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ADS8638: Reference output voltage

Hirotaka Matsumoto
Guru 27585 points
Part Number: ADS8638

Hi all

Would you mind if we ask ADS8638?

Our customer uses an internal voltage reference with  decaupling cap=10uF between REF and REFGND.

On our customer's boards, the result of REF voltage(9 boards all) were average 2.67V. Tolerance of the 9 boards are approxy ±0.6%.

On the datasheet, Typ voltage is 2.5V, tolerance is ±1.2%.

Why is there some difference?

And then, conditions are follows;

<Condition>

AVDD=5.0V,  DVDD=3.3V, HVDD=5.0V, HVSS=GND

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 40935 points

    Hello Matsumoto-san,

    Thanks for your query on E2E.

    The specifications in ADS8638's datasheet were characterized at HVDD=10V to 15V and HVSS=-10V to -15V, also there is a requirement for VHVDD and VHVSS,  10V<VHVDD - VHVSS<30V as shown in page 5 (Electrical Characteristics table) in the datasheet, but your customer's test condition(VHVDD - VHVSS=5V) didn't meet this requirement, please change the power supplies and check again.

    By the way,  the 10uF decoupling cap is good when using internal reference on ADS8638, also please place this cap close to REFP and REFGND pins to get better performance.

    Thanks.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Guru 27585 points

    Dale san

    Thank you for your reply!
    Our customer has an additional question for it.

    <Question>
    We tested using following conditions;
    AVDD=5.0V,  DVDD=3.3V, HVDD=12.0V, HVSS=GND
    (HVDD was changed from 5V to 12V to fit the value 10V ＜ VHVDD－VHVSS ＜ 30V.)
    It seems that meets the conditions of "10V ＜ VHVDD－VHVSS ＜ 30V" and  "HVDD = 10V to 15V".
    However, VREF voltage is 2.67V ave.
    So, unless HVSS is within –10V to –15V, will our customer not get VREF=2.5V(typ)?

    Or, in case of VHVSS=0V, does it require VHVDD=15V?
    VHVDD : MIN : 5V TYP: 10V MAX:15V
    VHVSS : MIN : -15V TYP:-10V MAX:0V

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 40935 points

    Hello Matsumoto-san,

    I just double-checked on ADS8638EVM board, the internal voltage reference is not related to HVDD and HVSS, and this internal voltage reference worked well(2.5V) for different supply situations: different HVDD/HVSS voltage even no HVDD/HVSS power supplies. The requirement for HVDD/HVSS is applied for internal front-end HV Mux.

    After checked datasheet again, the internal voltage reference is powered by AVDD, not HVDD and HVSS, please look at the block diagram as below also in datasheet:

    Definitely something else is wrong with the customer's device, maybe has been damaged.

    Thanks.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Guru 27585 points

    Dale san

    Thank you so much for your reply.

    We have questions about REFGND. 
    On our customer's board, REFGND doesn't connect to GND like as Figure73.
    Should it connect to GND?
    And according to it, is it possible to get VREF(typ)=2.5V(normal voltage)?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 40935 points
    Hello Matsumoto-san,
    Definitely REFGND should be connected to analog ground or the ground plane if a solid ground plane is used. This is the root reason for your customer's issue. Figure 37 in datasheet should be corrected and the ground connection for REFGND shown in Figure 38 is correct.

    Thanks&Regards
    Dale
  • 0 in reply to Dale Li
    Guru 27585 points

    Dale san

    Thank you for your reply.
    By connecting REFGND to analog ground, our customer got VREF voltage=2.50V.

    Figure 37 in datasheet should be corrected and the ground connection for REFGND shown in Figure 38 is correct.
    ->On the ADS8638's datasheet, each figures are follows;
    Figure37 : SINAD vs CHANNEL NUMBER
    Figure38 : SINAD vs FREE-AIR TEMPERATURE

    Is your mention which the datasheet shows Figure37 and Figure38 correct?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 40935 points

    Hello Matsumoto-san,

    This is a good news, thanks for update. I apologized for the mistake, the figure numbers should be 73 and 74 for REFEND connection.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Guru 27585 points

    Dale san

    Thank you for your reply!
    If you have the opportunity to update the datasheet, could you add the idication(REFGND-AGND should be connected) to the Figure73?
    This is cutomer's request.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 40935 points

    Hello Matsumoto-san,

    Sure, I will let System and Marketing colleagues know this and they will update it with other changes together next time. Thanks.

    Best Regards

    Dale