
11
A/D CONVERTER
11-43
32185/32186 Group Hardware Manual
Rev.1.10 REJ09B0235-0110 May 15, 07
11.3.5 Accuracy of A/D Conversion
The accuracy of the A/D Converter is indicated by an absolute accuracy. The absolute accuracy refers to
a difference expressed by LSB between the output code obtained by A/D converting the analog input
voltages and the output code expected for an A/D converter with ideal characteristics. The analog input
voltages used during accuracy measurement are the midpoint values of the voltage width in which an A/D
converter with ideal characteristics produces the same output code. If VREF0 = 5.12 V, for example, the
width of 1 LSB for a 10-bit A/D converter is 5 mV, so that 0 mV, 5 mV, 10 mV, 15 mV, 20 mV, 25 mV and so
on are selected as midpoints of the analog input voltage.
If an A/D converter is said to have the absolute accuracy of ±2 LSB, it means that if the input voltage is 25
mV, for example, the output code expected for an A/D converter with ideal characteristics is H’005, and the
actual A/D conversion result is in the range of H’003 to H’007. Note that the absolute accuracy includes
zero and full-scale errors.
When actually using the A/D Converter, the analog input voltages are in the range of AVSS0 to VREF0.
Note, however, that low VREF0 voltages result in a poor resolution. Note also that output codes for the
analog input voltages from VREF0 to AVCC0 are always H’3FF.
H'000
H'001
H'002
H'003
H'3FE
H'3FF
→
A/D
conversion
result
(hexadec
imal)
→ Analog input voltage [V]
VREF0
1024
× 1
Ideal A/D conversion characteristics
A/D conversion characteristics with infinite resolution
0
VREF0
1024
× 2
VREF0
1024
× 3
VREF0
1024
× 1022
VREF0
1024
× 1023
VREF0
1024
× 1024
Figure 11.3.7 Ideal A/D Conversion Characteristics Relative to the 10-bit A/D Converter’s Analog Input Voltages
11.3 Functional Description of A/D Converter