PIC18F4410 MICROCHIP [Microchip Technology], PIC18F4410 Datasheet - Page 218

• Manufacturer Part Number: PIC18F4410
• Description: 28/40/44-Pin Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2X1X/4X1X
18.1
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 shown in Figure 18-3. 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 2.5 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 18-1:
EQUATION 18-2:
EQUATION 18-3:
DS39636C-page 216
T
V
or
T
T
T
T
Temperature coefficient is only required for temperatures > 25°C. Below 25°C, T
T
T
ACQ
C
ACQ
AMP
COFF
C
ACQ
DD
Note:
HOLD
). The source impedance affects the offset voltage
=
=
A/D Acquisition Requirements
SS
=
=
=
=
=
SS
=
=
) impedance varies over the device voltage
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
) impedance directly affect the time
AMP
T
0.2 μs
(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)
1.05 μs
0.2 μs + 1 μs + 1.2 μs
2.4 μs
(V
-(C
AMP
+ T
REF
HOLD
HOLD
+ T
C
– (V
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
S
+ T
)(R
C
)(R
) and the internal sampling
+ T
COFF
REF
IC
IC
+ R
COFF
/2048)) • (1 – e
+ R
HOLD
SS
SS
HOLD
+ R
+ R
) must be allowed
S
S
) ln(1/2047)
. The sampling
) ln(1/2048)
(-T
C
/C
HOLD
Preliminary
(R
IC
+ R
SS
+ R
S
To
Equation 18-1 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.
Example 18-3 shows the calculation of the minimum
required acquisition time T
based
assumptions:
C
Rs
Conversion Error
V
Temperature
))
DD
HOLD
)
calculate
on
COFF
the
= 0 ms.
the
=
=
≤
=
=
following
minimum
© 2007 Microchip Technology Inc.
25 pF
2.5 kΩ
1/2 LSb
5V → Rss = 2 kΩ
85°C (system max.)
ACQ
. This calculation is
application
acquisition
system
time,