Datasheet

For minimal loss of accuracy due to current injection, pins adjacent to the analog input
pins should not be transitioning during conversions.
19.6.2 Sources of error
Several sources of error exist for A/D conversions. These are discussed in the following
sections.
19.6.2.1 Sampling error
For proper conversions, the input must be sampled long enough to achieve the proper
accuracy. Given the maximum input resistance of approximately 7 kΩ and input
capacitance of approximately 5.5 pF, sampling to within 1/4 LSB (at 12-bit resolution)
can be achieved within the minimum sample window (3.5 cycles at 8 MHz maximum
ADCK frequency) provided the resistance of the external analog source (R
AS
) is kept
below 2 kΩ.
Higher source resistances or higher-accuracy sampling is possible by setting
ADC_SC3[ADLSMP] (to increase the sample window to 23.5 cycles) or decreasing
ADCK frequency to increase sample time.
19.6.2.2 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
DDA
/ (2
N
*I
LEAK
) for less than 1/4 LSB leakage error (N = 8 in 8-bit, 10 in
10-bit or 12 in 12-bit mode).
19.6.2.3 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
.
Application information
MC9S08PA60 Reference Manual, Rev. 1, 9/2012
552 Freescale Semiconductor, Inc.