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ADS1248 Input impedance

Fred Wedemeier
Prodigy 75 points
Other Parts Discussed in Thread: ADS1248

Table 7 on page 25 of the data sheet defines +/- input current leakage as a function of sample rate. Is the leakage random from input to input on a given part, or does this spec reflect a variation from part to part? Would be great if there's "similar" leakage, regardless of value, on all inputs of a given part.

  • 0
    TI__Guru**** 181021 points

    Hi Fred!

    If you take a look at Figure 51 on page 24 of the ADS1248 data sheet you will see that the device contains a single PGA and each of the eight input channels gets muxed into its AINn and AINp inputs.  The impedance listed in Table 7 would be typical for all channels for a given device.

  • 0 in reply to Tom Hendrick
    Prodigy 75 points

    Tom,

    Thank you for the quick reply! What you described makes perfect sense when muxing inputs to one side of the PGA. I'll be reading high-impedance differential signals. From more study of the Electrical Characteristics table, I think my underlying question is answered by the "Differential input current" spec. My understanding of that number is, "regardless of the absolute value of leakage on PGA (+) and (-), they're typically matched to within 100pA." Correct??

    fcw

  • 0 in reply to Fred Wedemeier
    TI__Guru 55466 points

    Hello Fred,

    Yes, you are correct, the differential input current will be typically matched by 100pA.  The absolute input current is as described on Table 7, as a function of data rate and input voltage.  For example, at 20 SPS, the effective input impedance is in the order of 5000 MOhms with +/-(0.5nA + 0.1nA/V) input bias current.

    Best Regards,

    Luis