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LPC54005 ADC offset from zero

Question asked by David Kaplan on Jul 17, 2019
Latest reply on Jul 26, 2019 by xiangjun.rong

We have a custom board based on the LPC54005 MCU.

The board runs with FreeRTOS at 180M by calling BOARD_BootClockPLL180M().

We set the ADC to use the Synchronous mode with a CLKDIV of 3 to arrive at 60MHZ ADC clock.

We also set the  TSAMP to 3 and RESOL to 12Bit.

We added to symbols:

FSL_FEATURE_ADC_HAS_CTRL_RESOL=1
FSL_FEATURE_ADC_HAS_CTRL_ASYNMODE=1
FSL_FEATURE_ADC_HAS_CTRL_BYPASSCAL=1
FSL_FEATURE_ADC_HAS_CTRL_TSAMP=1

 

When we measure zero voltage at an ADC input we get a reading of 55 to 64 ADC units.

It seems that all ADC behave in the same manner with an offset from zero.

We use a VRef at +2.494VDC.

 

Is this normal and if not what am I doing wrong?

 

Thanks

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