参考手册：(reference manual) To improve conversion accuracy, the ADC has an independent power supply which can be separately filtered and shielded from noise on the PCB. ● The ADC voltage supply input is available on a separate VDDA pin. ● An isolated supply ground connection is provided on pin VSSA. When available (according to package), VREF- must be tied to VSSA.
On 100-pin packages: To ensure a better accuracy on low voltage inputs, the user can connect a separate external reference voltage ADC input on VREF+ and VREF-. The voltage on VREF+ can range from 0V to VDDA. On packages with 64 pins or less: The ADC voltage supply and ground are internally tied to the ADC reference voltage pins.
数据手册：(data sheet) VREF+ can be internally connected to VDDA and VREF- can be internally connected to VSSA, depending on the package.
VREF+ and VREF– inputs are available only on 100-pin packages
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