Choosing ADC Reference Voltage

[Yusuf]

Hello,

My problem is that I have a custom STM32WB55RG board and I want to use ADC reference voltage as 1V or as close as possible to 1 V. My questions are:

1- In datasheet, it states that "If VDDA> 2V, it should be 2<Vref<VDDA and if VDDA<2V, Vref should be equal to VDDA". If I decrease VDDA to 1.7 which is around its minimum,1.62V, could there be a problem with other peripherals etc?

2- Not in STM32WB55 Datasheet but in other datasheets such as  "STM32F103x8 Datasheet Table 9", it states that VDD-VDDA should be maximum 300 mV". Is it valid for STM32WB55RG too? And if it is, could there be problem if I decrease VDD to around 1.8V?

3- Would using an external ADC with SPI protocol be a feasible solution?

I use SPI1,SPI2,ADC1,RTC,USB,I2C1,TIM16 and RF peripherals in my software and there is a RGB LED in the board.

Sincerely,

Yusuf.

[AScha.3]

Hi,

>I want to use ADC reference voltage as 1V

Why you want this ? for what its needed?

+

1+2 ->

So if you want USB working, you need 3.0V minimum supply (VDD, VDDA, VUSB...all.)

and you could use Vref from external source as low as 2,0V . Thats it.

If you just want an 0..1V input range with best resolution, set VDD...VRef to 3.0V

and use an opamp (gain =3) to get , what you want.

[Yusuf]

Hello,

Thank you for your reply. Regarding noise and are problems, I don't want to use an pre-amplifier. My question is that I assume I can use VREFBUF in normal mode to get 2.048 V reference voltage internally or (VDDA-150 mV) if I use VREF in degraded mode. Is it feasible to use that by decreasing VDDA to 1.7 V?

Sincerely,

Yusuf.

[AScha.3]

No.

2,4V minimum, if using vrefbuf. (btw how should it do 2.x V ref, if supply is lower than that ? magic ? )

[Yusuf]

As you can see below which is taken from AN5690 Application Note, VREFBUF can be used in Degraded Mode if VDDA is lower than that value. In STM32WB55 Datasheet (Figure2), it states that it gives VDDA-150 mV as VREFBUF output which will be 1.55 for 1.7 V VDDA.

 

 

 

[AScha.3]

Right, but what is a "reference" good for, if its no more any exact reference voltage ????

Better dont play around with useless "reference" . 

Same as using a resistor to VDDA . :)

So you could : use a 1.8V linear regulator for supply and vdda+Vref+ , ADC working and clear defined reference voltage; like LP2985-1.8 , 1,8V +/- 0,5% .