PIC18LF252-ISP Datasheet, PDF (187/332 Pages) Microchip Technology – High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D

PIC18FXX2
To calculate the minimum acquisition time,
Equation 17-1 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.
EQUATION 17-1: ACQUISITION TIME
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
= TAMP + TC + TCOFF
EQUATION 17-2: A/D MINIMUM CHARGING TIME
VHOLD =
or
TC
=
(VREF – (VREF/2048)) • (1 – e^{(-Tc/CHOLD(RIC + RSS + RS))})
-(120 pF)(1 kΩ + RSS + RS) ln(1/2048)
Example 17-1 shows the calculation of the minimum
required acquisition time, TACQ. This calculation is
based on the following application system assump-
tions:
• 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
EXAMPLE 17-1: 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(1/2048)
-120 pF (1 kΩ + 7 kΩ + 2.5 kΩ) ln(0.0004883)
-120 pF (10.5 kΩ) ln(0.0004883)
-1.26 μs (-7.6246)
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
© 2006 Microchip Technology Inc.
DS39564C-page 185