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ADS8684: ADS8684 External Reference (/REFSEL, REFIO) behavior during power-up

Ryu Yamashita
Genius 4915 points

Part Number: ADS8684

Hi team,

Since I haven't received a reply, I'm reposting this.

I am evaluating the ADS8684 (ADS868x family) and have questions regarding the external reference configuration and its behavior during power-up.
Background:
  • External reference is applied to the REFIO pin.
  • /REFSEL is tied to DVDD as shown in datasheet section 8.3.8.2 “External Reference” and Figure 62.
  • Internal reference is disabled by /REFSEL = High.
Questions:
  1. During power-up, when /REFSEL is tied directly to DVDD, is there any condition where the internal reference could be briefly enabled before /REFSEL is recognized as High?
    In other words, is there any possibility that the internal reference output and the external reference applied to REFIO could conflict momentarily during DVDD ramp-up?
  2. If REFIO is driven by an external reference while AVDD = 0 V (for example, AVDD ramps after the reference is already present), is this condition supported?
    • If supported, are there any constraints such as maximum injection current, required series resistance, or ramp-rate considerations?
    • If not recommended, what is the preferred power-up sequencing between AVDD, DVDD, /REFSEL, and external VREF?
  3. If the /REFSEL pin is left floating (not connected), is its logic state defined internally?
    • Does it default to internal reference or external reference mode, or is this considered an undefined condition?

Best Regards,
Ryu.

  • +1
    TI__Expert 7750 points

    Hello Ryu, 

    Apologies for the delay. 

    There is currently no power-up sequence recommended for the device, nor limited maximum injection current, required series resistance, or ramp-rate considerations for this specific case. 

    If using something like REF5040, this issue should be of low concern. Could you please share the type of external reference being used? is there a concern of a high current output from it? 

    Because the device goes into idle mode after start-up, so long as there in no conversion initiated or anything else done before DVDD is fully powered up and the internal reference disabled, it should be ok in most cases. 

    Best regards, 

    Yolanda