PIC18CXX2
To calculate the minimum acquisition time,
Equation 16-1 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 16-1: Acquisition Time
TACQ
=
Amplifier Settling Time +
Holding Capacitor Charging Time +
Temperature Coefficient
=
TAMP + TC + TCOFF
Equation 16-2: 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(1/2047)
)
=
Example 16-3 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 16-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ =
TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25°C.
TACQ =
TC =
2 µs + Tc + [(Temp - 25°C)(0.05 µs/°C)]
-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
7/99 Microchip Technology Inc.
Preliminary
DS39026B-page 171
MICROCHIP [ MICROCHIP ]
