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ADS8638: thermal pad and pin #4 not connected and floating

Jon Bilbao
Intellectual 1415 points
Part Number: ADS8638

Hi, 

what happens if we left the thermal pad and the pin #4  not connected and floating in the ADS8638? 

it is recommended in the datasheet to connect them to: pin #4 to ground and thermal pad to the PCB. However, we found out that too late and we left both floating. 

What can happen now? 

Should we expect some failed operation or any parasitics influence in the ADC? 

Regards,

Jon

  • 0
    TI__Mastermind 40475 points

    Hello Jon,

    As you have seen the recommendation for the connections of thermal pad and NC pins in data sheet, these pins should not be floated and the package thermal pad must be soldered to the printed circuit board for thermal and mechanical performance. The thermal pad can significantly help to dissipate the thermal which can possibly damage the device, also it will make the performance worse because of the temperature drift caused by the self-heating on the device.

    Thanks.

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Intellectual 1415 points

    Hello Dale,

    Right now, customer wants to avoid changing the design because the product has been in product already several years.

    The impact of not connecting the thermal pad is clear. Now, what is the impact of connecting/disconnecting the NC pin (pin#4)?

    Is the NC pin connected internally to the silicon?

    As described above, the NC was left floating.

    What would change if they connect it to ground? Would there be any impact?

    For example, less noise, different behavior of the offset error? Especially, a change offset could be critical if in the floating design only positive/negative offset is possible and with the change the offset can be either positive or negative.

    Regards,

    Jon

  • 0 in reply to Jon Bilbao
    TI__Mastermind 40475 points
    Hi Jon,
    As I mentioned in previous post, one significant impact is drift to the performance, the self-heating of the device will lead to the worse drift performance(offset and gain drift etc.) if the therm pad is not connected to the ground. However, I hope you can understand that we only can guarantee what will not happen if the customer uses our device according to recommendation in the data sheet, for example, the ADC can work well and will not be damaged if thermal pad and NC pins are connected to the ground regarding the recommendation in data sheet. We can not guarantee what will happen if the customer does not follow up the recommendation in the data sheet. For NC pins, will have to check with design team if you really want to know internal circuit, it will take some time because this is an old database and I'm not sure about the design team's time slot.
    Best regards,
    Dale