Microchip Technology MA180025 Data Sheet
PIC18F87J90 FAMILY
DS39933D-page 294
2010 Microchip Technology Inc.
21.1
A/D Acquisition Requirements
For the A/D Converter to meet its specified accuracy,
the charge holding capacitor (C
the charge holding capacitor (C
HOLD
) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 21-2. The
source impedance (R
analog input model is shown in Figure 21-2. 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
maximum recommended impedance for analog
sources is 2.5 k
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.
at least the minimum acquisition time before starting a
conversion.
To calculate the minimum acquisition time,
Equation 21-1 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 21-3 shows the calculation of the minimum
required acquisition time, T
Equation 21-1 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 21-3 shows the calculation of the minimum
required acquisition time, T
ACQ
. This calculation is
based on the following application system
assumptions:
C
assumptions:
C
HOLD
=
25
pF
Rs
=
2.5 k
Conversion Error
1/2 LSb
V
DD
=
3V
Rss = 2 k
Temperature
=
85
C (system max.)
EQUATION 21-1:
A/D ACQUISITION TIME
EQUATION 21-2:
A/D MINIMUM CHARGING TIME
EQUATION 21-3:
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.
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 s + 1.2 s
2.4
s