PIC18F2480 MICROCHIP [Microchip Technology], PIC18F2480 Datasheet - Page 254

• Manufacturer Part Number: PIC18F2480
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2480/2580/4480/4580
19.1
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 19-2. 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 2.5 k . 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 19-1:
EQUATION 19-2:
EQUATION 19-3:
DS39637A-page 252
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.
) impedance directly affect the time
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
(V
-(C
T
5 s
(Temp – 25 C)(0.05 s/ C)
(50 C – 25 C)(0.05 s/ C)
1.25 s
-(C
-(120 pF) (1 k + 7 k + 2.5 k ) ln(0.0004883) s
9.61 s
5 s + 1.25 s + 9.61 s
12.86 s
AMP
AMP
REF
HOLD
HOLD
+ T
+ T
– (V
)(R
)(R
C
C
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
S
REF
+ T
+ T
) and the internal sampling
IC
IC
+ R
COFF
/2048)) • (1 – e
+ R
COFF
SS
SS
HOLD
+ R
+ R
HOLD
S
S
) must be allowed
) ln(1/2048)
) ln(1/2047) s
. The sampling
(-Tc/C
HOLD
(R
Preliminary
IC
+ R
SS
+ R
S
))
)
To
Equation 19-1 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 19-3 shows the calculation of the minimum
required acquisition time T
based
assumptions:
C
Rs
Conversion Error
V
Temperature
V
DD
HOLD
HOLD
calculate
on
COFF
the
= 0 ms.
the
=
=
=
=
=
following
minimum
 2004 Microchip Technology Inc.
120 pF
2.5 k
1/2 LSb
5V
50 C (system max.)
0V @ time = 0
ACQ
Rss = 7 k
. This calculation is
application
acquisition
system
time,