Accuracy Of A-D Conversion - Renesas M32R/ECU Series User Manual

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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'3FF
H'3FE
H'003
H'002
H'001
H'000
0
Figure 11.3.6 Ideal A-D Conversion Characteristics Relative to the 10-bit A-D Converter's Analog Input Voltages
Ideal A-D conversion characteristics
A-D conversion characteristics with infinite resolution
VREF VREF VREF
× 1 × 2
1024 1024
11.3 Functional Description of A-D Converters
× 3
1024
VREF
× 1022
1024
→ Analog input voltage [V]
11-39
A-D Converters
VREF
× 1023
1024
VREF
× 1024
1024
32180 Group User's Manual (Rev.1.0)