PIC18FXX8 Datasheet, PDF (247/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

PIC18FXX8
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
=
• Rs
=
• Conversion Error ≤
• VDD
=
• Temperature
=
• VHOLD
=
120 pF
2.5 kΩ
1/2 LSb
5V → Rss = 7 kΩ
50°C (system max.)
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
Note:
When using external voltage references with the A/D converter, the source impedance of the external
voltage references must be less than 20Ω to obtain the specified A/D resolution. Higher reference source
impedances will increase both offset and gain errors. Resistive voltage dividers will not provide a sufficiently
low source impedance.
To maintain the best possible performance in A/D conversions, external VREF inputs should be buffered with
an operational amplifier or other low output impedance circuit.
 2004 Microchip Technology Inc.
DS41159D-page 245