PIC18F242 MICROCHIP [Microchip Technology], PIC18F242 Datasheet - Page 187

• Manufacturer Part Number: PIC18F242
• Description: 28/40-pin High Performance, Enhanced FLASH Microcontrollers with 10-Bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

To
Equation 17-1 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 17-1:
EQUATION 17-2:
Example 17-1 shows the calculation of the minimum
required acquisition time, T
based on the following application system assump-
tions:
• C
• Rs
• Conversion Error
• V
• Temperature
• V
EXAMPLE 17-1:
© 2006 Microchip Technology Inc.
T
V
or
T
T
Temperature coefficient is only required for temperatures > 25 C.
T
T
T
ACQ
C
ACQ
ACQ
C
ACQ
HOLD
DD
HOLD
HOLD
calculate
=
=
=
=
=
=
=
=
T
2 s + T
-C
-120 pF (1 k + 7 k + 2.5 k ) ln(0.0004883)
-120 pF (10.5 k ) ln(0.0004883)
-1.26 s (-7.6246)
9.61 s
2 s + 9.61 s + [(50 C – 25 C)(0.05 s/ C)]
11.61 s + 1.25 s
12.86 s
AMP
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
(V
-(120 pF)(1 k + R
T
HOLD
AMP
REF
the
+ T
+ T
C
– (V
(R
C
+ [(Temp – 25 C)(0.05 s/ C)]
=
=
=
=
=
IC
ACQUISITION TIME
C
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
+ T
minimum
REF
+ T
+ R
COFF
120 pF
2.5 k
1/2 LSb
5V
50 C (system max.)
0V @ time = 0
/2048)) • (1 – e
COFF
SS
ACQ
+ R
SS
Rss = 7 k
. This calculation is
S
+ R
) ln(1/2048)
acquisition
S
) ln(1/2048)
(-Tc/C
HOLD
time,
(R
IC
+ R
SS
+ R
S
))
)
PIC18FXX2
DS39564C-page 185