K60P100M100SF2RM Datasheet, PDF (851/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 34 Analog-to-Digital Converter (ADC)
• For software triggered conversions, immediately follow the write to the SC1
register with a wait instruction or stop instruction.
• For Normal Stop mode operation, select ADACK as the clock source. Operation
in Normal Stop reduces V_DD noise but increases effective conversion time due to
stop recovery.
• There is no I/O switching, input or output, on the MCU during the conversion.
There are some situations where external system activity causes radiated or conducted
noise emissions or excessive V_DD noise is coupled into the ADC. In these situations, or
when the MCU cannot be placed in Wait or Normal Stop or I/O activity cannot be halted,
these recommended actions may reduce the effect of noise on the accuracy:
• Place a 0.01 μF capacitor (C_AS) on the selected input channel to V_REFL or V_SSA (this
improves noise issues, but affects the sample rate based on the external analog source
resistance).
• Average the result by converting the analog input many times in succession and
dividing the sum of the results. Four samples are required to eliminate the effect of a
1 LSB, one-time error.
• Reduce the effect of synchronous noise by operating off the asynchronous clock
(ADACK) and averaging. Noise that is synchronous to ADCK cannot be averaged
out.
34.6.2.4 Code width and quantization error
The ADC quantizes the ideal straight-line transfer function into 65536 steps (in 16-bit
mode). Each step ideally has the same height (1 code) and width. The width is defined as
the delta between the transition points to one code and the next. The ideal code width for
an N bit converter (in this case N can be 16, 12, 10, or 8), defined as 1 LSB, is:
LSB
Figure 34-99. Ideal code width for an N bit converter
There is an inherent quantization error due to the digitization of the result. For 8-bit, 10-
bit, or 12-bit conversions, the code transitions when the voltage is at the midpoint
between the points where the straight line transfer function is exactly represented by the
actual transfer function. Therefore, the quantization error will be ± 1/2 LSB in 8-bit, 10-
bit, or 12-bit modes. As a consequence, however, the code width of the first (0x000)
conversion is only 1/2 LSB and the code width of the last (0xFF or 0x3FF) is 1.5 LSB.
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
851