PIC18F87J93 Microchip Technology, PIC18F87J93 Datasheet - Page 34

• Manufacturer Part Number: PIC18F87J93
• Description: LCD PIC18F Microcontroller
• Manufacturer: Microchip Technology

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PIC18F87J93 FAMILY
2.1
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 2-2. The source
impedance (R
impedance directly affect the time required to charge
the capacitor C
impedance varies over the device voltage (V
source impedance affects the offset voltage at the ana-
log 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 2-1:
EQUATION 2-2:
EQUATION 2-3:
DS39948A-page 32
T
V
or
T
T
T
T
Temperature coefficient is only required for temperatures > 25°C. Below 25°C, T
T
T
ACQ
Note:
C
ACQ
AMP
COFF
C
ACQ
HOLD
=
=
A/D Acquisition Requirements
=
=
=
=
=
=
=
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
S
AMP
) and the internal sampling switch (R
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) μs
1.05 μs
0.2 μs + 1 μs + 1.2 μs
2.4 μs
(V
-(C
AMP
HOLD
HOLD
+ T
REF
HOLD
+ T
C
. The sampling switch (R
– (V
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
+ T
)(R
C
)(R
+ T
REF
COFF
IC
IC
+ R
COFF
/2048)) • (1 – e
+ R
HOLD
SS
SS
+ R
+ R
) must be allowed
S
S
) ln(1/2048) μs
) ln(1/2048)
(-T
C
DD
/C
). The
HOLD
Preliminary
SS
SS
(R
)
)
IC
+ R
SS
+ R
S
To calculate the minimum acquisition time, Equation 2-1
may be used. This equation assumes that 1/2 LSb error
is used (1,024 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 2-3 shows the calculation of the minimum
required acquisition time, T
based
assumptions:
))
C
Rs
Conversion Error
V
Temperature
)
HOLD
DD
on
COFF
the
= 0 ms.
=
=
≤
=
=
following
© 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
system