PIC18CXX2_13 Datasheet, PDF (169/304 Pages) Microchip Technology – High Performance Microcontrollers with 10-bit A/D

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 k + RSS + RS) ln(1/2047)
Example 16-1 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 k
1/2 LSb
5V  Rss = 7 k
50C (system max.)
0V @ time = 0
EXAMPLE 16-1: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25C.
TACQ = 2 s + Tc + [(Temp - 25C)(0.05 s/C)]
TC = -CHOLD (RIC + RSS + RS) ln(1/2047)
-120 pF (1 k + 7 k + 2.5 k) ln(0.0004885)
-120 pF (10.5 k) ln(0.0004885)
-1.26 s (-7.6241)
9.61 s
TACQ = 2 s + 9.61 s + [(50C - 25C)(0.05 s/C)]
11.61 s + 1.25 s
12.86 s
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DS39026D-page 169