MMC2107 Datasheet, PDF (497/618 Pages) –

Freescale Semiconductor, Inc.
Queued Analog-to-Digital Converter (QADC)
Pin Connection Considerations
18.11.7 Analog Input Pins
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:
• 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.
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 18.11.7.2 Error Resulting from Leakage). In some
cases, the size of the capacitor at the pin may be very small.
Figure 18-54 is a simplified model of an input channel. Refer to this
model in the following discussion of the interaction between the external
circuitry and the circuitry inside the QADC.
SOURCE
R_SRC
EXTERNAL FILTER
RF
V_SRC
C_F
INTERNAL CIRCUIT MODEL
S1
AMP
C_P
S2
S3
C_SAMP
V_I
V _SRC = Source voltage
R_SRC = Source impedance
RF = Filter impedance
CF = Filter capacitor
CP = Internal parasitic capacitance
C_SAMP= Sample capacitor
VI = Internal voltage source during sample and hold
Figure 18-54. Electrical Model of an A/D Input Pin
MMC2107 – Rev. 2.0
MOTOROLA
Queued Analog-to-Digital Converter (QADC)
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Technical Data
497