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參數資料
| 型號： | QADCRM |
| 英文描述： | Queued Analog-to-Digital Converter Reference Manual |
| 中文描述： | 排隊模擬到數字轉換器參考手冊 |
| 文件頁數： | 40/122頁 |
| 文件大小： | 940K |
| 代理商： | QADCRM |
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MOTOROLA
5-8
PIN CONNECTION CONSIDERATIONS
For More Information On This Product,
Go to: www.freescale.com
QADC
REFERENCE MANUAL
5.6 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 practi-
cal range of capacitors that still have good high frequency characteristics). This capac-
itor 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. (re-
fer to
5.6.2 Error Resulting from Leakage
). In some cases, the size of the capacitor
at the pin may be very small.
Figure 5-7
is a simplified model of an input channel. Refer to this model in the follow-
ing discussion of the interaction between the external circuitry and the circuitry inside
the QADC.
Figure 5-7 Electrical Model of an A/D Input Pin
In
Figure 5-7
, R
F
and C
F
comprise the external filter circuit. C
S
is the internal sample
capacitor. Each channel has its own capacitor. C
S
is never precharged; it retains the
value of the last sample. V
I
is an internal voltage source used to precharge the DAC
QADC SAMPLE AMP MODEL
S1
S2
AMP
R
F
S4
S3
C
DAC
V
I
C
S
C
F
V
SRC
INTERNAL CIRCUIT MODEL
EXTERNAL CIRCUIT
= SOURCE VOLTAGE
= FILTER IMPEDANCE (SOURCE IMPEDANCE INCLUDED)
= FILTER CAPACITOR
= INTERNAL CAPACITANCE (FOR A BYPASSED CHANNEL, THIS IS THE C
DAC
CAPACITANCE)
= DAC CAPACITOR ARRAY
= INTERNAL VOLTAGE SOURCE FOR PRECHARGE (V
DDA
/2)
V
SRC
R
F
C
F
C
S
C
DAC
V
I
F
Freescale Semiconductor, Inc.
n
.
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