Microchip Technology MCP1630DM-DDBS1 Data Sheet
©
2007 Microchip Technology Inc.
DS41211D-page 67
PIC12F683
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 9-4. The source
impedance (R
Input model is shown in Figure 9-4. The source
impedance (R
S
) and the internal sampling switch (R
SS
)
impedance directly affect the time required to charge the
capacitor C
capacitor C
HOLD
. The sampling switch (R
SS
) impedance
varies over the device voltage (V
DD
The maximum recommended impedance for analog
sources is 10 k
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),
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, Equation 9-1 may be used. This equation
assumes that 1/2 LSb error is used (1024 steps for the
ADC). The 1/2 LSb error is the maximum error allowed
for the ADC to meet its specified resolution.
can be started. To calculate the minimum acquisition
time, Equation 9-1 may be used. This equation
assumes that 1/2 LSb error is used (1024 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
HOL D
R
IC
R
S S
R
S
+
+
(
) ln(1/2047)
–
=
10pF 1k
Ω 7kΩ 10kΩ
+
+
(
)
–
ln(0.0004885)
=
1.37
=
µs
T
AC Q
2µ
S
1.37µ
S
50°C- 25°C
(
) 0.05µ
S
/°C
(
)
[
]
+
+
=
4.67µ
S
=
V
AP P L I ED
1
e
Tc
–
RC
---------
–
⎝
⎠
⎜
⎟
⎛
⎞
V
A P PL I E D
1
1
2047
------------
–
⎝
⎠
⎛
⎞
=
V
AP PL I E D
1
1
2047
------------
–
⎝
⎠
⎛
⎞
V
C H O L D
=
V
AP P LI ED
1
e
T
C
–
RC
----------
–
⎝
⎠
⎜
⎟
⎛
⎞
V
C H O L D
=
;[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 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.