PIC18F6393_09 Datasheet, PDF (36/58 Pages) Microchip Technology – 64/80-Pin High Performance, Flash Microcontrollers with LCD Driver, 12-Bit ADC and nanoWatt Technology

PIC18F6393/6493/8393/8493
2.1 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 2-3. 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 ana-
log 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), the channel must be sampled for at least
the minimum acquisition time before starting a
conversion.
Note:
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
To calculate the minimum acquisition time, Equation 2-1
may be used. This equation assumes that 1/2 LSb error
is used (4096 steps for the 12-bit A/D). The 1/2 LSb error
is the maximum error allowed for the A/D to meet its
specified resolution.
Equation 2-3 shows the calculation of the minimum
required acquisition time, TACQ. This calculation is
based on the following application system
assumptions:
CHOLD
= 25 pF
Rs
= 2.5 kΩ
Conversion Error ≤ 1/2 LSb
VDD
= 3V → Rss = 4 kΩ
Temperature
= 85°C (system max.)
EQUATION 2-1: ACQUISITION TIME
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
EQUATION 2-2: A/D MINIMUM CHARGING TIME
VHOLD =
or
TC
=
(VREF – (VREF/4096)) • (1 – e^{(-TC/CHOLD(RIC + RSS + RS))})
– (CHOLD)(RIC + RSS + RS) ln(1/4096)
EQUATION 2-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = TAMP + TC + TCOFF
TAMP = 0.2 µs
TCOFF =
(Temp – 25°C)(0.02 µs/°C)
(85°C – 25°C)(0.02 µs/°C)
1.2 µs
Temperature coefficient is only required for temperatures > 25°C. Below 25°C, TCOFF = 0 µs.
TC
= -(CHOLD)(RIC + RSS + RS) ln(1/4096) µs
-(25 pF) (1 kΩ + 4 kΩ + 2.5 kΩ) ln(0.0002441) µs
1.56 µs
TACQ = 0.2 µs + 1.56 μs + 1.2 µs
2.96 µs
DS39896B-page 36
© 2009 Microchip Technology Inc.