PIC18F6X2X Datasheet, PDF (242/386 Pages) Microchip Technology – 64/80-Pin High Performance, 64-Kbyte Enhanced FLASH Microcontrollers with A/D

PIC18F6X2X/8X2X
20.1 A/D Acquisition Requirements
For the A/D converter 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 20-2. 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). The source impedance affects the offset voltage
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 2.5 kΩ. After the analog input channel is
selected (changed), this acquisition must be done
before the conversion can be started.
Note:
When the conversion is started, the hold-
ing capacitor is disconnected from the
input pin.
To calculate the minimum acquisition time,
Equation 20-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Example 20-1 shows the calculation of the minimum
required acquisition time, TACQ. This calculation is
based on the following application system
assumptions:
CHOLD
= 120 pF
Rs
= 2.5 kΩ
Conversion Error ≤ 1/2 LSb
VDD
= 5V → Rss = 7 kΩ
Temperature
= 50°C (system max.)
VHOLD
= 0V @ time = 0
EQUATION 20-1: ACQUISITION TIME
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
EQUATION 20-2: A/D MINIMUM CHARGING TIME
VHOLD = (VREF – (VREF/2048)) • (1 – e^{(-Tc/CHOLD(RIC + RSS + RS))})
or
Tc
= -(120 pF)(1 kΩ + RSS + RS) ln(1/2047)
EXAMPLE 20-1: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25°C.
TACQ = 2 µs + TC + [(Temp – 25°C)(0.05 µs/°C)]
TC
= -CHOLD (RIC + RSS + RS) ln(1/2047)
-120 pF (1 kΩ + 7 kΩ + 2.5 kΩ) ln(0.0004885)
-120 pF (10.5 kΩ) ln(0.0004885)
-1.26 µs (-7.6241)
9.61 µs
TACQ = 2 µs + 9.61 µs + [(50°C – 25°C)(0.05 µs/°C)]
11.61 µs + 1.25 µs
12.86 µs
DS39612A-page 240
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