ADC REFSEL VGB Behavior

[nate]

Chip: NRF51822

Per the documentation the band gap voltage is 1.2V however on my board when I apply the voltage 1.5V to AIN2 i read the following value from the ADC: 209

This would imply that the total voltage that can be measured is ~1.8V

Is this expected? Shouldn't only report 255 for any value above 1.2V?

The following is my ADC config:

NRF_ADC->CONFIG	= 	  (ADC_CONFIG_EXTREFSEL_None << ADC_CONFIG_EXTREFSEL_Pos) 						
					| (ADC_CONFIG_PSEL_AnalogInput2 << ADC_CONFIG_PSEL_Pos)							/*!< Use analog input 2 as analog input. */
					| (ADC_CONFIG_REFSEL_VBG << ADC_CONFIG_REFSEL_Pos)								/*!< Use internal 1.2V (really 1.8V?) bandgap voltage as reference for conversion. */
					| (ADC_CONFIG_INPSEL_AnalogInputNoPrescaling << ADC_CONFIG_INPSEL_Pos) 			/*!< Analog input specified by PSEL with no prescaling used as input for the conversion. */
					| (ADC_CONFIG_RES_8bit << ADC_CONFIG_RES_Pos);									/*!< 8 bit ADC resolution. */

[nate]

Per the link you added Stefan the following comment was made: "the other hand, when applying voltage source to the ADC input pin with high impedance, additional gain and offset error is introduced which is different for different prescalers. Choosing high impedance voltage divider is on the other hand desirable to prevent high leakage current though the voltage divider. "

Is there an equation you have access to that can help me determine what the offset and gain will be based off introduced impedance from my circuit?