68HC705SP7 datasheet(130/230 Pages) FREESCALE

130 / 230 page

Analog Subsystem

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Analog Subsystem

8.7.1.2 External Absolute Reference

If a stable external source is provided, the reference measurement point

can be any one of the channel selected pins from PB1–PB4. In this case

the reference reading can be taken by setting the MUX bit in the AMUX

which connects channel selection bus to the pin connected to the

external reference source. If the external reference is greater than

VDD –1.5 volts, then the DHOLD bit should be used to select the 1/2

divided input.

Table 8-6. Absolute Voltage Reading Errors

Error Source

Accuracy Improvements Possible

In Hardware

In Software

Change in reference

voltage

Provide closer tolerance reference

Calibration and storage of reference source

over temperature and supply voltage

Change in magnitude of

ramp current source

Not adjustable

Compare unknown with recent

measurement from reference

Non-linearity of ramp

current source vs.

voltage

Not adjustable

Calibration and storage of voltages at 1/4,

1/2, 3/4, and FS

Change in magnitude of

ramp capacitor

Provide closer tolerance ramp capacitor

Compare unknown with recent

measurement from reference

Frequency shift in

internal low-power

oscillator

Use external oscillator with crystal

Compare unknown with recent

measurement from reference

Frequency shift in

external oscillator

Provide closer tolerance crystal

Compare unknown with recent

measurement from reference

Sampling capacitor

leakage

Use faster conversion times

Compare unknown with recent

measurement from reference

Internal voltage divider

ratio

Not adjustable

Compare unknown with recent

measurement from reference OR avoid

use of divided input

Input offset voltage of

comparator 2

Not adjustable

Sum two readings on reference or

unknown using INV and INV control bit

and divide by 2 (average of both)

Noise internal to MCU

Close decoupling at VDD and VSS pins

and reduce supply source impedance

Average multiple readings on both the

reference and the unknown voltage

Noise external to MCU

Close decoupling of power supply, low

source impedances, good board

layout, use of multi-layer board

Average multiple readings on both the

reference and the unknown voltage

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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