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Please tell me about Video DAC (ADV7125) current limit.

Category: Datasheet/Specs
Product Number: ADV7125
Software Version: non

I believe that the current control of the RGB analog signal output can be controlled using the RSET pin.
Is it possible to make it smaller than 530Ω under the following conditions?
If it is possible to make it smaller, could you please tell me how small it can be made? (I want to increase the current flowing.)

SYNC pin=low, VAA=3.3V

(Intent of question)
The value of RSET can be calculated using the following formula
I interpret it as IOR, IOB,IOG (mA) = 7989.6 × VREF (V)/RSET (Ω)

FEATURES has the following information
It says DAC output current range:2.0mA to 26.5mA, is max26.5mA the limit?

The value listed in FEATURES is range:2.0mA to 26.5mA, so
According to this description, if the SYNC pin is set to High, up to 26.5mA can be used.

When SYNC pin is set to low, it can be used up to 18.5mA ←When SYNC pin is set to low, 18.5mA is the limit value?
In the circuit example in the data sheet, the resistance value of RSET is 530Ω.
7989.6 × VREF (V)/RSET (Ω)=7989.6 x 1.235 / 530 =18.61mA ←Exceeds 18.5mA

I think my interpretation is fundamentally wrong, so I'd like to confirm.
Increase the current (analog signal voltage value Vp-p) as much as possible.
The purpose is to strengthen noise resistance.

  • Hi,

    Please note " DAC output current is proportional to Rset and Rload. DAC is a current output based on digital input values, Rset and Rload.
    If you increase Rload by 20% the effective output voltage would increase by 20% with the caveat that the output voltage does not exceed limit.
    We do not test this or specify these limits beyond what is defined in the data sheet.

    When Rset = 510 Ohms, the DAC outputs expect to see a 37.5 Ohm load. The load consists of a 75 Ohm resistor to ground right next to the pin and a 75 Ohm resistor to ground at the sink. Normal cables plus sinks are set up this way.

    The encoders expect to be terminated with 75 ohms on your PCB with another 75 ohms at the end of the transmission line, so the total load resistance is 37.5 Ohms.

    A resistor connected between the RSET pin and AGND is used to control the full-scale output current and, therefore, the output voltage levels of DAC .

     Note : The output current is based on the equations given in the data sheet

    The Max output current are defined in the Reference Input equations with the caveat you do not exceed the absolute voltages and values.


    Dharani S

  • Dear Dharani S

    Thank you for answering.

    I'm sorry for my poor understanding, I don't want to change the Rload value, but only the Rset value change.

    For example, change the Rset value from 530Ω to 470Ω. Is this possible if the maximum rated voltage is met?

    The voltage value is less than 0.8V.

  • Hi,

    From the formula,

    IOG (mA) = 11,444.8 × VREF (V)/RSET (Ω) (SYNC being asserted)
    with Rset = 530 and Rload = 37.5Ohm

    gives you (37.5 * 11444 * 1.235 / 530) / 1000 = 1.000 Vpp

    See Figure 5 in the data sheet.

    Assuming green channel with sync included,

    510 --> 1.039 Vpp
    530 --> 1.000 Vpp
    560 --> 0.946 Vpp

    Dharani S

  • Dear Dharani S

    Thank you for your kind calculation formula.

    I can understand how to calculate the expected output voltage from the set value.

    How can I determine whether this setting either output voltage or output current are okay for the ADV7125?

    Or is there no limit for ADV7125?

  • Hi,

    As stated, we do not test this or specify these limits beyond what is defined in the data sheet in our eval board.

    While looking at the ADV7125 datasheet, Figure 5 and Table 7 we see that the red and blue DAC output has will output has a voltage range of 0 - 0.975V. The green has the same output except it is positively offset by 0.271V to give room for the sync pulse.

    Make note that the white level output current level, for red and blue it's 0-18.67mA. For green it's 7.2-26mA for black to white.

    This is all done with Rset=560 Ohms. The DAC outputs are limited to 1.4. so this is what we'd do.

    Could you please check from your side for this case and share us your results if it is not as expected?

    Dharani S