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ADS1120: VQFN thermal pad layout

JAP
Intellectual 2410 points
Part Number: ADS1120
Other Parts Discussed in Thread: TIDA-00095,

Dear Experts,

Is there any further instructions how to design layout for ADS1120 thermal pad and ground pins? According to datasheet p.4 thermal pad should be left unconnected or be connected only with AVSS. On the other hand, according to layout guidelines (p.61) AVSS and DGND should be connected as close to the device as possible. When looking TIDA-00095's layout, AVSS and DGND are connected to thermal pad which is connected to ground plane with vias. Is this the best way to route ground pins and thermal pad?

Thanks!

Rgds,

Juha

  • 0
    TI__Guru** 112725 points

    Hi Juha,

    The ADS1120 in the QFN package had the large center pad for mechanical stability reasons and not thermal.  The ADS1120 is considered a low-power device and does not generate a lot of heat requiring dissipation to a thermal plane.

    If the pad is connected to a signal/source, it should be AVSS.  In a unipolar supply case, AVSS would be connected to GND.  However, in a bipolar supply case, AVSS could be below GND by as much as -2.5V.  How you connect the device depends on the supply source.  As far as routing, you make the connections that make the most sense for the PCB layout.  In a unipolar supply case, you could connect all the grounds to the center pad, and then connect the center pad to GND if that makes the most reasonable PCB routing.  But you may not want to connect the device that way, as there is not requirement that the pad be connected to a source.  The only requirement is the pad be there and the device soldered to it for mechanical stability reasons.

    Best regards,

    Bob B