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EThom.3
Graduate II
April 2, 2025
Solved

Two VREF+ pins on STM32G474Q?

  • April 2, 2025
  • 4 replies
  • 1303 views

When I first glanced through the STM32G474 datasheet, I saw VREF+ and VREF- on pins 43 and 44 on the LQFP128 package. "Cool", I thought, although I had no significant need for an actual VREF- pin.

However, when I was making a schematics symbol, I noticed that the two pins are in fact VREF+ and VREF+. Apparently, the exact same thing on two pins next to each other.

EThom3_0-1743606269300.png

Does anyone know why this is? It makes perfect sense to have multiple pins for supply voltage and ground. But for a reference voltage? I have found no explanation in the datasheet or the reference manual.

This question was asked in another thread in December, but no answer was provided. Thread with original question:

https://community.st.com/t5/stm32-mcus-products/vref-stm32g474qe/m-p/750453 

 

Edit: added screenshot

    This topic has been closed for replies.
    Best answer by mƎALLEm

    Hello all,

    Our feedback regarding this subject:

    The purpose to have two VREF+ pins is to decrease (to 1/2) the inductance of the bonding wires + lead frame. VREF+ is very sensitive to perfect decoupling (strong voltage) - but when the inductance of wire from pin to the pad on chip is too high then the VREF+ voltage on the die is not perfectly decoupled (there is large inductance). The inductance magnitude is too high on large packages - like LQFP128 (larger wires from pin to pad on chip). Smaller packages inductance is acceptable but for LQFP128 we should decrease this inductance and two VREF+ pins were designed there.

    So it's recommended to connect both VREF+ pins externally (even they are internally connected).

    I've already suggested (internally) to add a note on the pinout table 2 on both VREF+ pins to connect them externally.

     

    4 replies

    M
    mƎALLEm
    Technical Moderator
    April 2, 2025

    Hello @EThom.3 

    Are you sure this is the correct link? 

    It doesn't mention any VREF+ pin problem.

    Also check the package LQFP128 ?

    Could you please share a screenshot of the issue?

    M
    mƎALLEm
    Technical Moderator
    April 2, 2025
    @mƎALLEm wrote:

    Hello @EThom.3 

    Also check the package LQFP128 ?

    OK I was pointing to another STM32G4 datsheet.

    I will check internally about these two VREF+ pins on that package (I think one of them is VREF-). I'll get back to you as soon as I have an answer (Internal ticket 206667).

     

    E
    EThom.3Author
    Graduate II
    April 2, 2025

    Thanks.

    According to the datasheet, VREF- is connected to VSSA. But in case one of the pins really is VREF-, it would be crucial to know which is which.

    It seems a bit unlikely that no-one has raised the issue before, if that is really the case.

      M
      MasterT
      Explorer
      April 2, 2025

      Data sheet clearly states that VFRE- is bounded to VSSA, so the reason why two VREF+ I'd think to separate VREF for manny internal analog consumers, ADC/ DAC-slow/ DAC-fast/ OPA/ COMP.

      Can you measure resistance between pin 43 & 44, when IC is unpowered?

      E
      EThom.3Author
      Graduate II
      April 2, 2025

      I don't have a device yet. I am in the beginning of the schematic drawing phase.

      M
      MasterT
      Explorer
      April 2, 2025

      I find stm32g474QET6 on STM32G474E-EVAL evaluation board, schematics indicates that 43 & 44 externally connected. 

      Chances both pins likely connected inside as well, than reasoning to minimize inductance/ resistance/ interference in between analog peripheral devices. 15 msps DAC's potentially could significantly impact adc SNR performance.

        E
        EThom.3Author
        Graduate II
        April 2, 2025

        Thanks! :thumbs_up:

        I tried to find the schematic, but didn't succeed.

        I agree with you that they are most likely internally connected. The reason for it, I won't even try to guess.

        M
        markproplayers
        Visitor II
        April 10, 2025

        Great observation, it seem amazing

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