MC68HC705JP7 Datasheet, PDF (135/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
Voltage Measurement Methods
will depend on the variety of sources, but will generally be better than for
absolute readings. Many of these error sources can be taken into
account using the features of the analog subsystem and appropriate
software as described in Table 8-7. As with absolute measurements,
most of the errors can be reduced by frequent comparisons to the
reference voltage, use of the inverted comparator inputs, and averaging
of multiple samples.
Table 8-7. Ratiometric Voltage Reading Errors
Error Source
Accuracy Improvements Possible
In Hardware
In Software
Change in reference
voltage
Not required for ratiometric
Compare unknown with recent
measurement from reference
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
Frequency shift in internal
low-power oscillator
Not required for ratiometric
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
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 V_DD and V_SS pins Average multiple readings on both the
and reduce supply source impedance reference and the unknown voltage
8.7.2.1 Internal Ratiometric Reference
If readings are to be ratiometric to V_DD, the reference measurement
point can be internally selected. In this case the reference reading can
be taken by setting the V_REF bit and clearing the MUX1:4 bits in the
AMUX register which connects the channel selection bus to the V_DD pin.
In order to stay within the V_MAX range, the DHOLD bit should be used to
select the 1/2 divided input.
MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
Analog Subsystem
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