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AM2434: How to calculate the input impedance of ADC

Hideaki Matsumoto
Genius 9090 points
Part Number: AM2434

Tool/software:

Hi,

The customer wants to place a low pass filter at the ADC input. Could you tell them the input impedance of ADC interface ?

 

According to the Note (2) of 6.7.8 ADC10B Electrical Characteristics (ALX package) in the datasheet,  it seems that the user can calculate an input impedance.

Could you tell them how it can be calculated ?

 

(2)  The ADC0_AIN pins are connected to an internal sampling capacitor for a user configurable acquisition time and acquisition frequency.

      The input impedance of the ADC0_AIN pins is a function of the sampling capacitance along with user configurable acquisition time and

      acquisition frequency. The designer must understand the time required for the source impedance of each ADC0_AIN pin to charge the

      internal sampling capacitor. The acquisition time must be set long enough for the internal sampling capacitor to settle to greater than

      14 bits of accuracy.

 

 Thanks and regards,

Hideaki

  • 0
    Mastermind 8945 points

    The ADC input model can be found in the TRM:

    The parameterisation values can be found in the datasheet. For example (AM263Px):

  • 0
    TI__Prodigy 410 points

    Hi Hideaki,

    The AM243x ADC input impedance depends on how well the source impedance charges the internal sampling capacitance within the programmed acquisition time and sampling frequency.

    Here is the procedure to get the equation for maximum source impedance and configurable number of acquisition cycles to fill the sampling capacitor (highlighted part is the final equation):




    Note: 

    • The formula is derived derived using 14-bit settling to ensure the system maintains12-bit system accuracy, as recommended in the datasheet.
    • Minimum/maximum t_ACQ values are given in the AM243x Datasheet, Section 6.7.8 ADC10B Electrical Characteristics (ALX package).
    • The associated register field is given in the AM243x TRM, Section 12.1.1.6.16 ADC_STEPDELAY_j Register.


    Regards,
    Karam Haddad

  • 0 in reply to Kier
    TI__Prodigy 410 points

    Thank you for the response, Kier! Unfortunately, the parameters you have shown don't exactly line up with what we provide in the AM243x datasheet and TRM. I have responded with a more tailored response to AM243x.

    Thanks,
    Karam