PIC18LF24K Datasheet, PDF (433/594 Pages) –

PIC18(L)F26/45/46K40
31.3 ADC 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
Input model is shown in Figure 31-4. The source
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 Figure 31-4. The maximum recommended
impedance for analog sources is 10 k. As the
EQUATION 31-1: ACQUISITION TIME EXAMPLE
source impedance is decreased, the acquisition time
may be decreased. After the analog input channel is
selected (or changed), an ADC acquisition must be
completed before the conversion can be started. To
calculate the minimum acquisition time, Equation 31-1
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.
Assumptions: Temperature = 50°C and external impedance of 10k 5.0V VDD
TACQ = Amplifier Settling Time + Hold Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
= 2µs + TC + Temperature - 25°C0.05µs/°C
The value for TC can be approximated with the following equations:
V
A
P
P
LI
E
D


1
–
^{---2---n----+--1--1--}^----–^{----1--}
=
VCHOLD

VAPPLIED1
–
e^{–-R---T--C-C--}
=
VCHOLD


;[1] VCHOLD charged to within 1/2 lsb
;[2] VCHOLD charge response to VAPPLIED

VAPPLIED1
–
e^{–-R---T-C--c-}


=
V
A
P
P
L
I
E
D


1
–
^{---2---n----+--1--1--}^----–^{----1--}
;combining [1] and [2]
Note: Where n = number of bits of the ADC.
Solving for TC:
TC = –CHOLDRIC + RSS + RS ln(1/2047)
= –10pF1k + 7k + 10k ln(0.0004885)
= 1.37µs
Therefore:
TACQ = 2µs + 892ns + 50°C- 25°C0.05µs/°C
= 4.62µs
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.
 2016 Microchip Technology Inc.
Preliminary
DS40001816C-page 433