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ADS124S06: Unused pins

Oba
Genius 17275 points
Part Number: ADS124S06

Hello,

 

My customer has some questions about ADS124S06.

 

1.

The datasheet recommends to connect unused analog input pins to floating, AVDD or mid-supply for lower leakage current.

To minimize the noise (from external) effect, what is the best way for these unused analog pins?

2.

For unused GPIO pins, they are going to put 47Kohm pull-down.

Do you have any recommended pull-down resister value?

 

Regards,

Oba

  • +1
    TI__Mastermind 23235 points

    Hi Obata-san,

    I would argue that tying unused analog inputs to AVSS would be the preferred option if noise coupling from external sources might be a concern.

    47kOhm sounds like a reasonable pulldown resistor value for unused GPIOs. There is no specific value required. But if you are concerned about noise pickup then of course you should not make the resistor 1MOhm or anything that large.

    Regards,
    Joachim

  • 0 in reply to Joachim Wuerker
    TI__Genius 17275 points

    Hello

    The customer has an additional question related to this topic.

    When connecting unused analog input pins to AVSS, will the AVDD current increase?

    Regards,
    Oba

  • 0 in reply to Oba
    TI__Guru** 113765 points

    Hi Obata-san,

    Leakage current is a complex discussion.  When connecting the analog input to AVSS the highest leakage current occurs at high temperature with the bulk of the leakage current coming from the ESD structures.

    As an example, note the datasheet figure shown below that as the absolute input voltage is near zero and the ADS124S06 is operating at high temperature the leakage current is the highest.

    Also note that the current is less than 2nA.  As I already stated this is a complex topic and the graph also includes all leakage and currents for the device while sampling.

    Consider that all diodes have a reverse leakage.  Some ESD protection diodes can be 10 to 100uA of leakage current, so 2nA is many orders of magnitude less and is insignificant to the overall supply current required in the system.  If an ESD cell is connected to DVDD then the leakage would come from the DVDD supply.  For the analog inputs, the ESD cell is connected to the AVDD supply so the leakage current would flow from AVDD.

    There tends to be a lot of confusion regarding the recommendations in the datasheet regarding the unused analog input connections.  At room temperature notice that the total current is perhaps a couple of pA or less.  In contrast the higher temp leakage current is much higher but in the end is insignificant to the overall system current consumption.  Just to be clear, the leakage shown in the graph above happens at high temp, and is inconsequential to the overall current required by the ADS124S06 for device operation.  Total system leakage from switches, capacitors and diodes will be much higher than the ESD cell leakage current of the ADS124S06.  So although the current appears to be significant relative to the room temperature, the current is really not that great even in worst case.  So this should be a non-issue.

    Best regards,

    Bob B