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ADS1278-SP: bottom exposed pad electrical connectivity

Erik Laurila
Prodigy 180 points
Part Number: ADS1278-SP

Is the exposed thermal pad on the bottom of the package electrically connected to anything?

  • 0
    TI__Mastermind 46200 points

    Hello Erik,

    Yes, the bottom thermal pad is electrically connected to the ground pins of the device.  The pad should be thermally connected to the board for heat dissipation.  It can be soldered, in which case it can use the board ground plane for heat dissipation, or it can be mounted using a thermal epoxy.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 180 points

    Hi Keith,

    On my board, I was planning to thermal bond the thermal pad to CHASSIS ground.  The thermal expoxy is an electrical isolator.  However, the ground pins of the device are connected to the GND plane of my card.  CHASSIS and GND are electrically isolated on my card (there is a 1megahom resistor between them).  Is this acceptable?

    Thanks, Erik

  • 0 in reply to Erik Laurila
    TI__Mastermind 46200 points

    Hello Erik,

    Connecting a different 'isolated' ground can couple noise into the ADC, especially if there are any high currents or ESD currents that can flow on the chassis ground.  Functionally, this will work, but we recommend connecting the thermal pad to the ADC ground plane for best noise performance.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 180 points

    Is the thermal pad electrically connected to the case lid or some other metal surface on the package?  We could run a wire from the case lid to GND for the electrical connection for example.  Then, we would still thermal bond the pad to Chassis to pull the heat out.

  • 0 in reply to Erik Laurila
    Prodigy 180 points

    Remember the thermal bond is an electrical isolator.  So the thermal pad will not be electrically tied to chassis.  Per the datasheet the lid and pad are connected to GND pins on the device.  The GND pins are soldered to the ground plane not chassis in my case.

  • 0 in reply to Erik Laurila
    TI__Mastermind 46200 points

    Hello Erik,

    Both the thermal pad and the lid are electrically connected to the ground pins of the IC package.

    Yes, using a thermal bond material that is an electrical isolator will prevent the flow of DC current, but the thermal pad and chassis will form a capacitor that can couple higher frequency noise from the chassis into the device package, and ultimately into the ADC readings.

    If the chassis will be electrically quiet, no high frequency current flowing, then you should be O.K. with this configuration.

    Regards,
    Keith