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ADS131M08: LDO/PMIC recommendation for a 3.3 volts supply

Drake Jest
Prodigy 240 points
Part Number: ADS131M08
Other Parts Discussed in Thread: ISOW7841, , LP5907, , ISO7741

Hello,

I would like to ask some recommendations for a PMIC/LDO for the ADC with only 3.3 volt power supply available, The power supply comes from a ISOW7841 digital isolator with power , which specifies a 100mV pk-pk. Are there LDOs available can suppress this noise for the ADC to perform its best.

The ADC will be using the internal reference for its reference voltage.

  • 0
    TI__Guru* 97825 points

    Drake,


    One possible solution would be to use the LDO that is on the ADS131M08EVM. If you look at the schematic, the EVM uses an LP5907MFX-3.3/NOPB. This device outputs 3.3V but power is derived from the 5V usb power which is likely going to be noisy.

    Looking at the datasheet, the LDO has good PSRR specifications and the EVM seems to perform well with this LDO.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 240 points

    Hi Joseph,

    It does have good PSRR specs, but since my the supply and signal voltages from the ISOW7841 are 3.3v i am skeptical if the PSRR of the LP5907(3.3 volt version) would still be present.

    On the other hand if i go with a smaller voltage lets say LP5907(3 volt version) im afraid the data pins of the ADC would be stressed compared to the  50mV dropout at 100mA  of the 3.3 version.

    So i guess my question now is would the PSRR of the LP5907 still be active for inputs lesser or equal to its output voltage?

  • 0 in reply to Drake Jest
    TI__Guru* 97825 points

    Drake,


    Ok, now I understand. I just wasn't clear on the operation of the ISOW device. There are a couple of different options that I think you have.

    Because the ISOW7841 integrates the power and sets the voltage to 3.3V, I don't think any LDO could effectively do the cleanup if you tried to keep the voltage at 3.3V. One recommendation would be to use some sort of boost after the ISOW7841 and then use the LP5907 to lower it back down to the 3.3V level. I don't know enough about those devices to make a recommendation.

    Another solution would be to keep the LP5907 and use a 3V supply for the device. One issue that might be a problem is the isolation side input for the ISOW7841. You would need to make sure that the digital outputs for the ADS131M08 go high enough to be detected as a high by the isolation. Looking at the ISOW7841, it looks like the input detection is 0.7 of the supply. Assuming this is 0.7*VISO, it looks like it would still work, but I would still check with the apps engineers that support this device.

    Lastly, you could skip the ISOW7841 and separate the solutions for power and digital interface. You would use something like an SN6505 for isolated power and then an ISO7741 for the digital isolation. You'd still need to use an LDO to clean up supply noise, but the supply for the isolation and ADC would be the same.

    I'm not really that familiar with these isolation products, so I would post in the appropriate forum if you had questions on any of those solutions.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 240 points

    The second solution is the most favorable to me since little would be changed from the current prototype design. Im worried about the data pins of the ADC though since it will be receiving  VISO – 0.1 (3.2V) just 0.1 volt shy fom the VDD + 0.3 absolute maximum rating of the ADC. Is the ADC built to handle that extra bit of stress for a continous time?

  • 0 in reply to Joseph Wu
    Prodigy 240 points

    the ISOW7841 has to go, and i will be replacing it with a digital isolators that has no integrated power, and will be placing an additional isolator that is power only. This is the best compromise i can think of.

    Thank you very much for the LP5907 recommendation. Say does TI has a better regulator than this? Not that it is not enough, im just curious if you guys have a series that perfoms better.

    Have a great day.

  • 0 in reply to Drake Jest
    TI__Guru* 97825 points

    Drake,

    That's probably the solution that I would have used. For the regulator, that might be worth posting on the power management forum. I not particularly familiar with their newer devices.

    Good luck on your project. Post back if you have more questions about the device, or if you need to have your schematic reviewed.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 240 points

    Hi Joseph

    I would actually take you on that offer. 

    So in the isolated section of the circuit is powered by the SN6505 (5v out) , there are two thing that needs to be powered the ADS131 and a signal only digital isolator  ISO7741 would it be alright to power the ISO7741 using the same 3.3volt regulator(LP5907) as the adc? or its better to use another regulator (coming from the same source SN6505) so that the ADC is the only one connected to the ultra low noise LDO ?

    Thank you very much

  • 0 in reply to Drake Jest
    TI__Guru* 97825 points

    Drake,

    I'd be comfortable using the same regulator for the ISO7741 as the ADC. The ISO device doesn't appear to take much more than a handful of milliAmps of current. This seems like a rather light load for the LDO.

    However, I generally support data converter devices. I'm not an expert with any of the ISO or LDO devices, including the SN6505. You may want to post in the forums for those devices just to get a definitive answer.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 240 points

    Hi Joseph,

    Thanks for that, I was worried that transients from other devices under the same LDO as ADC and it might affect the accuracy.

    If you are comfortable then so will i be, afterall you know better than me (my first high-end ADC design).

    Thanks again Joseph, have a great day !