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ADC0820-N: Vref-/Vref+

Shuji Ishiwata
Mastermind 7845 points
Part Number: ADC0820-N

Tool/software:

Hi All,


I have some questions about the ADC0820-N.

(1) There is an internal resistor of 1.9kΩ between pin 11 (Vref-) and pin 12 (Vref+).
     Is there any documentation showing the accuracy of this internal resistor? For example, 1.9kΩ ±1%.

(2) I am thinking of setting Vref- and Vref+ by voltage division using external resistors. Is this okay?
     Will this affect the internal resistance?

(3) Is it better to supply an LDO or reference power supply directly to Vref?

Best Regards,
Ishiwata

  • 0
    TI__Genius 15150 points

    Hello Ishiwata-san,

    1. I did not see where the 1.9kΩ resistance was shown. Can you share the figure or line where that is found?

    2. Because the DAC uses precision matched resistors, they will have the same voltage drop across them, so a voltage divider to provide the voltage on Vref(+) is okay.

    3. Voltage references in general will be cleaner supplies, but for an 8-bit converter, an LDO should be sufficiently low enough in noise. As a rule of thumb, the LDO noise should be at least 10dB lower than the LSB size of the ADC. For example, if the LSB size is 1mV, the LDO noise should be less than 1/sqrt(10)mV.

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Mastermind 7845 points

    Hello Joel-san,


    Thank you for your reply.

    When a customer measured the resistance between pin 11 (Vref-) and pin 12 (Vref+), it was found to be 1.9kΩ.

    Which is better for the power supply to Vref(+): a resistor divider or an LDO?

    Best Regards,
    Ishiwata

  • 0 in reply to Shuji Ishiwata
    TI__Genius 15150 points

    Hi Ishiwata-san,

    Apologies for the delay.

    Understood. That looks to be the equivalent resistance of the resistor ladder. The error in the matching of the resistor network is included in the total unadjusted error (TUE) spec in the datasheet, but they are not specified directly.

    One of the concerns that might come by connecting the resistive divider directly to the Vref+ pin is that it presents a load, so you should add a voltage follower to buffer the voltage. You also have concerns with the tolerance of the resistors. With that said, an LDO or precision reference will likely be more accurate. At 8-bit accuracy though, it is not too great of a concern.

    Regards,
    Joel