MC68HC05JJ6 Datasheet, PDF (130/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

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 through 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_DD –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
Change in reference
voltage
Change in magnitude of
ramp current source
Non-linearity of ramp
current source vs.
voltage
Change in magnitude of
ramp capacitor
Frequency shift in
internal low-power
oscillator
Frequency shift in
external oscillator
Sampling capacitor
leakage
Internal voltage divider
ratio
Input offset voltage of
comparator 2
Noise internal to MCU
Noise external to MCU
Accuracy Improvements Possible
In Hardware
In Software
Provide closer tolerance reference
Calibration and storage of reference source
over temperature and supply voltage
Not adjustable
Compare unknown with recent
measurement from reference
Not adjustable
Calibration and storage of voltages at 1/4,
1/2, 3/4, and FS
Provide closer tolerance ramp capacitor
Compare unknown with recent
measurement from reference
Use external oscillator with crystal
Compare unknown with recent
measurement from reference
Provide closer tolerance crystal
Compare unknown with recent
measurement from reference
Use faster conversion times
Compare unknown with recent
measurement from reference
Not adjustable
Compare unknown with recent
measurement from reference OR avoid
use of divided input
Not adjustable
Sum two readings on reference or
unknown using INV and INV control bit
and divide by 2 (average of both)
Close decoupling at V_DD and V_SS pins Average multiple readings on both the
and reduce supply source impedance reference and the unknown voltage
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
130
Analog Subsystem
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor