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ADS8504 internal voltage reference

Tom Saluzzo
Expert 1280 points
Other Parts Discussed in Thread: ADS7804, ADS8504, ADS8515

The TI analog to digital converter, ADS8504IBDW is used in an application where the device is connected to use the internal voltage reference. In the datasheet, there is a note that states:

 (8) The internal reference may not be started correctly beyond the industrial temperature range (-40 Deg. C to 85 Deg. C), therefore use of an external reference is recommended.

 This device is a direct upgrade for the ADS7804 which was previously in the design.  The ADS7804 does not have a similar note.   It may be possible for the device to exceed this temperature range in the application.

 Can someone at TI to elaborate on that note, specifically the following questions please?

 The latest datasheet I find on the TI site is dated 2007.   Has there been any updates to the datasheet since then?

 Is the temperature of -40 Deg. C to 85 Deg. C an ambient temperature or an equivalent device junction temperature?

 At approximately what temperature does the reference fail to start up?

 What exactly is it about the reference that causes it to not start up?  What type of reference is it that it will not start up?

 What is the different between the ADS7804 and ADS8504 references such that one works over a wider temperature and one does not?

 Under the REFERENCE section of the datasheet, it also talks about adding the external reference to Pin 5, but pin 5 is AGND2.   I assume this should be pin 3, the REF pin.   Is the datasheet incorrect here?

 Is there a pin for pin compatible device in TI’s lineup that is suggested for upgrading to from the ADS8504 which does not have this issue?

 There is a 16-bit equivalent ADC, the ADS8515 which has a similar note.   Does this part have a similar reference as the ADS8504 device and therefore the same issue?

  • 0
    TI__Expert 6925 points

    Hello Tom,

    This thread will be moved to the precision data converters forum.

    Since the ADS8504 was specifically designed to be the upgrade for the ADS7804, there isn’t another device that is pin-to-pin compatible with the ADS7804.

    Please note that the ADS8504 doesn’t have a reference issue. Any device used outside its specified operational temperature range is not guaranteed to work or meet datasheet specs. This applies to both the ADS8504 and ADS7804 that have the same operational temperature range of -40°C to +85°C.

    We are currently investigating the differences between ADS8504 and ADS7804 references and should be able to provide an update by the end of next week.

    Datasheet information:
    - The one revised in 2007 is the latest one. No other revision has been done since then.
    - There is a typo in the reference section. It is incorrect to mention pin 5 as reference. Both ADS7804 and ADS8504 datasheets will be submitted for update, but it could take more than a month to have both datasheets updated in the TI website.

    Regards,
    Rafael

  • 0
    TI__Expert 6925 points

    Hello Tom,

    I forgot to clarify that the operational temperature range of -40°C to +85°C is ambient temperature for both the ADS7804 and ADS8504. Also, sorry about the “investing” typo in my previous post.

    Regards,
    Rafael

  • 0 in reply to Rafael Ordonez
    Expert 1280 points

    Please reply with the results of your investigation of the differences between the ADS8504 and ADS7804 references.

  • 0 in reply to Tom Saluzzo
    TI__Expert 6925 points

    Hello Tom,

    I'm sorry for the delay.

    The reference voltage is generated by a band-gap reference in both ADCs. The design team informed that the reference style is the same for both ADCs and confirmed that the ADS8504 band-gap reference is able to start-up even outside the operational temperature range. This also applies to the ADS8515. However, the overall device performance specs cannot be guaranteed outside the operational temperature range.

    Regards,
    Rafael