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frackers
frackers
Senior
November 3, 2016
Question

stm32f3 ADC input conflict with opamp output?

  • November 3, 2016
  • 2 replies
  • 1386 views
Posted on November 03, 2016 at 09:28

I have a problem (I think) with PA6 in an STM32F303VCTx using it as an analog input. This pin is shared (along with other things) with opamp2_vout and even though I have not enabled any opamps, I think what I'm reading on ADC2 channel 3 is a steady level from the opamp and not my signal. It appears to operate in digital GPIO modes correctly.

The manual is very ambiguous about the opamp outputs - from the manual

''Important note: the amplifier output pin is directly connected to the output pad to minimize

the output impedance. It cannot be used as a general purpose I/O, even if the amplifier is

configured as a PGA and only connected to the ADC channel.''

Is this connection present even if the opamp is not enabled (I assume the default power-up state is not enabled) or is there some specific action I have to take to use this pin other than set it as an analog input in the GPIO setup and allocate the relevant ADC channel?

Thanks

#adc #stm32f3 #opamp
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2 replies

Amel NASRI
Amel NASRI
ST Technical Moderator
November 15, 2016
Posted on November 15, 2016 at 15:15

Hi frackers,

This note is applicable in following conditions:

  • OPAMP clock is enabled
  • OPAMP configured in follower or PGA mode

This is not the case for you as OPAMP clock is disabled.

Is it possible for you to check the same using another pin as ADC channel?

-Mayla-

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stefanwirt9
stefanwirt9
Visitor II
November 30, 2016
Posted on November 30, 2016 at 09:46

I have the same problem as frackers. I can’t connect the OPAMP output to ADC input.

I’m using a STM32f303CB with ADC3 and OPAMP3 in follower mode.

ADC and OPAMP work separately correct, but not if channel 17(OPAMP) is connected to the ADC.

I’ve been trying to solve the problem for a few weeks.

Is there any example code for this case? Or are there known bugs?

Thanks

hemogloben
hemogloben
Associate II
January 13, 2017
Posted on January 13, 2017 at 22:51

tuberide:

Just resolved this problem myself.  Channel 17 is for the OPAMP Reference Voltage, NOT the Opamp output (as described in the

http://www.st.com/content/ccc/resource/technical/document/application_note/0d/7c/06/14/ef/43/46/5b/DM00157785.pdf/files/DM00157785.pdf/jcr:content/translations/en.DM00157785.pdf

document (page 20)).  Instead, once your OPAMP is configured you need to configure the ADC to use a specific channel (for OPAMP3 and ADC3 it should be ADC3_IN1).
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