PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 243

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

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.
Example 20-1 shows the calculation of the minimum
required acquisition time T
based on the following application system assumptions:
• C
• Rs
• Conversion Error ≤
• V
• Temperature
• V
EQUATION 20-1:
EQUATION 20-2:
EXAMPLE 20-1:
 2002 Microchip Technology Inc.
T
V
or
Tc
T
Temperature coefficient is only required for temperatures > 25°C.
T
T
T
ACQ
ACQ
ACQ
C
ACQ
HOLD
DD
HOLD
HOLD
calculate
=
=
=
=
=
=
=
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
T
2 µs + T
-C
-120 pF (1 kΩ + 7 kΩ + 2.5 kΩ) ln(0.0004885)
-120 pF (10.5 kΩ) ln(0.0004885)
-1.26 µs (-7.6241)
9.61 µs
2 µs + 9.61 µs + [(50°C - 25°C)(0.05 µs/°C)]
11.61 µs + 1.25 µs
12.86 µs
AMP
AMP
HOLD
(V
-(120 pF)(1 kΩ + R
the
REF
+ T
+ T
=
=
=
=
=
C
(R
C
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
– (V
C
minimum
+ [(Temp - 25°C)(0.05 µs/°C)]
+ T
IC
+ T
120 pF
2.5 kΩ
1/2 LSb
5V → Rss = 7 kΩ
50°C (system max.)
0V @ time = 0
+ R
REF
COFF
COFF
ACQ
SS
/2048)) • (1 - e
. This calculation is
+ R
SS
acquisition
S
) ln(1/2047)
+ R
S
) ln(1/2047)
(-Tc/C
HOLD
time,
Preliminary
(R
IC
+ R
SS
+ R
S
))
)
PIC18FXX8
DS41159B-page 241