Linearity errors for ADC are usually measured in LSB error. Why is the ADC INL measured in ppm?
ADCs accuracy specifically in DC is measured relative to LSB. DNL and INL are the most common DC accuracy parameter. When an ADC converts voltage to digital form, one digital code consist of a range of voltage. This voltage range that is equivalent to a single code is called an LSB,or least significant bit. An LSB is define as the smallest step width voltage of a converter. One of the common ADC parameter is DNL. The DNL or Differential Non-Linearity measures the maximum deviation of the code width from the ideal for an specific code of ADC conversion. On the otherhand, Integral non-linearity or INL, is the summation of all the DNL error from Code 1 up to code n, where n is the maximum code of the ADC.Both the DNL and INL are measured in reference to an LSB. LSB are computed as reference voltage over total number of codes. The total number of codes would be dependent on the converter resolution. 1 LSB in a 16-bit ADC is different from 1 LSB of a 20-bit converter. Example using a 3.3V reference, 1 LSB error for a 16-bit would mean 50.3uV error but 1 lsb error in 20-bit would mean 3.14uV. Although in this example, 16-bit and 20-bit will have the same 1 lsb error, they would differ measuring them in relation to voltage.