mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 672

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Application information
frequency characteristics. This capacitor is connected between V
and V
and
REFH
REFL
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 (parasitic only).
29.6.1.3 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
and V
. If the input
REFH
REFL
is equal to or exceeds V
, the converter circuit converts the signal to 0xFFF (full scale
REFH
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
, the converter circuit converts it
REFL
to 0x000. Input voltages between V
and V
are straight-line linear conversions.
REFH
REFL
There is a brief current associated with V
when the sampling capacitor is charging.
REFL
For minimal loss of accuracy due to current injection, pins adjacent to the analog input
pins should not be transitioning during conversions.
29.6.2 Sources of error
Several sources of error exist for A/D conversions. These are discussed in the following
sections.
29.6.2.1 Sampling error
For proper conversions, the input must be sampled long enough to achieve the proper
accuracy.
RAS + RADIN =SC / (FMAX * NUMTAU * CADIN)
Figure 29-49. Sampling equation
Where:
RAS = External analog source resistance
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Preliminary
672
Freescale Semiconductor, Inc.