參數(shù)資料
型號: MC56F8000RM
廠商: 飛思卡爾半導體(中國)有限公司
英文描述: 16-bit Digital Signal Controllers
中文描述: 16位數(shù)字信號控制器
文件頁數(shù): 111/124頁
文件大?。?/td> 1880K
代理商: MC56F8000RM
Equivalent Circuit for ADC Inputs
56F8013 Technical Data, Rev. 2
Freescale Semiconductor
Preliminary
111
10.15 Equivalent Circuit for ADC Inputs
Figure 10-17
illustrates the ADC input circuit during sample and hold. S1 and S2 are always open/closed
at the same time that S3 is closed/open. When S1/S2 are closed & S3 is open, one input of the sample and
hold circuit moves to (V
REFH
-V
REFL
)/2, while the other charges to the analog input voltage. When the
switches are flipped, the charge on C1 and C2 are averaged via S3, with the result that a single-ended
analog input is switched to a differential voltage centered about (V
REFH
-V
REFL
)/2. The switches switch
on every cycle of the ADC clock (open one-half ADC clock, closed one-half ADC clock). Note that there
are additional capacitances associated with the analog input pad, routing, etc., but these do not filter into
the S/H output voltage, as S1 provides isolation during the charge-sharing phase.
One aspect of this circuit is that there is an on-going input current, which is a function of the analog input
voltage, V
REF
and the ADC clock frequency.
1.
2.
3.
4.
Parasitic capacitance due to package, pin-to-pin and pin-to-package base coupling; 1.8pF
Parasitic capacitance due to the chip bond pad, ESD protection devices and signal routing; 2.04pF
Equivalent resistance for the channel select mux; 100 ohms
Sampling capacitor at the sample and hold circuit. Capacitor C1 is normally disconnected from the input and is only
connected to it at sampling time; 1.4pF
Equivalent input impedance, when the the input is selected =
5.
Figure 10-17 Equivalent Circuit for A/D Loading
1
2
3
Analog Input
4
S1
S2
S3
C1
C2
S/H
C1 = C2 = 1pF
(V
REFH
- V
REFL )
/ 2
125
ESD Resistor
8pF noise damping capacitor
1
(ADC Clock Rate) x 1.4 x 10
-12
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