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ADS1131 Mean of Zeff=500M

masahiko shimizu
Guru 11235 points

Hi,

Could you let me know the mean of Zeff=500M, please?
Is this value at opening three switch?
Or this value at closing three switch?
Or another mean?

best regards,
Shimizu

  • 0
    TI__Guru** 113765 points

    Hi Shimizu-san,

    The reference is sampled by a switched capacitor input stage sampling at the modulator rate of 76.8kHz.  The charge/discharge of the sampling cap will create charge to move from the VREF supply.  As this is a capacitor charging and related to a frequency we model this as average charge over a period of time to determine a value of impedance.  The reference input will appear as though 500M ohms is connected across the reference inputs. 

    Maybe another way to view this block diagram is if you were to replace the CBUF and switches (enclosed by the dotted line in Figure 2) with an impedance value (resistor), you would have an equivalent input impedance of 500M ohms.  This makes modeling of the reference input less complicated.

    Best regards,

    Bob B