PIC18F25J11-I/PT MICROCHIP [Microchip Technology], PIC18F25J11-I/PT Datasheet - Page 350

• Manufacturer Part Number: PIC18F25J11-I/PT
• Description: 28/44-Pin, Low-Power, High-Performance Microcontrollers with nanoWatt XLP Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F46J11 FAMILY
20.1
For the A/D Converter to meet its specified accuracy,
the charge holding capacitor (C
to fully charge to the input channel voltage level. The
analog input model is illustrated in Figure 20-2. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 10 kΩ. After the analog input channel is
selected (changed), the channel must be sampled for
at least the minimum acquisition time before starting a
conversion.
EQUATION 20-1:
EQUATION 20-2:
EQUATION 20-3:
DS39932C-page 350
T
V
or
T
T
T
T
Temperature coefficient is only required for temperatures > 25°C. Below 25°C, T
T
T
ACQ
DD
Note:
C
ACQ
AMP
COFF
C
ACQ
HOLD
). The source impedance affects the offset voltage
=
=
A/D Acquisition Requirements
SS
=
=
=
=
=
SS
=
=
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
) impedance varies over the device voltage
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
) impedance directly affect the time
AMP
T
0.2 ms
(Temp – 25°C)(0.02 μs/°C)
(85°C – 25°C)(0.02 μs/°C)
1.2 μs
-(C
-(25 pF) (1 kΩ + 2 kΩ + 2.5 kΩ) ln(0.0004883) μs
1.05 μs
0.2 μs + 1.05 μs + 1.2 μs
2.45 μs
(V
-(C
AMP
HOLD
+ T
REF
HOLD
+ T
C
– (V
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
S
+ T
)(R
C
) and the internal sampling
)(R
+ T
REF
COFF
IC
IC
+ R
COFF
/2048)) • (1 – e
+ R
HOLD
SS
SS
HOLD
+ R
+ R
) must be allowed
S
S
) ln(1/2048) μs
. The sampling
) ln(1/2048)
(-T
C
/C
HOLD
(R
IC
+ R
SS
+ R
S
To
Equation 20-1 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.
Equation 20-3 provides the calculation of the minimum
required acquisition time, T
based
assumptions:
C
Rs
Conversion Error
V
Temperature
))
HOLD
DD
)
calculate
on
COFF
the
= 0 μs.
the
=
=
≤
=
=
following
minimum
© 2009 Microchip Technology Inc.
25 pF
2.5 kΩ
1/2 LSb
3V → Rss = 2 kΩ
85°C (system max.)
ACQ
. This calculation is
application
acquisition
system
time,