How can I use Vdda/Vref pin to get better range for ADC

[RGima.1]

Hello.

I need STM with external Vref 3V. But I don't need so much pins like 100pins in ST where we have special pin Vref. Could I use VDDA/Vref pin in STM32F103RC according to drawing belloow? I wanna connect precision voltage reference to VDDA on 3V, and normal 3V from other high power source for VDD. Is it possible or not? If is not possible could you explane why? I don't want use external ADC for this application because it is expansive.

Thank you.

[Ozone]

Reference voltage sources are designed for stability over a wide temperature range, but very limited currents.

The supply requirements of the MCU analog peripherals are way beyond those of reference sources.

I think getting an external version as stable as the internal reference in the way you want to do it, an expernal ADC would be cheaper.

[RGima.1]

I will compare this, thank you. Any way for me still interesting quesiton is it possible or not?

[Amel NASRI · Accepted solution]

Hi @RGima.1​ ,

Yes it is possible as long as you are aligned with operating conditions described in product datasheet.

My confirmation is also based on the content of AN2566 Getting started with STM32F10xxx hardware development:

To improve conversion accuracy, the ADC has an independent power supply that can be filtered separately, 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 the VSSA pin
• On packages with 64 pins or less, the VREF+ and VREF- pins are not available, they are internally connected to the ADC voltage supply (VDDA) and ground (VSSA).

But in such case, you need also to pay attention to the details of the section "Power supply schemes": The VDDA pin must be connected to two external decoupling capacitors (100 nF Ceramic + 1 µF Tantalum or Ceramic).

And to complete my answer, I suggest you to have a look to AN2834 for tips on How to get the best ADC accuracy in STM32 microcontrollers.

-Amel

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[waclawek.jan]

When constructing the power source for VDDA, you have to observe this:

JW