MMC2114 MOTOROLA [Motorola, Inc], MMC2114 Datasheet - Page 521

• Manufacturer Part Number: MMC2114
• Description: M CORE Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

19.11.7 Analog Input Pins
MMC2114 • MMC2113 • MMC2112 — Rev. 1.0
MOTOROLA
V
R
SRC
SRC
= Source voltage
= Source impedance
SOURCE
V
SRC
R
SRC
EXTERNAL FILTER
Figure 19-53. Electrical Model of an A/D Input Pin
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 pin of the part. 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 pin 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
cases, the size of the capacitor at the pin may be very small.
Figure 19-53
model in the following discussion of the interaction between the external
circuitry and the circuitry inside the QADC.
R F
•
•
Freescale Semiconductor, Inc.
C
F
For More Information On This Product,
Queued Analog-to-Digital Converter (QADC)
R
C
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.
F
F
= Filter impedance
= Filter capacitor
INTERNAL CIRCUIT MODEL
Go to: www.freescale.com
is a simplified model of an input channel. Refer to this
S1
19.11.7.2 Error Resulting from
C
P
AMP
C
C
V
P
I
SAMP
= Internal voltage source during sample and hold
= Internal parasitic capacitance
= Sample capacitor
Queued Analog-to-Digital Converter (QADC)
S2
Pin Connection Considerations
C
SAMP
Leakage.) In some
S3
Advance Information
V
I
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