Datasheet

 1999-2013 Microchip Technology Inc. DS39026D-page 169
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
EQUATION 16-2: A/D MINIMUM CHARGING TIME
Example 16-1 shows the calculation of the minimum
required acquisition time T
ACQ. This calculation is
based on the following application system assump-
tions:
•C
HOLD = 120 pF
•Rs = 2.5 k
• Conversion Error  1/2 LSb
•V
DD = 5V  Rss = 7 k
• Temperature = 50C (system max.)
•V
HOLD = 0V @ time = 0
EXAMPLE 16-1: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
T
ACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=T
AMP + TC + TCOFF
VHOLD = (VREF - (VREF/2048)) • (1 - e
(-Tc/CHOLD(RIC + RSS + RS))
)
or
TC = -(120 pF)(1 k + RSS + RS) ln(1/2047)
TACQ =TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25C.
T
ACQ =2 s + Tc + [(Temp - 25C)(0.05 s/C)]
T
C =-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
T
ACQ =2 s + 9.61 s + [(50C - 25C)(0.05 s/C)]
11.61 s + 1.25 s
12.86 s