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ADS117L11: Properties of reference sampling circuit

Part Number: ADS117L11

Hello,

I am designing a board that includes the ADS117L11 AD-converter. Right now I'm having a hard time dimensioning the reference driver (I want it to be fully differential, so I need to buffer Refp and Refn). In Page 30 in the datasheet a simplified circuit for the reference input is given, while the internal switsches switch with fmod (in my case 26 MHz)

Now depnding on the values of C_REF and the parasitic switch resistance (and maybe also pin parasitic inductance), I need to design the driver different. Is there any Informaltion on these values?  Especially the capacitance is hard to guess.

Thank You and best regards,

Christian

  • Hello Christian,

    Cref is approximately 9.5pF.  The internal switch resistance is around 50Ohm, but this does not really impact the external driver behavior as this value determines internal settling to an ideal input source.

    Assuming you have a relatively large external reference capacitance of 100nF or more, then you can model the input as an effective resistance:

    Rref-eff = 16k || (1/Cref*f-MOD)  Note: there is an additional load in parallel with the switched capacitance equivalent to 16kOhm.

    Cref = 9.5pF

    f-MOD = f-CLK/2 for high speed mode, internal clock divider 1

    For example, the equivalent input resistance with f-CLK=25.6MHz, high speed mode.

    Rref-eff = 16k || 1/(9.5p*12.8M) = 5.43kOhm

    Reference input current with Vref = 2.5V, Iref = 2.5/5.43k = 460uA, which agrees closely with Figure 6-56.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • Thank you, that helps! The resistance is important as it dampens the serial resonance between the external capacitance, the sampling capacitor and the parasictic inductance of pins, and the cap. But with 50 ohms this shouldnt be a problem.