Datasheet

PIC12(L)F1501
DS41615A-page 124 Preliminary  2011 Microchip Technology Inc.
15.3 A/D 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 (RSS)
impedance directly affect the time required to charge
the capacitor C
HOLD. The sampling switch (RSS)
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 A/D 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
TACQ Amplifier Settling Time Hold Capacitor Charging Time Temperature Coefficient++=
T
AMP TC TCOFF++=
2µs T
C Temperature - 25°C0.05µs/°C++=
TC CHOLD RIC RSS RS++ ln(1/2047)–=
12.5pF 1k
 7k  10k  ++– ln(0.0004885)=
1.12= µs
VAPPLIED 1e
Tc–
RC
---------
–



VAPPLIED 1
1
2
n1+
1–
--------------------------–


=
VAPPLIED 1
1
2
n1+
1–
--------------------------
–


VCHOLD=
VAPPLIED 1e
TC–
RC
----------
–



VCHOLD=
;[1] VCHOLD charged to within 1/2 lsb
;[2] V
CHOLD charge response to VAPPLIED
;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 VDD=
Assumptions:
Note: Where n = number of bits of the ADC.
T
ACQ 5µs 1.12µs 50°C- 25°C0.05µs/°C++=
7.37µs=
Note 1: The reference voltage (VREF+) has no effect on the equation, since it cancels itself out.
2: The charge holding capacitor (CHOLD) 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.