Microchip Technology AC244045 Data Sheet
PIC16F72X/PIC16LF72X
DS41341E-page 106
© 2009 Microchip Technology Inc.
9.3
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 Figure 9-3. The source imped-
ance (R
Input model is shown in Figure 9-3. The source imped-
ance (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 Figure 9-3. The maximum recommended imped-
ance for analog sources is 10 k
ance for analog sources is 10 k
Ω. 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 mini-
mum acquisition time, Equation 9-1 may be used. This
equation assumes that 1/2 LSb error is used (256 steps
for the ADC). The 1/2 LSb error is the maximum error
allowed for the ADC to meet its specified resolution.
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 mini-
mum acquisition time, Equation 9-1 may be used. This
equation assumes that 1/2 LSb error is used (256 steps
for the ADC). The 1/2 LSb error is the maximum error
allowed for the ADC to meet its specified resolution.
EQUATION 9-1:
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(1/511)
–
=
10pF 1k
Ω 7kΩ 10kΩ
+
+
(
)
–
ln(0.001957)
=
1.12
=
µs
T
ACQ
2
ΜS
1.12
ΜS
50°C- 25°C
(
) 0.05
ΜS
/°C
(
)
[
]
+
+
=
4.42
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
Ω 5.0V V
DD
=
Assumptions:
Note: Where n = number of bits of the ADC.
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
Ω. This is required to meet the pin
leakage specification.