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68HC705SP7 数据表(PDF) 130 Page - Freescale Semiconductor, Inc |
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68HC705SP7 数据表(HTML) 130 Page - Freescale Semiconductor, Inc |
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130 / 230 page  Analog Subsystem General Release Specification MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0 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–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 VDD –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 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 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 VDD and VSS pins and reduce supply source impedance Average multiple readings on both the reference and the unknown voltage 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 Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com |
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