Microchip Technology PIC12F1501, Manual

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
2011-2015 Microchip Technology Inc. DS40001609E-page 139
PIC16(L)F1508/9
15.4 ADC Acquisition Requirements
For the ADC to meet its specified accuracy, 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 15-4 . 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
), refer
to Figure 15-4 .
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 ADC acquisition must be
done before the conversion can be started. To calculate
the minimum acquisition time,
Equation 15-1 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.
EQUATION 15-1: 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)
–
=
12.5pF 1k

7k

10k

+ +
 
–
ln(0.0004885) =
1.72
= µs
V
AP P LI ED
1 e
Tc –
RC
---------
–
 
 
 
V
A PP L I E D
1
1
2
n 1 +
 
1 –
-------------------------- –
 
 
=
V
AP P LI ED
1
1
2
n 1 +
 
1 –
-------------------------- –
 
 
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

5.0V V
DD
=
Assumptions:
Note: Where n = number of bits of the ADC.
T
A C Q
2µs
1.72
µs 50°C- 25°C
 
0.05
µs/°C
   
+ + =
4.97µs =
Note 1:
The reference voltage (V
RPOS
) 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.