PIC18F23K20-E/MLQTP MICROCHIP [Microchip Technology], PIC18F23K20-E/MLQTP Datasheet - Page 267

• Manufacturer Part Number: PIC18F23K20-E/MLQTP
• Description: 28/40/44-Pin Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

19.3
For the ADC to meet its specified accuracy, the charge
holding capacitor (C
charge to the input channel voltage level. The Analog
Input model is shown in Figure 19-5. The source
impedance (R
impedance directly affect the time required to charge the
capacitor C
varies over the device voltage (V
The maximum recommended impedance for analog
sources is 10 kΩ. As the source impedance is
decreased, the acquisition time may be decreased.
After the analog input channel is selected (or changed),
EQUATION 19-1:
© 2007 Microchip Technology Inc.
Note 1: The reference voltage (V
Assumptions:
The value for T
Solving for T
Therefore:
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 kΩ. This is required to meet the pin
A/D Acquisition Requirements
HOLD
leakage specification.
T
V
S
V
V
ACQ
T
) and the internal sampling switch (R
AP PLIE D
T
AP P LI ED
. The sampling switch (R
ACQ
AP P LIED
C
C
=
=
=
=
=
:
C
HOLD
=
=
=
–
ACQUISITION TIME EXAMPLE
Temperature
–
5µ
7.45µ
can be approximated with the following equations:
1.20
13.5pF 1k
⎛
⎝
C
Amplifier Settling Time
T
5µs
⎛
⎜
⎝
⎛
⎜
⎝
S
1
) must be allowed to fully
AMP
HOLD
1 e
1 e
+
µs
–
S
–
+
–
1.20µ
----------- -
2047
(
+
T
(
1
R
–
--------- -
C
–
-------- -
DD
RC
RC
T
T
Tc
IC
Ω
+
C
C
S
), see Figure 19-5.
⎞
⎠
⎞
⎟
⎠
⎞
⎟
⎠
+
+
+
[
+
REF
(
=
T
Temperature - 25°C
=
=
=
R
700
[
SS
(
COFF
) has no effect on the equation, since it cancels itself out.
SS
50°C- 25°C
50°C and external impedance of 10k
) impedance
V
V
V
Ω
Advance Information
+
CHOLD
CHOLD
A P PLIE D
HOLD
+
R
S
10k
) ln(1/2047)
+
) is discharged after each conversion.
SS
Hold Capacitor Charging Time
Ω
)
) 0.05µ
⎛
⎝
)
(
1
ln(0.0004885)
–
) 0.05µs/°C
----------- -
2047
(
S
1
/°C
an A/D acquisition must be done before the conversion
can be started. To calculate the minimum acquisition
time, Equation 19-1 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.
PIC18F2XK20/4XK20
⎞
⎠
)
]
;[2] V
;combining [1] and [2]
;[1] V
)
]
CHOLD
CHOLD
Ω
3.0V V
+
charged to within 1/2 lsb
charge response to V
Temperature Coefficient
DD
DS41303B-page 265
APPLIED