NXP Semiconductors K22F series, Reference Manual, Page: 812 / 1407

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
CFG1[ADLSMP] and changing CFG2[ADLSTS] to increase the sample window, or
decreasing ADCK frequency to increase sample time.
34.6.2.2 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, where N = 8 in
8-bit mode, 10 in 10-bit mode, 12 in 12-bit mode, or 16 in 16-bit mode.
34.6.2.3 Noise-induced errors
System noise that occurs during the sample or conversion process can affect the accuracy
of the conversion. The ADC accuracy numbers are guaranteed as specified only if the
following conditions are met:
• There is a 0.1 μF low-ESR capacitor from V
REFH
to V
REFL
.
• There is a 0.1 μF low-ESR capacitor from V
DDA
to V
SSA
.
• If inductive isolation is used from the primary supply, an additional 1 μF capacitor is
placed from V
DDA
to V
SSA
.
• V
SSA
, and V
REFL
, if connected, is connected to V
SS
at a quiet point in the ground
plane.
• Operate the MCU in Wait or Normal Stop mode before initiating (hardware-triggered
conversions) or immediately after initiating (hardware- or software-triggered
conversions) the ADC conversion.
Application information
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
812 NXP Semiconductors