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DAC60004: Reference input impedance

Jbjb
Prodigy 30 points
Part Number: DAC60004

Hello,

I plan to use some DAC6004 in one of my design.

I will have 4 DACs. All of them will be using the same 2.5V-reference. This reference will be shared with ADCs (for these parts, the accuracy of the reference is extremely important and it must not be influenced by other components). The REFIN input current is critical in my design as it may create a voltage drop (few µV) in the PCB tracks.

I have, thus, several questions regarding the REFIN pin of your DAC.

1) What is the impedance of the REFIN terminal ?

I found two information in the datasheet that seem different :

And :

What is the impedance of the REFIN terminal ? Shouldn't it be 30Kohm/4  as there are 4 buffers in parrallel ?  What is the difference between these two values ?

2) Is this impedance really constant ?
This leakage current may not be a problem if it does not change over time and temperature.

So :

- is the REFIN impedance the same no matter the mode  (normal mode or power-down mode) ?
- is the REFIN impedance constant over time (no matter the DAC output) ?
- do you have information about the temperature coefficient of this impedance ?

I thank  you for your time,

Best regards,

Jean-Baptiste

  • 0
    TI__Guru* 92655 points

    Jean-Baptiste,

    I wasn't able to find much information on the DAC reference input impedance. I've sent an email to the design group for more information. 

    This may take a couple of days, but I should be able to get some answers to your questions.

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points

    Jean-Baptiste,


    I did hear back from the designer, and the input impedance is 15kΩ. This impedance comes from a resistive voltage divider at the front end before the reference buffer.

    We estimate that the absolute value of this resistance is likely ±10%. As a resistor, it should be stable and constant over time. I don't have an exact temperature coefficient, but I would guess that it is about 20ppm/°C.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Hello Joseph,

    Thanks for your time.

    The fact that this impedance comes from a resistive divider answers my questions.

    However, I still want to point at the "30kΩ (typical)" value given in the datasheet (see pciture above). What does it mean, is it a msitake ?

    Have a nice day,

    Jean-Baptiste

  • 0 in reply to Jbjb
    TI__Guru* 92655 points

    Jean-Baptiste,


    You are correct, that should read 15kΩ typical. There are two places mentioning that in the text, but the electrical characteristics table is correct.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Thanks again for your answers Joseph. Now I fully understand the REFIN input impedance.

    Best regards,

    Jean-Baptiste