参数资料
型号: PIC16LF818T-I/SS
元件分类: 微控制器/微处理器
英文描述: 8-BIT, FLASH, 10 MHz, RISC MICROCONTROLLER, PDSO20
封装: 0.209 INCH, PLASTIC, MO-150, SSOP-20
文件页数: 162/176页
文件大小: 3124K
代理商: PIC16LF818T-I/SS
PIC16F818/819
DS39598D-page 84
2003 Microchip Technology Inc.
11.1
A/D Acquisition Requirements
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 11-2. 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 varies over the device voltage (VDD), see
Figure 11-2. The maximum recommended imped-
ance for analog sources is 2.5 k
. As the impedance
is decreased, the acquisition time may be decreased.
After the analog input channel is selected (changed),
this acquisition must be done before the conversion
can be started.
To
calculate
the
minimum
acquisition
time,
Equation 11-1 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.
To calculate the minimum acquisition time, TACQ, see
the “PICmicro Mid-Range MCU Family Reference
Manual” (DS33023).
EQUATION 11-1:
ACQUISITION TIME
FIGURE 11-2:
ANALOG INPUT MODEL
TACQ
TC
TACQ
=
Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
TAMP + TC + TCOFF
2
s + TC + [(Temperature – 25°C)(0.05 s/°C)]
CHOLD (RIC + RSS + RS) In(1/2047)
-120 pF (1 k
+ 7 k + 10 k) In(0.0004885)
16.47
s
2
s + 16.47 s + [(50°C – 25°C)(0.05 s/°C)
19.72
s
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
. This is required to meet the pin
leakage specification.
4: After a conversion has completed, a 2.0 TAD delay must complete before acquisition can begin again.
During this time, the holding capacitor is not connected to the selected A/D input channel.
CPIN
VA
RS
ANx
5 pF
VDD
VT = 0.6V
ILEAKAGE
RIC
≤ 1K
Sampling
Switch
SS RSS
CHOLD
= DAC Capacitance
VSS
6V
Sampling Switch
5V
4V
3V
2V
567 8 9 10 11
(k
)
VDD
= 120 pF
± 500 nA
Legend: CPIN
VT
ILEAKAGE
RIC
SS
CHOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
various junctions
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