Microchip Technology AC244045 Data Sheet
2010-2012 Microchip Technology Inc.
DS41440C-page 159
PIC16(L)F1825/1829
16.3
A/D Acquisition Requirements
For the ADC to meet its specified accuracy, the charge
holding capacitor (C
holding capacitor (C
HOLD
) must be allowed to fully
charge to the input channel voltage level. The Analog
Input model is shown in
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
the capacitor C
HOLD
. The sampling switch (R
SS
)
impedance varies over the device voltage (V
DD
), refer
to
. The maximum recommended
impedance for analog sources is 10 k
. As the
source impedance is decreased, the acquisition time
may be decreased. After the analog input channel is
selected (or changed), an A/D acquisition must be
done before the conversion can be started. To calculate
the minimum acquisition time,
may be decreased. After the analog input channel is
selected (or changed), an A/D acquisition must be
done before the conversion can be started. To calculate
the minimum acquisition time,
may be
used. This equation assumes that 1/2 LSb error is used
(1,024 steps for the ADC). The 1/2 LSb error is the
maximum error allowed for the ADC to meet its
specified resolution.
(1,024 steps for the ADC). The 1/2 LSb error is the
maximum error allowed for the ADC to meet its
specified resolution.
EQUATION 16-1:
ACQUISITION TIME EXAMPLE
T
ACQ
Amplifier Settling Time
Hold Capacitor Charging Time
Temperature Coefficient
+
+
=
T
AMP
T
C
T
COFF
+
+
=
2µs
T
C
Temperature - 25°C
0.05µs/°C
+
+
=
T
C
C
HOLD
R
IC
R
SS
R
S
+
+
ln(1/2047)
–
=
12.5pF 1k
7k 10k
+
+
–
ln(0.0004885)
=
1.12
=
µs
V
APPLIED
1 e
Tc
–
RC
---------
–
V
APPLIED
1
1
2
n
1
+
1
–
--------------------------
–
=
V
APPLIED
1
1
2
n
1
+
1
–
--------------------------
–
V
CHOLD
=
V
APPLIED
1 e
T
C
–
RC
----------
–
V
CHOLD
=
;[1] V
CHOLD
charged to within 1/2 lsb
;[2] V
CHOLD
charge response to V
APPLIED
;combining [1] and [2]
The value for T
C
can be approximated with the following equations:
Solving for T
C
:
Therefore:
Temperature
50°C and external impedance of 10k
5.0V V
DD
=
Assumptions:
Note: Where n = number of bits of the ADC.
T
ACQ
5µs
1.12
µs
50°C- 25°C
0.05
µs/°C
+
+
=
7.37µs
=
Note 1:
The reference voltage (V
REF
+) has no effect on the equation, since it cancels itself out.
2:
The charge holding capacitor (C
HOLD
) is not discharged after each conversion.
3:
The maximum recommended impedance for analog sources is 10 k
. This is required to meet the pin
leakage specification.