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16.6

A/D Conversion Precision Definitions

The medium-speed A/D converter converts analog values input from analog input channels to 10-

bit digital values by comparing them with an analog reference voltage. In this operation, the

absolute precision of the A/D conversion (i.e. the deviation between the input analog value and the

output digital value) includes the following kinds of error.

(1) Offset error

(2) Full-scale error

(3) Quantization error

(4) Nonlinearity error

The above four kinds of error are described below with reference to figure 16.7. For the sake of

clarity, this figure shows 3-bit medium-speed A/D conversion rather than 10-bit medium-speed

A/D conversion. Offset error (see figure 16.7 (1)) is the deviation between the actual A/D

conversion characteristic and the ideal A/D conversion characteristic when the digital output value

changes from the minimum value (zero voltage) of 0000000000 (000 in the figure) to 0000000001

(001 in the figure ). Full-scale error (see figure 16.7 (2)) is the deviation between the actual A/D

conversion characteristic and the ideal A/D conversion characteristic when the digital output value

changes from 1111111110 (110 in the figure) to the maximum value (full-scale voltage) of

111111111 (111 in the figure). Quantization error is the deviation inherent in the medium-speed

A/D converter, given by 1/2 LSB (see figure 16.7 (3)). Nonlinearity error is the deviation between

the actual A/D conversion characteristic and the ideal A/D conversion characteristic from zero

voltage to full-scale voltage (see figure 16.7 (4)). This does not include offset error, full-scale

error, and quantization error.

1/8

2/8

3/8

4/8

5/8

6/8

7/8

111

110

101

100

011

010

001

000

Analog input

voltage

(3) Quantization error

(4) Nonlinearity error

(2) Full-scale error

Ideal A/D conversion

Characteristic

Ideal A/D conversion

Characteristic

Digital output

FS : Full-scale voltage

Analog input

voltage

Actual A/D conversion

characteristic

(1) Offset error

Figure 16.7 A/D Conversion Precision Definitions

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