixel buses.  If the traces are short enough you may not need the series resistors and control the integrity with the drive settings only.

         3) Do signal integrity analysts using the IBIS files and tweaking the series resistor and drive strength as needed for best signal integrity.

        We are in the High-Performance Media group, are designing boards based on 50 Ohm trace impedances.  This better match's design requirements for HDMI, USB, DDRs, SDRAMs and DSPs we use on our boards.  For the pixel bus we generally use 33 Ohms and tweak the output drive strengths to get good signal integrity.

    Note: Normally receivers have on-die termination resistors.  For schematics you can do point to point routing as long as you maintain the 50 Ohms, 100 Ohm differential layout for the TMDS pairs.

    Thanks,

    Poornima

  • Hi,

    Thank you for your reply.

    The ADV7480 TVDD was supplied with 3.3 V (ranging from 3.14 to 3.46 V) and Vicm1 was measured.

    After all, is it a problem that Vicm1 is 2.55V in ADV7480?

    Also, what kind of circuit does the external device generate 2.55V?

    The TMDS trace between the ADV7480 and the board's HDMI connector has only ESD protection diodes to GND.

    HDMI data was received and the TMDS eye pattern met the HDMI specification.

    Best regards.

  • Hi Tatsuya San,

          As stated earlier, I don't know what type of impact this 2.55V(Vicm1) will create on ADV7480 part.

          Could you please measure the same in our EVAL-ADV7480 board since we have been validated our part across different compliance standard.

          Also if voltage range is different then you can change the divider network as long as the new voltage levels meet the ADV7480 specs.

    Thanks,

    Poornima