Renesas M32R/ECU Series, User Manual

Page: 484 / 839

11
11-39
A-D Converters
32180 Group User's Manual (Rev.1.0)
11.3.5 Accuracy of A-D Conversion
The accuracy of the A-D Converters 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 char-
acteristics produces the same output code. If VREF = 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 Converters, the analog input voltages are in the range of AVSS to VREF. Note,
however, that low VREF voltages result in a poor resolution. Note also that output codes for the analog input
voltages from VREF to AVCC are always H’3FF.
H'000
H'001
H'002
H'003
H'3FE
H'3FF
→
A-D con
v
ersion res
ult (he
xade
cimal)
→
Analog input voltage [V]
VREF
1024
×
1
Ideal A-D conversion characteristics
A-D conversion characteristics with infinite resolution
0
VREF
1024
×
2
VREF
1024
×
3
VREF
1024
×
1022
VREF
1024
×
1023
VREF
1024
×
1024
Figure 11.3.6 Ideal A-D Conversion Characteristics Relative to the 10-bit A-D Converter’s Analog Input Voltages
11.3 Functional Description of A-D Converters