MC68HC705MC4MP datasheet(73/180 Pages) FREESCALE

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Analog Subsystem

Analog Section

General Release Specification

Analog Subsystem

8.3 Analog Section

The following subsections describe the analog section of this

subsystem.

8.3.1 Ratiometric Conversion

The A/D is ratiometric, with two dedicated pins supplying the reference

voltages (VREFH and VREFL). An input voltage equal to VREFH converts

to $FF (full scale) and an input voltage equal to VREFL converts to $00.

An input voltage greater than VREFH will convert to $FF with no overflow

indication. For ratiometric conversions, the source of each analog input

should use VREFH as the supply voltage and be referenced to VREFL.

8.3.2 VREFH and VREFL

The reference supply for the converter uses two dedicated pins rather

than being driven by the system power supply lines because the voltage

drops in the bonding wires of those heavily loaded pins would degrade

the accuracy of the A/D conversion. VREFH and VREFL are internally

wired to the analog supply voltages AVDD and AVSS. These pins are

located next to each other to permit optimal decoupling.

8.3.3 Accuracy and Precision

The 8-bit conversions shall be accurate to within

± 11/2 LSB including

quantization.

8.4 Conversion Process

The A/D reference inputs are applied to a precision internal

digital-to-analog converter. Control logic drives this D/A and the analog

output is successively compared to the selected analog input that was

sampled at the beginning of the conversion time. The conversion

process is monotonic and has no missing codes.

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