VREF+ in STM32L1xx

[mmsya]

Posted on May 18, 2018 at 00:53

Dear all,

I have question regarding the VREF+ please.  My understanding is that in STM32L we only have one VREFINT which is about 1.2xx Volts and this is not a reference Voltage to the ADC.  It is directly connected to the ADC_IN17 input. 

. It enables accurate monitoring of the VDD value (when no external voltage, VREF+, is available for ADC).

I am not sure what does this statement '

when no external voltage, VREF+, is available for ADC' mean?  Does this mean the VREF+ pin should be floating?

If for other channels I need a reference Voltage let's say some where between 1.8 and 2.4V we have to supply it externally?  Basically is there any reference Voltage  (Not VREFINT) available internally?

My understanding is VREF+ should always be less than or equal VDDA.  I believe we can select internally VREF+ to VDDA but I am not sure how.Can we select the ADC_IN17  channel reference Voltage VDDA internally and other channel other VREF?

Thanks,

ALi.

[T J]

Posted on May 19, 2018 at 01:17

not quite.

there are 2 references we are talking about,

an external reference coming into pin 21 on your chip.

it has to be close to the Vdda rail voltage.

Are you using 1.5V  Vdda rail ?

the Internal Reference is Called VREFINT and is approx 1.2V, You can look it up.

If you dont have an external Reference Pin, you can use the Internal reference.

You have both, they are mutually exclusive.

[T J]

Posted on May 19, 2018 at 01:25

It seems to be different than usual, I will admit, I haven't used your selected processor before.

in the absolute maximum ratings, Vref + cannot exceed Vdda by more than 0.4V, but there is no lower limit.

In section 6.3.1 it shows that Vssa can be as low as 1.8V., below that and you can expect some issues in the resultant ADC/ DAC values.