參數(shù)資料
型號： MC68HC705SB7
廠商： FREESCALE SEMICONDUCTOR INC
元件分類： 微控制器/微處理器
英文描述： 8-BIT, OTPROM, 2.1 MHz, MICROCONTROLLER, PDSO28
封裝： SOIC-28
文件頁數(shù)： 30/170頁
文件大小： 2161K
代理商： MC68HC705SB7
August 27, 1998
GENERAL RELEASE SPECIFICATION
MC68HC05SB7
ANALOG SUBSYSTEM
MOTOROLA
REV 2.1
15-27
15.5
VOLTAGE MEASUREMENT METHODS
The various methods for obtaining a voltage measurement can use software tech-
niques to express these voltages as absolute or ratiometric readings.
NOTE
All A/D conversion methods should include a test for a maximum elapsed time in
order to detect error cases where the inputs may be outside of the design
specication.
15.5.1 Absolute Voltage Readings
The absolute value of a voltage measurement can be calculated in software by
rst taking a reference reading from a xed source and then comparing subse-
quent unknown voltages to that reading as a percentage of the reference voltage
multiplied times the known reference value.
The accuracy of absolute readings will depend on the error sources taken into
account using the features of the analog subsystem and appropriate software as
described in Table 15-6. As can be seen from this table, most of the errors can be
reduced by frequent comparisons to a known voltage, use of the inverted compar-
ator inputs, and averaging of multiple samples.
Table 15-6. Absolute Voltage Reading Errors
Error Source
Accuracy Improvements Possible
In Hardware
In Software
Change in
Reference Voltage
Provide closer tolerance reference
Calibration and storage of
reference source over temperature
and supply voltage
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
Change in Magnitude of
Ramp Capacitor
Provide closer tolerance ramp capacitor
Compare unknown with recent
measurement from reference
Frequency Shift in Internal
Low-Power Oscillator
Use external oscillator with crystal
Compare unknown with recent
measurement from reference
Frequency Shift in
External Oscillator
Provide closer tolerance crystal
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
OR avoid use of divided input
Noise internal to MCU
Close decoupling at VDD and VSS pins
and reduce supply source impedance
Average multiple readings on both the
reference and the unknown voltage
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