How to handle conflicts between reference manual and reference design about ADC of STM32

adcstm32

On page 207 of stm32 reference manual, it says that VSSA(power supply of ADC, negative) must be tied to VREF-(analog reference, negative). But when I refer to reference design of STM32(Evaluation board).On page 39, it shows a conflic design:
enter image description here

when i choose not to solder R41 and apply a real negative voltage (-3v3) to VREF-, the two pins(VREF- and VSSA) do not connect. So, which one is the right solution?

In addition, VREF+ VREF- VDDA VSSA have relations to accuracy, so I choose to add two magnetic bead:
enter image description here

Is that reasonable?

Best Answer

You must not connect a negative voltage to VREF-. The "-" only denotes that it's the lower voltage. If you check the manual, p. 209 it will state:

VREF-: The lower/negative reference voltage for the ADC,VREF- = VSSA

and for VSSA:

VSSA: Ground for analog power supply equal to VSS

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