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ADS8028: REF pin will keep 2V when POR

Gary Zeng
Intellectual 1105 points
Part Number: ADS8028

Tool/software:

Hi Ti experts,

My customer is using ADS8028 in PLC IO module and as Analog input measurement.

Now the issue is that REF pin will keep 2V and DVDD/AVDD keep 1V when IC PORs. They use external REF, which is 3V. They use same 3.3V for powering AVDD and DVDD and they are behind of external 3V.

Ch1 is AVDD and DVDD; Ch2 is External REF

1. When they disconnect the external REF with IC, AVDD and DVDD will not have 1V step.

2. When they disconnect the AVDD/DVDD, figure is same as pic1 and REF will keep 2V and AVDD/DVDD will keep 1V.

As datasheet mentions, customer needs to config EXT_REF bit register after 500us of DVDD POR. Default ref is internal ref. So I think it is caused that when IC PORs, internal ref and external ref are working at the same time. Voltage flows from External REF to AVDD/DVDD through internal REF.

This issue now have no effect to system but customer want to know if there is any potential risk. Like 1V DVDD will cause ADC core can not start when 3.3V DVDD is coming.

  • 0
    TI__Guru**** 180771 points

    Hi Gary,

    Can you provide a bit more detail on the o'scope capture?  Is the reference powered on before AVdd/DVdd is powered up?  What is the time base (it's hard to read in the picture)? What causes that jump up to 3.3 on CH1?

  • 0 in reply to Tom Hendrick
    TI__Intellectual 1105 points

    AVDD/DVDD is 750us behind of VREF powered as pic shows.

    The 1V of AVDD/DVDD you see is caused by VREF through IC inside. After 750us, AVDD/DVDD is powered by a 3.3V LDO.

    Does it have any risk such as IC can not successfully power on?

  • 0 in reply to Gary Zeng
    TI__Guru**** 180771 points

    Hi Gary,

    You are violating the ABS MAX spec of the ADS8028 - Vref is not to exceed AVdd.  Since you essentially are backdriving the ADC through the REF pin there is no way to guarantee proper operation.  It is recommended to bring up the AVdd first and then enable the external reference.