Datasheet

Chapter 10 Analog-to-Digital Converter (S08ADC12V1)
MC9S08DN60 Series Data Sheet, Rev 3
190 Freescale Semiconductor
10.6.1.3 Analog Input Pins
The external analog inputs are typically shared with digital I/O pins on MCU devices. The pin I/O control
is disabled by setting the appropriate control bit in one of the pin control registers. Conversions can be
performed on inputs without the associated pin control register bit set. It is recommended that the pin
control register bit always be set when using a pin as an analog input. This avoids problems with contention
because the output buffer is in its high impedance state and the pullup is disabled. Also, the input buffer
draws DC current when its input is not at V
DD
or V
SS
. Setting the pin control register bits for all pins used
as analog inputs should be done to achieve lowest operating current.
Empirical data shows that capacitors on the analog inputs improve performance in the presence of noise
or when the source impedance is high. Use of 0.01 μF capacitors with good high-frequency characteristics
is sufficient. These capacitors are not necessary in all cases, but when used they must be placed as near as
possible to the package pins and be referenced to V
SSA
.
For proper conversion, the input voltage must fall between V
REFH
and V
REFL
. If the input is equal to or
exceeds V
REFH
, the converter circuit converts the signal to 0xFFF (full scale 12-bit representation), 0x3FF
(full scale 10-bit representation) or 0xFF (full scale 8-bit representation). If the input is equal to or less
than V
REFL
, the converter circuit converts it to 0x000. Input voltages between V
REFH
and V
REFL
are
straight-line linear conversions. There is a brief current associated with V
REFL
when the sampling
capacitor is charging. The input is sampled for 3.5 cycles of the ADCK source when ADLSMP is low, or
23.5 cycles when ADLSMP is high.
For minimal loss of accuracy due to current injection, pins adjacent to the analog input pins should not be
transitioning during conversions.
10.6.2 Sources of Error
Several sources of error exist for A/D conversions. These are discussed in the following sections.
10.6.2.1 Sampling Error
For proper conversions, the input must be sampled long enough to achieve the proper accuracy. Given the
maximum input resistance of approximately 7kΩ 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 @
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 ADLSMP (to increase the
sample window to 23.5 cycles) or decreasing ADCK frequency to increase sample time.
10.6.2.2 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
DDAD
/(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).