Freescale Semiconductor, Inc.
Ana log Sub syste m
Table 8-7. Ratiometric Voltage Reading Errors
Accuracy Improvements Possible
Error Source
In Hardware
In Software
Change in reference
voltage
Compare unknown with recent
measurement from reference
Not required for ratiometric
Change in magnitude of
ramp current source
Compare unknown with recent
measurement from reference
Not adjustable
Non-linearity of ramp
current source vs. voltage
Calibration and storage of voltages at
1/4, 1/2, 3/4, and FS
Not adjustable
Change in magnitude of
ramp capacitor
Compare unknown with recent
measurement from reference
Not required for ratiometric
Not required for ratiometric
Not required for ratiometric
Frequency shift in internal
low-power oscillator
Compare unknown with recent
measurement from reference
Frequency shift in external
oscillator
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference
Sampling capacitor leakage Use faster conversion times
Internal voltage divider ratio Not adjustable
Compare unknown with recent
measurement from reference
Sum two readings on reference or
unknown using INV and INV control bit
and divide by 2 (average of both)
Input offset voltage of
Not adjustable
comparator 2
Close decoupling at V and V pins
and reduce supply source impedance
Average multiple readings on both the
reference and the unknown voltage
DD
SS
Noise internal to MCU
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
General Release Specification
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
Analog Subsystem
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