XC68HC705JJ7 Motorola, XC68HC705JJ7 Datasheet - Page 135

• Manufacturer Part Number: XC68HC705JJ7
• Description: MICROCONTROLLER
• Manufacturer: Motorola

8.7.2.1 Internal Ratiometric Reference
MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
Change in reference
voltage
Change in magnitude of
ramp current source
Non-linearity of ramp
current source vs. voltage
Frequency shift in internal
low-power oscillator
Sampling capacitor leakage Use faster conversion times
Internal voltage divider ratio Not adjustable
Input offset voltage of
comparator 2
Noise internal to MCU
Error Source
Table 8-7. Ratiometric Voltage Reading Errors
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
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.
If readings are to be ratiometric to V
point can be internally selected. In this case the reference reading can
be taken by setting the V
AMUX register which connects the channel selection bus to the V
In order to stay within the V
select the 1/2 divided input.
Not required for ratiometric
Not adjustable
Not adjustable
Not required for ratiometric
Not adjustable
Close decoupling at V
and reduce supply source impedance
In Hardware
Analog Subsystem
DD
Accuracy Improvements Possible
and V
REF
Table
MAX
SS
bit and clearing the MUX1:4 bits in the
pins
range, the DHOLD bit should be used to
8-7. As with absolute measurements,
DD
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference
Calibration and storage of voltages at
1/4, 1/2, 3/4, and FS
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference
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)
Average multiple readings on both the
reference and the unknown voltage
, the reference measurement
Voltage Measurement Methods
In Software
Advance Information
Analog Subsystem
DD
pin.
135