Analog-to-Digital Converter - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 593

Voltage

Figure 12.2 ■ An ideal A/D converter output characteristic

2 n -1

Voltage

V DD

V DD /2 n

Figure 12.3 ■ Output characteristic of an ideal n -bit A/D converter

(the staircase may have unequal steps in some values) and nonmonotonicity (higher voltage
may have smaller code).
Obviously, the more bits used in representing the A/D conversion result, the smaller the con-
version error will be. Most microcontrollers use 8, 10, or 12 bits to represent the conversion result.
Some microcontrollers (mainly 8051 variants from Silicon Laboratory, TI, and Analog Devices)
use 16 bits or even 24 bits to represent conversion results. Whenever the on-chip A/D converter
cannot provide the required accuracy, an external A/D converter should be considered.