NXP Semiconductors K32 L2A Series, Reference Manual, Page: 222 / 1379

selected using SC2[REFSEL]. The alternate voltage reference pair, V
ALTH
and V
ALTL
,
may select additional external pins or internal sources based on MCU configuration. See
the chip configuration information on the voltage references specific to this MCU.
In some packages, the external or alternate pairs are connected in the package to V
DDA
and V
SSA
, respectively. One of these positive references may be shared on the same pin
as V
DDA
on some devices. One of these ground references may be shared on the same pin
as V
SSA
on some devices.
If externally available, the positive reference may be connected to the same potential as
V
DDA
or may be driven by an external source to a level between the minimum Ref
Voltage High and the V
DDA
potential. The positive reference must never exceed V
DDA
. If
externally available, the ground reference must be connected to the same voltage
potential as V
SSA
. The voltage reference pairs must be routed carefully for maximum
noise immunity and bypass capacitors placed as near as possible to the package.
AC current in the form of current spikes required to supply charge to the capacitor array
at each successive approximation step is drawn through the V
REFH
and V
REFL
loop. The
best external component to meet this current demand is a 0.1 μF capacitor with good
high-frequency characteristics. This capacitor is connected between V
REFH
and V
REFL
and must be placed as near as possible to the package pins. Resistance in the path is not
recommended because the current causes a voltage drop that could result in conversion
errors. Inductance in this path must be minimum, that is, parasitic only.
11.6.1.3 Analog input pins
The external analog inputs are typically shared with digital I/O pins on MCU devices.
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, which is
full scale 12-bit representation, 0x3FF, which is full scale 10-bit representation, or 0xFF,
which is 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.
Application information
K32 L2A Reference Manual, Rev. 2, 01/2020
222 NXP Semiconductors