MKL04Z32VFK4 Datasheet, PDF (460/658 Pages) Freescale Semiconductor, Inc – KL04 Sub-Family Reference Manual

Application information
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 mode, or I/O activity cannot be
halted, the following 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, that
is, ADACK, and averaging. Noise that is synchronous to ADCK cannot be averaged
out.
28.6.2.4 Code width and quantization error
The ADC quantizes the ideal straight-line transfer function into 4096 steps in the 12-bit
mode). Each step ideally has the same height, that is, 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, where N can be 12, 10, or 8, defined as 1 LSB, is:
LSB
Figure 28-50. 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.
28.6.2.5 Linearity errors
The ADC may also exhibit non-linearity of several forms. Every effort has been made to
reduce these errors, but the system designers must be aware of these errors because they
affect overall accuracy:
KL04 Sub-Family Reference Manual, Rev. 3.1, November 2012
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Freescale Semiconductor, Inc.