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Uwe Bonnes
Uwe Bonnes
Chief
April 15, 2021
Solved

STM32 ADC limited common mode range

  • April 15, 2021
  • 2 replies
  • 1790 views

Hello,

for a switched capacitor ADC with differential inputs, I would expect no limits on common mode voltage as long as input voltage is in the GND/VDDA range. However STM32 ADCs only allow a small range of common mode voltage around (Vref- VDDa) /2. In some test circuit with common mode voltages outside that range I got (stable) values several percent off.

  • Why has the STM32 ADC this requirements
  • Is there characteristic data what happens outside the allowed range?

This topic has been closed for replies.
Best answer by Imen.D

Hello @Uwe Bonnes​ ,

STM32 ADC support only fully differential input signals (we do not support pseudo diff signals). This is by design.

It is forbidden to use the ADC outside of the specified VCM as the functional behavior is not guaranteed.

Hope this answer your question.

Imen

2 replies

I
Imen.DBest answer
ST Technical Moderator
April 16, 2021

Hello @Uwe Bonnes​ ,

STM32 ADC support only fully differential input signals (we do not support pseudo diff signals). This is by design.

It is forbidden to use the ADC outside of the specified VCM as the functional behavior is not guaranteed.

Hope this answer your question.

Imen

In order to give better visibility on the answered topics, please click on 'Best answer' on the reply which solved your issue or answered your question. Thanks
LaoMa
LaoMa
Associate III
January 28, 2022

Hello @Imen DAHMEN ,

I have a similar and bigger issue....

In case I have to sample a signal on In+ biased at Vdda/2 (Vdda/2 is then connected to In-) I suppose I won't have any trouble .....

But in case I have to make a differential conversion of a signal coming out from a thermopile sensor, whose values can be positive up to 2.7 V and negative down to -300 mV (signals are scaled from 0 to 3000 mV). According to the reply above, it appears that I cannot use the +300 mV bias as the In- reference, can I?

All DS are showing the differential conversion like a big feature, but really it has this limitation that should be outlined in the 1st page in a way that the designers will know that since the beginning.... in my case, I have no big problem to compute the differential value by the code but I need 2 conversions instead than 1.... it is not a negligible detail, is it?

Thanks for your advises

Maurizio

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