11 Replies Latest reply on Nov 19, 2013 4:31 PM by user_14586677

    SAR ADC Oscillations

    content.librarian

       Hello, I am trying to use an SAR ADC for an application that requires low power and battery power.  I have the SAR input setup from an analog pin(high z).  Going into the pin, I have a DC power supply that is supplying 2.5VDC.  I have the ADC output reading into a uint16, and then going directly to the LCD.  

         

      -12-bit

         

      -100kSPS(tried 60k and 900k with bypass cap)

         

      -vref internal(tried bypass and external vref)

         

      -played around with IMO frequency a bit, but left it at 3MHz and PLL at 24MHz

         

      -software trigger(use startconv and stopconv then read)

         

       

         

      I took several readings, delaying 4 seconds between each reading.  The max and min decimal values are below.  2154 is 2.5V at my vdd of 4.75VDC.  

         

       

         

      Min/max

         

      -------

         

      2154/2165

         

      2156/2161

         

      2155/2168

         

      2153/2163

         

       

         

      Why are the outputs oscillating 4 bits?  I am really only getting 8 bits of resolution with this setup because of the oscillation..