MC68HC05JJ6 Datasheet, PDF (132/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
Table 8-7. Ratiometric Voltage Reading Errors
Error Source
Accuracy Improvements Possible
In Hardware
In Software
Change in reference
voltage
Change in magnitude of
ramp current source
Not required for ratiometric
Not adjustable
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference
Non-linearity of ramp
current source vs. voltage
Not adjustable
Change in magnitude of
ramp capacitor
Not required for ratiometric
Calibration and storage of voltages at
1/4, 1/2, 3/4, and FS
Compare unknown with recent
measurement from reference
Frequency shift in internal
low-power oscillator
Frequency shift in external
oscillator
Not required for ratiometric
Not required for ratiometric
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
Compare unknown with recent
measurement from reference
Input offset voltage of
comparator 2
Noise internal to MCU
Not adjustable
Close decoupling at
V_DD and V_SS pins and
reduce supply source impedance
Sum two readings on reference or
unknown using INV and INV control bit
and divide by 2 (average of both)
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
General Release Specification
132
Analog Subsystem
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor