Freescale Semiconductor, Inc.
Ana log Sub syste m
8.7.1.2 Exte rna l Ab so lute Re fe re nc e
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
V
–1.5 volts, then the DHOLD bit should be used to select the 1/2
DD
divided input.
Table 8-6. Absolute Voltage Reading Errors
Accuracy Improvements Possible
Error Source
In Hardware
In Software
Change in reference
voltage
Calibration and storage of reference source
over temperature and supply voltage
Provide closer tolerance reference
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
Provide closer tolerance ramp capacitor
Use external oscillator with crystal
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
Provide closer tolerance crystal
Use faster conversion times
Sampling capacitor
leakage
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference OR avoid
use of divided input
Internal voltage divider
ratio
Not adjustable
Not adjustable
Sum two readings on reference or
unknown using INV and INV control bit
and divide by 2 (average of both)
Input offset voltage of
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
For More Information On This Product,
Go to: www.freescale.com
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