From the data sheet - this refers to bit 6 of the Status byte
"ADC error bit. This bit is written to at the same time as the RDY bit. This bit is set to indicate that the result
written to the ADC data register is clamped to all 0s or all 1s. Error sources include overrange or underrange,
or the absence of a reference voltage. This bit is cleared when the result written to the data register
is within the allowed analog input range again."
Are there any other conditions besides these that would cause the bit to be set?