NXP Semiconductors KL25 Series - Sources of Error

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.
28.6.1.3 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.
For minimal loss of accuracy due to current injection, pins adjacent to the analog input
pins must not be transitioning during conversions.
28.6.2 Sources of error
28.6.2.1 Sampling error
For proper conversions, the input must be sampled long enough to achieve the proper
accuracy.
RAS + RADIN =SC / (FMAX * NUMTAU * CADIN)
Figure 28-65. Sampling equation
Where:
Chapter 28 Analog-to-Digital Converter (ADC)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 503