AN217
Rev. 0.2 9
The ADC’s digital filter gives two options for output: FAST and SINC3. The FAST filter uses results from only the
current conversion cycle or period, while the SINC3 filter uses three conversion cycles. The fast filter output reacts
more quickly to changes on the analog input, while the SINC3 filter output produces lower-noise results.
Additionally, the SINC3 filter is designed to reject 50 and 60 Hz frequencies (see Figure 4 on page 7). When it is
necessary to respond quickly to signal changes or when fast channel switching is required (between different
measured signals at the AMUX), the FAST filter should be used. In most cases, the SINC3 filter output is used for
best noise performance.
Using the FAST Filter: Set the ADC0 Interrupt Select bit (AD0ISEL) bit to 1 in the ADC0 Configuration Register
(ADC0CF). This will set the ADC0 end-of-conversion interrupt bit to be set upon a ready result in the FAST filter.
The output of the FAST filter is placed in the ADC0FH, ADC0FM, and ADC0FL registers. NOTE: When using
the FAST filter, the Decimation Ratio must be divisible by 8. (See SFR Definition 2.4.)
Using the SINC3 Filter: Clear the ADC0 Interrupt Select bit (AD0ISEL) bit to 0 in the ADC0 Configuration
Register (ADC0CF). This will set the ADC0 end-of-conversion interrupt bit to be set upon a ready result in the
SINC3 filter. The output of the SINC3 filter is placed in the ADC0H, ADC0M, and ADC0L registers.
2.2.7. Calibration
The C8051F35x family of devices has two, built-in calibration functions: internal and system calibration. Each of
these calibration options can calibrate for both offset and gain errors. Offset calibration should be performed prior
to gain calibration.
Calibration coefficients are stored in special function registers (constants are 24-bits) and are user accessible.
However, the calibration adjustments are made automatically by the ADC, and so no further user calibration
adjustment is necessary once the ADC calibration function is complete (i.e., the MCU does not have to condition
the ADC outputs for calibration).
Internal calibrations (offset and gain) require no external signals to the ADC inputs, but do require the voltage
reference input. System calibration uses external signals, and is useful for removing errors due to temperature-
induced offset/gain drift, reference voltage drift, offset, and gain errors from the analog front-end circuit.
System calibration is performed as follows:
Offset: This is done first by applying an external “zero” voltage to the AIN+/AIN– pins.
Gain: This is done after offset calibration. A full-scale positive voltage is applied to the AIN+/AIN– pins, for the
given current programmable gain amplifier setting.
Calibrations are initiated by writing to the ADC0 System Mode Bits (AD0SM) in the ADC0CF register (see SFR
Definition 2.5). Section 5.2 of the data sheet details complete information concerning the calibration of the ADC. It
is recommended that a full self or system calibration be performed at system start-up. An MCU-scheduled regular
calibration can be periodically performed for best performance of the measurement system.
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