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AM3352: ADC input resistance

Mike Pettus
Prodigy 210 points
Part Number: AM3352

The AM3352 8-channel A/D has an input impedance specification that is defined as a function of frequency.  Snip from the data sheet attached (page 97).  Using that formula for example, at 100 Hz input frequency, the input Z comes out to over 150 Meg ohms.  Implied by the formula is that at 0 Hz (DC) the input Z is infinite.  Is this correct?  Is there a DC leakage current specification?

Thanks -

M. Pettus

CTO, Vubiq Networks

  • 0
    TI__Guru 62835 points

    Input leakage is defined in the same table.  Parameter name "Pullup and pulldown switch current leakage Ileak", where the maximum value is defined as 0.5uA.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 210 points

    Paul,

    OK - we'll use that leakage current value.  We saw the switch current leakage under "Touch Screen Switch Drivers", which we would not have thought to be the A/D input specification.  Does this also mean that there are pull up or pull down resistance values associated with the A/D inputs?  This is a bit confusing.

    Thanks,

    Mike 

  • 0 in reply to Mike Pettus
    TI__Guru 62835 points

    During acquisition, the input is connected to a capacitor via an internal analog switch to sample the applied voltage.  The switch that connects the sample capacitor to the input and the capacitor itself is physically small, so the leakage associated with them is very small.

    The ADC has additional analog switches that connect to the inputs and are used to bias a resistive touch screen when using the touch screen feature.  These switches need to be significantly larger to bias the touch screen fast enough to provide a reasonable response time.  Therefore, they are the major contribution to input leakage.  This is why the input leakage parameter references the pullup and pulldown switches.

    The pullup/pulldown switches are very low impedance and should only be used for biasing a resistive touch screen.

    The input impedance parameter is primarily a function of the sampling capacitor impedance which is frequency dependent.  That is why it is defined with a formula that is a function of frequency.

    Hopefully this helps you understand.

    Regards,
    Paul