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PIC18F4520-I/PT 参数 Datasheet PDF下载

PIC18F4520-I/PT图片预览
型号: PIC18F4520-I/PT
PDF下载: 下载PDF文件 查看货源
内容描述: 28 /40/ 44引脚增强型闪存微控制器与10位A / D和纳瓦技术 [28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology]
分类和应用: 闪存微控制器
文件页数/大小: 412 页 / 6898 K
品牌: MICROCHIP [ MICROCHIP ]
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PIC18F2420/2520/4420/4520  
To calculate the minimum acquisition time,  
19.1 A/D Acquisition Requirements  
Equation 19-1 may be used. This equation assumes  
that 1/2 LSb error is used (1024 steps for the A/D). The  
1/2 LSb error is the maximum error allowed for the A/D  
to meet its specified resolution.  
For the A/D Converter to meet its specified accuracy,  
the charge holding capacitor (CHOLD) must be allowed  
to fully charge to the input channel voltage level. The  
analog input model is shown in Figure 19-3. The  
source impedance (RS) and the internal sampling  
switch (RSS) impedance directly affect the time  
required to charge the capacitor CHOLD. The sampling  
switch (RSS) impedance varies over the device voltage  
(VDD). The source impedance affects the offset voltage  
at the analog input (due to pin leakage current). The  
maximum recommended impedance for analog  
sources is 2.5 kΩ. After the analog input channel is  
selected (changed), the channel must be sampled for  
at least the minimum acquisition time before starting a  
conversion.  
Example 19-3 shows the calculation of the minimum  
required acquisition time TACQ. This calculation is  
based on the following application system  
assumptions:  
CHOLD  
Rs  
Conversion Error  
VDD  
Temperature  
=
=
=
=
25 pF  
2.5 kΩ  
1/2 LSb  
5V Rss = 2 kΩ  
85°C (system max.)  
Note:  
When the conversion is started, the  
holding capacitor is disconnected from the  
input pin.  
EQUATION 19-1: ACQUISITION TIME  
TACQ  
=
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient  
TAMP + TC + TCOFF  
EQUATION 19-2: A/D MINIMUM CHARGING TIME  
VHOLD  
or  
TC  
=
=
(VREF – (VREF/2048)) • (1 – e(-TC/CHOLD(RIC + RSS + RS))  
)
-(CHOLD)(RIC + RSS + RS) ln(1/2048)  
EQUATION 19-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME  
TACQ  
TAMP  
TCOFF  
=
=
=
TAMP + TC + TCOFF  
0.2 μs  
(Temp – 25°C)(0.02 μs/°C)  
(85°C – 25°C)(0.02 μs/°C)  
1.2 μs  
Temperature coefficient is only required for temperatures > 25°C. Below 25°C, TCOFF = 0 μs.  
TC  
=
-(CHOLD)(RIC + RSS + RS) ln(1/2047) μs  
-(25 pF) (1 kΩ + 2 kΩ + 2.5 kΩ) ln(0.0004883) μs  
1.05 μs  
TACQ  
=
0.2 μs + 1 μs + 1.2 μs  
2.4 μs  
DS39631E-page 228  
© 2008 Microchip Technology Inc.