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?

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    \$\begingroup\$ The documentation states that those pins must be connected, and the reference design does so via that 0 Ohm resistor. Where is the conflict? \$\endgroup\$ – Wouter van Ooijen Jul 1 '13 at 9:36
  • \$\begingroup\$ @WoutervanOoijen. what if i remove 0 ohm resistor and apply a negative voltage to VREF-? the VSSA and VREF- can not be connected. \$\endgroup\$ – oilpig Jul 2 '13 at 0:57
  • \$\begingroup\$ Then you violate the instructions in the reference manual and you will live in interesting times. \$\endgroup\$ – Wouter van Ooijen Jul 2 '13 at 5:37
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    \$\begingroup\$ There are valid reasons why the reference design has R41. There may be other chips that use the same reference design but do not tie VREF- to VSSA. The board designers may have left an option to add a chip inductor in place of R41. The board may have been designed before the reference manual was written. In any event, the reference manual always takes precedence over any particular reference board. \$\endgroup\$ – Joe Hass Sep 8 '13 at 13:16
  • \$\begingroup\$ There is no conflict, it's connected. \$\endgroup\$ – KyranF Jan 27 '16 at 5:47

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

  • \$\begingroup\$ Exactly, all references to "vref-" in both the datasheet and EVM board manual show that "Vref- = VSSA = GND" \$\endgroup\$ – Passerby Sep 8 '13 at 13:21
  • \$\begingroup\$ There was never any question of connecting a negative reference - rather the possibility (ruled out by the manual) was of connecting a positive voltage greater than the potential of VSSA. \$\endgroup\$ – Chris Stratton Jan 27 '16 at 5:49

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