Microchip Technology MA160014 数据表
PIC18(L)F2X/4XK22
DS41412F-page 308
2010-2012 Microchip Technology Inc.
17.4
A/D Acquisition Requirements
For the ADC to meet its specified accuracy, the charge
holding capacitor (C
holding capacitor (C
HOLD
) must be allowed to fully
charge to the input channel voltage level. The Analog
Input model is shown in
Input model is shown in
. The source
impedance (R
S
) and the internal sampling switch (R
SS
)
impedance directly affect the time required to charge the
capacitor C
capacitor C
HOLD
. The sampling switch (R
SS
) impedance
varies over the device voltage (V
DD
), see
.
The maximum recommended impedance for analog
sources is 10 k
sources is 10 k
. As the source impedance is
decreased, the acquisition time may be decreased.
After the analog input channel is selected (or changed),
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,
can be started. To calculate the minimum acquisition
time,
may be used. This equation
assumes that 1/2 LSb error is used (1024 steps for the
ADC). The 1/2 LSb error is the maximum error allowed
for the ADC to meet its specified resolution.
ADC). The 1/2 LSb error is the maximum error allowed
for the ADC to meet its specified resolution.
EQUATION 17-1:
ACQUISITION TIME EXAMPLE
T
ACQ
Amplifier Settling Time
Hold Capacitor Charging Time
Temperature Coefficient
+
+
=
T
AMP
T
C
T
COFF
+
+
=
5µs
T
C
Temperature - 25°C
0.05µs/°C
+
+
=
T
C
C
HOL D
R
IC
R
S S
R
S
+
+
ln(1/2047)
–
=
13.5pF 1k
700 10k
+
+
–
ln(0.0004885)
=
1.20
=
µs
T
ACQ
5µs
1.20µs
50°C- 25°C
0.05
s/
°C
+
+
=
7.45µs
=
V
AP P L I ED
1
e
Tc
–
RC
---------
–
V
A P PL I E D
1
1
2047
------------
–
=
V
AP PL I E D
1
1
2047
------------
–
V
C H O L D
=
V
AP P LI ED
1
e
T
C
–
RC
----------
–
V
C H O L D
=
;[1] V
CHOLD
charged to within 1/2 lsb
;[2] V
CHOLD
charge response to V
APPLIED
;combining [1] and [2]
The value for T
C
can be approximated with the following equations:
Solving for T
C
:
Therefore:
Temperature
50°C and external impedance of 10k
3.0V V
DD
=
Assumptions:
Note 1: The reference voltage (V
REF
) has no effect on the equation, since it cancels itself out.
2: The charge holding capacitor (C
HOLD
) is discharged after each conversion.
3: The maximum recommended impedance for analog sources is 10 k
. This is required to meet the pin
leakage specification.