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参数资料
| 型号： | MCV18E-I/P |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 32-BIT, FLASH, 20 MHz, RISC MICROCONTROLLER, PDIP18 |
| 封装： | 0.300 INCH, LEAD FREE, PLASTIC, DIP-18 |
| 文件页数： | 51/108页 |
| 文件大小： | 1509K |
| 代理商： | MCV18E-I/P |
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2009 Microchip Technology Inc.
DS41399A-page 47
MCV18E
7.3
A/D Acquisition Requirements
For the ADC 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 7-2. 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
The maximum recommended impedance for analog
sources is 10 k
Ω. As the source impedance is
decreased, the acquisition time may be decreased.
After the analog input channel is selected (or changed),
an A/D acquisition must be done before the conversion
can be started. To calculate the minimum acquisition
time, Equation 7-1 may be used. This equation
assumes that 1/2 LSb error is used. The 1/2 LSb error is
the maximum error allowed for the ADC to meet its
specified resolution.
EQUATION 7-1:
ACQUISITION TIME EXAMPLE
TACQ
Amplifier Settling Time
Hold Capacitor Charging Time
Temperature Coefficient
++
=
TAMP
TC
TCOFF
++
=
2s
TC
Temperature - 25°C
() 0.05s/°C
()
[]
++
=
TC
CHOLD RIC
RSS
RS
++
() ln(1/2047)
–
=
10pF 1k
Ω 7kΩ 10kΩ
++
()
–
ln(0.0004885)
=
1.37
=
s
TACQ
2s
1.37s
50°C- 25°C
() 0.05s/°C
()
[]
++
=
4.67s
=
VAPPLIED 1e
Tc
–
RC
---------
–
VAPPLIED 1
1
2047
------------
–
=
VAPPLIED 1
1
2047
------------
–
VCHOLD
=
VAPPLIED 1e
TC
–
RC
----------
–
VCHOLD
=
;[1] VCHOLD charged to within 1/2 lsb
;[2] VCHOLD charge response to VAPPLIED
;combining [1] and [2]
The value for TC can be approximated with the following equations:
Solving for TC:
Therefore:
Temperature
50°C and external impedance of 10k
Ω 5.0V VDD
=
Assumptions:
Note 1: The reference voltage (VREF) has no effect on the equation, since it cancels itself out.
2: The charge holding capacitor (CHOLD) is not discharged after each conversion.
3: The maximum recommended impedance for analog sources is 10 k
Ω. This is required to meet the pin
leakage specification.
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