PIC18F-LF1XK50 MICROCHIP [Microchip Technology], PIC18F-LF1XK50 Datasheet - Page 219

• Manufacturer Part Number: PIC18F-LF1XK50
• Description: 20-Pin USB Flash Microcontrollers
• Manufacturer: MICROCHIP [Microchip Technology]

17.3
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
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 k. As the source impedance is
decreased, the acquisition time may be decreased.
After the analog input channel is selected (or changed),
EQUATION 17-1:
 2010 Microchip Technology Inc.
Note 1: The reference voltage (V
The value for T
Solving for T
Therefore:
Assumptions:
2: The charge holding capacitor (C
3: The maximum recommended impedance for analog sources is 10 k. 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
. The sampling switch (R
AP P LI ED
ACQ
AP P LIED
C
C
=
=
=
=
=
:
C
HOLD
=
=
=
–
ACQUISITION TIME EXAMPLE
5µs
Temperature
–
7.45µs
can be approximated with the following equations:
1.20
13.5pF 1k


C
Amplifier Settling Time
T
5µs






1
) must be allowed to fully
AMP
HOLD
1 e
1 e
+
µs
–
Figure
–
+
–
1.20µs
----------- -
2047

+
T

1
–
--------- -
R
–
-------- -
C
DD
RC
RC
T
T
Tc
IC

+
C
C
17-5. The source
), see








+
+
+

+
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
) impedance
50°C and external impedance of 10k
Figure
V
V
V

+
CHOLD
CHOLD
A P PLIE D
HOLD
+
R
S
10k
 ln(1/2047)
17-5.
+
) is discharged after each conversion.
Preliminary
SS
Hold Capacitor Charging Time

)
 0.05µs/°C




1
ln(0.0004885)
–
 0.05µs/°C
----------- -
2047

1
an A/D acquisition must be done before the conversion
can be started. To calculate the minimum acquisition
time,
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.




Equation 17-1
;[2] V
;combining [1] and [2]
;[1] V


PIC18F/LF1XK50
CHOLD
CHOLD

3.0V V
+
charged to within 1/2 lsb
charge response to V
may be used. This equation
Temperature Coefficient
DD
DS41350E-page 219
APPLIED