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                   ADC Channels DMA Configuration function
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  This section provides a function allowing to configure the DMA for ADC 
  channel.
  Since converted channel values are stored into a unique data register, 
  it is useful to use DMA for conversion of more than one channel. This 
  avoids the loss of the data already stored in the ADC Data register. 
  
  When the DMA mode is enabled (using the ADC_DMACmd() function), after each
  conversion of a channel, a DMA request is generated.