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EThom.3
EThom.3
Senior II
April 2, 2025
Solved

Two VREF+ pins on STM32G474Q?

  • April 2, 2025
  • 13 replies
  • 4321 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.

 

13 replies

mƎALLEm
mƎALLEm
ST 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?

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mƎALLEm
mƎALLEm
ST 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).

 

To give better visibility on the answered topics, please click "Best answer" on the reply which solved your issue or answered your question.
EThom.3
EThom.3Author
Senior 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.

    MasterT
    MasterT
    Lead II
    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?

    EThom.3
    EThom.3Author
    Senior II
    April 2, 2025

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

    MasterT
    MasterT
    Lead II
    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.

      EThom.3
      EThom.3Author
      Senior 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.

      markproplayers
      Visitor II
      April 10, 2025

      Great observation, it seem amazing

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