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LMT85: LMT85 capacitor loads

Lenna Yan
Intellectual 1960 points
Part Number: LMT85

Hi Engineers,

In LMT85 datasheet, when driving a switched sampling input on an ADC. It may necessary to add some filtering to minimize noise coupling. And there are two methods we can see. I'm not very sure the application for the two different output connections.

When we should choose a Cload greater than 1100pF?

And why we need a resistor when the Cload greater than 1100pF? Is Rs used for limiting the current? 

Thanks & best regards.

Lenna

  • 0
    TI__Expert 5695 points
    Hi Lenna,
    The Cload described in the data sheet includes the capacitance in the traces (parasitics) and the component ones. When this value exceeds the 1100pF there is a need to add a series resistor.

    Regards,
    Jose
  • 0
    TI__Expert 6835 points

    Lenna,

    To answer your questions:

    When we should choose a Cload greater than 1100pF?

    That's up to the designer.  The external capacitor size will depends on filtering and ADC requirements.

    And why we need a resistor when the Cload greater than 1100pF? Is Rs used for limiting the current? 

    Rs is being used as an isolation resistor for stability due to large capacitive load.  The output cap affects the phase margin of the circuit.

    Larger Rs can create an offset due to bias current so keep that in mind when sizing the components.

    -Kelvin