2.1 Accuracy

All analog values are presumed to exist at the input to the A/D converter. The A/D converter quantizes or encodes specific values of the analog input into equivalent digital codes as an output. These digital codes have an inherent uncertainty or quantization error of ± 1/2LSB. That is, the quantized digital code represents an analog voltage that can be anywhere within ± 1/2LSB from the mid-point between adjacent digital codes. An A/D converter can never be more accurate than the inherant ± 1/2LSB quantizing error. Analog errors such as gain, offset, and linearity errors also affect A/D converter accuracy. Usually, gain and offset errors can be trimmed to zero, but linearity error is not adjustable because it is caused by the fixed-value ladder resistor network and network switch matching. Most quality A/D converters have less than ± 1/2LSB linearity error. Another major error consideration is differential linearity error. The size of steps between adjacent transition points in an ideal A/D converter is one LSB. Differential linearity error is the difference between adjacent transition points in an actual A/D converter and an ideal one LSB step. This error must be less than one LSB in order to guarantee that there are no missing codes. An A/D converter with ± 1/2 LSB linearity error does not necessarily imply that there are no missing codes.