Information

For minimal loss of accuracy due to current injection, pins adjacent to the analog input
pins should not be transitioning during conversions.
31.6.2 Sources of error
Several sources of error exist for A/D conversions. These are discussed in the following
sections.
31.6.2.1 Sampling error
For proper conversions, the input must be sampled long enough to achieve the proper
accuracy.
RAS + RADIN =SC / (FMAX * NUMTAU * CADIN)
Figure 31-65. Sampling equation
Where:
RAS = External analog source resistance
SC = Number of ADCK cycles used during sample window
CADIN = Internal ADC input capacitance
NUMTAU = -ln(LSBERR / 2
N
)
LSBERR = value of acceptable sampling error in LSBs
N = 8 in 8-bit mode, 10 in 10-bit mode, 12 in 12-bit mode or 16 in 16-bit mode
Higher source resistances or higher-accuracy sampling is possible by setting ADLSMP
and changing the ADLSTS bits (to increase the sample window) or decreasing ADCK
frequency to increase sample time.
31.6.2.2 Pin leakage error
Leakage on the I/O pins can cause conversion error if the external analog source
resistance (R
AS
) is high. If this error cannot be tolerated by the application, keep R
AS
lower than V
REFH
/ (4 × I
LEAK
× 2
N
) for less than 1/4 LSB leakage error (N = 8 in 8-bit
mode, 10 in 10-bit mode, 12 in 12-bit mode, or 16 in 16-bit mode).
Application information
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
628 Freescale Semiconductor, Inc.