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ADS7870: Using external reference above 2.5 volts resulting in inaccurate measurements.

Jens Trydal
Prodigy 10 points
Part Number: ADS7870
Other Parts Discussed in Thread: REF3130

Hi!

We have been using the ADS7870 for a few years. Originally we used the internal 2.5 volt reference. However we recently redesigned and started to use an external 3.0 volt reference.
We then discovered that the measurements became very inaccurate in the range between 2.5-3.0 volts. The internal reference voltage has been deactivated via the internal registers. Do anyone have an explanation for this behavior? Can this ADC be used with external references above 2.5volts? The limits considering use of external references are not mentioned in the datasheet.


I look forward to any reply and help resolving this issue.

Kind regards Jens Trydal

  • 0
    TI__Mastermind 24970 points

    Hello,

    How are you connecting the external reference? is it connected through pin 27 or through Pin 28?

    I suggest connecting it through pin 27, this will provide a driving buffer to the ADC to make sure the input is well settled through the conversion process. 

    If using pin 28, what reference are you using? is there a driving buffer?

    I believe the accuracy is due to degradation of the reference voltage. 

    Also, note that VDD needs to be at or higher than the reference voltage.

    Regards

    Cynthia

  • 0
    Prodigy 10 points

    Hi

    Thanks for your answer, the delayed response for my part is due to summer vacation.

    We have been using pin 28 as the connection for the external reference. The VDD that are being used are 3.3 Volts, witch is higher that the reference voltage of 3.0 Volts.  We are using the REF3130 from TI, https://www.ti.com/store/ti/en/p/product/?p=REF3130AIDBZT .

    Another observation that are possibly unrelated: 

             - When using the internal reference, at 2.5 Volts, and the internal buffer is activated.

             - The output from the buffer is measured to be 1.87 Volts, not 2.5 as I think it should be.(PS: This is of course with the external reference disconnected)

             - This fact does not affect the results. The ADC operates as it should with an 2.5 Volt reference.

             - When using an internal reference of 1.15 Volts, the output from the buffer is 1.15 Volts.

    Schematic of connection to ADS7870:

    Measured error on ADS7870 with 3.0V ext ref:

  • 0 in reply to Jens Trydal
    TI__Mastermind 24970 points

    Hello Jens,

    No worries, 

    Unfortunately many file sharing methods are not supported on our machines. I can send you an invitation to use a file sharing method that is supported, or we can take this conversation to an email thread where sharing individuals files is easier. Please me know which method to use, and I will send to your TI registered email. 

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 10 points

    Well, it's easiest for me to share a OneDrive-link.

  • 0 in reply to Jens Trydal
    TI__Mastermind 24970 points

    This has now been taken offline.