Microchip Technology MCU PIC PIC18F87K22-I/PTRSL TQFP-80 MCP PIC18F87K22-I/PTRSL Datenbogen
Produktcode
PIC18F87K22-I/PTRSL
PIC18F87K22 FAMILY
DS39960D-page 362
2009-2011 Microchip Technology Inc.
23.3
A/D Acquisition Requirements
For the A/D Converter to meet its specified accuracy,
the Charge Holding (C
the Charge Holding (C
HOLD
) capacitor must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in
analog input model is shown in
. The
source impedance (R
S
) and the internal sampling
switch (R
SS
) impedance directly affect the time
required to charge the capacitor C
HOLD
. The sampling
switch (R
SS
) impedance varies over the device voltage
(V
DD
).
The source impedance affects the offset voltage at the
analog input (due to pin leakage current). The maxi-
mum recommended impedance for analog sources
is 2.5 k
analog input (due to pin leakage current). The maxi-
mum recommended impedance for analog sources
is 2.5 k
. After the analog input channel is selected or
changed, the channel must be sampled for at least the
minimum acquisition time before starting a conversion.
minimum acquisition time before starting a conversion.
To calculate the minimum acquisition time,
can 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.
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
shows the calculation of the minimum
required acquisition time, T
ACQ
. This calculation is
based on the following application system
assumptions:
assumptions:
EQUATION 23-1:
ACQUISITION TIME
EQUATION 23-2:
A/D MINIMUM CHARGING TIME
EQUATION 23-3:
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
Note:
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
holding capacitor is disconnected from the
input pin.
• C
HOLD
= 25
pF
• Rs
= 2.5 k
• Conversion Error
1/2 LSb
• V
DD
= 3V
Rss = 2 k
• Temperature
= 85
C
T
ACQ
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=
T
AMP
+ T
C
+ T
COFF
V
HOLD
= (V
REF
– (V
REF
/2048)) • (1 – e
(-T
C
/C
HOLD
(R
IC
+ R
SS
+ R
S
))
)
or
T
T
C
=
-(C
HOLD
)(R
IC
+ R
SS
+ R
S
) ln(1/2048)
T
ACQ
=
T
AMP
+ T
C
+ T
COFF
T
AMP
=
0.2
s
T
COFF
=
(Temp – 25
C)(0.02 s/C)
(85
C – 25C)(0.02 s/C)
1.2
s
Temperature coefficient is only required for temperatures > 25
C. Below 25C, T
COFF
= 0 ms.
T
C
=
-(C
HOLD
)(R
IC
+ R
SS
+ R
S
) ln(1/2048)
s
-(25 pF) (1 k
+ 2 k + 2.5 k) ln(0.0004883) s
1.05
s
T
ACQ
=
0.2
s + 1.05 s + 1.2 s
2.45
s