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ADS124S06: About REFCOM pin connection in bipolar supply case

Umut Kilci
Prodigy 10 points
Part Number: ADS124S06

Tool/software:

Hi all,

I'm debugging a power supply issue on a board. It seems that the power supply chip sees low impedance (or a soft short) at the output, which limits the current and causes the output voltage to droop.

This voltage rail is used as AVSS for the ADS124S06. I've discovered that the REFCOM pin is connected to GND instead of AVSS. Could this cause such behavior? I'm suspecting there may be a low impedance path to GND in this case.

  • 0
    Prodigy 10 points

    Also AVSS-SW pin is connected to GND.

  • 0 in reply to Umut Kilci
    TI__Guru 51726 points

    Hi Umut Kilci,

    The design of the internal reference is based on a connection to AVSS and if connected to some other point (such as DGND using bipolar supplies) the REFOUT could oscillate and not be stable. So you should modify your board to connect both REFCOM and AVSS-SW to DGND. It is possible that this miswiring is causing the issue you are seeing

    Are you generating higher voltage bipolar supplies and then using LDOs to regulate them? Or some other supply configuration?

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 10 points

    Hi Brian,

    Thanks for the prompt reply. Actually, two switching regulators are used to generate AVCC (2.5 V) and AVSS (–2.5 V).

    When I removed the AVSS connection to the ADC, the output voltage error disappeared. Then I reversed the setup and tried connecting REFCOM to AVSS instead of GND, while keeping AVSS-SW unchanged—but that didn’t work. I’m beginning to suspect that there might be an internal diode or PN junction between AVSS-SW and AVSS, with the anode connected to AVSS-SW. If AVSS-SW is connected to GND, this diode could become forward-biased, potentially shorting the voltage or creating a low-impedance path.

  • +1 in reply to Umut Kilci
    TI__Guru 51726 points

    Hi Umut Kilci,

    Thanks for the additional info. Yes this is why both pins (REFCOM and AVSS-SW) are required to be tied to AVSS in the datasheet

    -Bryan