
2011 Microchip Technology Inc.
Preliminary
DS41455B-page 125
PIC16LF1902/3
15.3
A/D Acquisition Requirements
For the ADC to meet its specified accuracy, the charge
holding capacitor (CHOLD) must be allowed to fully
charge to the input channel voltage level. The Analog
impedance (RS) and the internal sampling switch (RSS)
impedance directly affect the time required to charge
the capacitor CHOLD. The sampling switch (RSS)
impedance varies over the device voltage (VDD), 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
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
++
=
TAMP
TC
TCOFF
++
=
2s
TC
Temperature - 25°C
0.05s/°C
++
=
TC
CHOLD RIC
RSS
RS
++
ln(1/511)
–
=
10pF 1k
7k 10k
++
–
ln(0.001957)
=
1.12
=
s
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] VCHOLD charge response to VAPPLIED
;combining [1] and [2]
The value for TC can be approximated with the following equations:
Solving for TC:
Therefore:
Temperature
50°C and external impedance of 10k
5.0V VDD
=
Assumptions:
Note: Where n = number of bits of the ADC.
TACQ
2s
1.12s
50°C- 25°C
0.05s/°C
++
=
4.42s
=
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.