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7721 Group User’s Manual

12–12

A-D CONVERTER

12.4 Absolute accuracy and differential non-linearity error

The A-D converter’s accuracy is described below. Refer to section “Appendix 12.3 A-D converter standard

characteristics,” also.

12.4.1 Absolute accuracy

The absolute accuracy is the difference expressed in the LSB between the actual A-D conversion result

and the output code of an A-D converter with ideal characteristics. The analog input voltage when measuring

the accuracy is assumed to be the mid point of the input voltage width that outputs the same output code

from an A-D converter with ideal characteristics. For example, when VREF = 5.12 V, 1 LSB width is 20 mV,

and 0 mV, 20 mV, 40 mV, 60 mV, 80 mV, ... are selected as the analog input voltages.

The absolute accuracy = ±3 LSB indicates that when the analog input voltage is 100 mV, the output code

expected from an ideal A-D conversion characteristics is “00516,” however the actual A-D conversion result

is between “00216” to “00816.”

The absolute accuracy includes the zero error and the full-scale error.

The absolute accuracy is degraded when VREF is lowered. Any of the output codes for analog input voltages

from VREF to AVCC is “FF16.”

0016

0116

0216

0316

0416

0516

0616

100

120

140

160

180

200

220

0716

0816

0916

0A16

0B16

+3 LSB

–3 LSB

Ideal A-D conversion

characteristics

Analog input voltage (mV)

Output code

(A-D conversion result)

Fig. 12.4.1 Absolute accuracy of A-D converter

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