mcf5282 Freescale Semiconductor, Inc, mcf5282 Datasheet - Page 604

• Manufacturer Part Number: mcf5282
• Manufacturer: Freescale Semiconductor, Inc

Queued Analog-to-Digital Converter (QADC)
28.9.7
Analog inputs should have low AC impedance at the pins. Low AC impedance can be realized by placing
a capacitor with good high frequency characteristics at the input signal of the device. Ideally, that capacitor
should be as large as possible (within the practical range of capacitors that still have good high-frequency
characteristics). This capacitor has two effects:
Series resistance can be used with the capacitor on an input signal to implement a simple RC filter. The
maximum level of filtering at the input pins is application dependent and is based on the bandpass
characteristics required to accurately track the dynamic characteristics of an input. Simple RC filtering at
the pin may be limited by the source impedance of the transducer or circuit supplying the analog signal to
be measured. (See
capacitor at the pin may be very small.
Figure 28-53
the interaction between the external circuitry and the circuitry inside the QADC.
In
C
used to provide charge to C
The following paragraphs provide a simplified description of the interaction between the QADC and the
user's external circuitry. This circuitry is assumed to be a simple RC low-pass filter passing a signal from
a source to the QADC input signal. These paragraphs make the following assumptions:
28-66
SAMP
Figure
•
•
•
•
•
•
V
R
SRC
SRC
It helps attenuate any noise that may exist on the input.
It sources charge during the sample period when the analog signal source is a high-impedance
source.
The external capacitor is perfect (no leakage, no significant dielectric absorption characteristics,
etc.).
All parasitic capacitance associated with the input signal is included in the value of the external
capacitor.
Inductance is ignored.
The “on” resistance of the internal switches is 0 ohms and the “off” resistance is infinite.
is the capacitor array used to sample and hold the input voltage. V
= Source Voltage
= Source Impedance
28-53, R
Analog Input Pins
Source
is a simplified model of an input channel. Refer to this model in the following discussion of
V
SRC
R
SRC
F
, R
Section 28.9.7.2, “Error Resulting from
SRC
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3
External Filter
, and C
Figure 28-53. Electrical Model of an A/D Input Signal
SAMP
R F
C
F
R F
C F
F
during sample phase.
comprise the external filter circuit. C
= Filter Impedance
= Filter Capacitor
Internal Circuit Model
S1
C
P
AMP
C P
C
V I
SAMP
= Internal Voltage Source During Sample and Hold
= Internal Parasitic Capacitance
Leakage.”) In some cases, the size of the
= Sample Capacitor
S2
P
is the internal parasitic capacitor.
I
C
is an internal voltage source
SAMP
Freescale Semiconductor
S3
V
I