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ADS131M04: ADS131M04 Input Impedance

Elliot Greenwald96
Prodigy 40 points
Part Number: ADS131M04

Hi,

We're considering using the ADS131M04 for an application in which the ADC would be driven by an active filter.  We're interested if there is a more detailed model of the input impedance of the chip.  It seems from the input impedance section of the data sheet that the input is a switched capacitor network when the gain is set to less than four.  Is this correct? And if so, would it be possible to get information on the capacitor size and switch resistances?

Thanks,

Elliot

  • 0
    TI__Mastermind 24970 points

    Hello,

    Our expert matter on this is currently not available, and will get back to your question when they are.

    But the device does have a difference input impedance depending on the gain is being used.

    – 300-kΩ input impedance for gains of 1, 2, and 4

    – 1-MΩ input impedance for gains of 8, 16, 32, and 64

    In the datasheet, Equation 4 is provided to show how the input impedance behaves for gain setting of 4 or less, perhaps this can be of help while we get back to you.

    317 kΩ × 4.096 MHz / fMOD

    where:

    • fMOD is the ΔΣ modulator frequency, fCLKIN / 2

     

    Regards

    Cynthia

  • 0
    TI__Guru* 94285 points

    Elliot,


    I only have some of the information that you asked for in the post. You would start with what Cynthia provided for her answer.

    In this ADC, the input is sampled for at the modulator rate (fMOD). With the input oscillator frequency, this is what you get:

    fCLKIN = 8.192MHz
    fMOD = fCLKIN/2 = 4.096MHz

    The input equivalent resistance is 317kΩ. Because the input is a switched capacitor sampled, you can start with the resistance to calculate the input sampling capacitor size:

    Equivalent resistance: R = 1/(C*f)
    C = 1/(R*f) = 1/(317kΩ * 4.096MHz) = 1.3pF

    For the transmission gate switches, I don't have too much to go on for that size. Generally for this type of device, the sizes are small to reduce charge injection in the sampling. The resistances are also dependent on input voltage and supply voltage. However, I would estimate the resistances to be 500Ω - 1kΩ. Note that this applies to gains 1, 2, and 4. The internal settings are a bit different for higher gains.

    I hope that helps.


    Joseph Wu