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.
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