Datasheet

© 2006 Microchip Technology Inc. DS41159E-page 245
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 T
ACQ. This calculation is
based on the following application system assumptions:
•C
HOLD = 120 pF
• Rs = 2.5 kΩ
• Conversion Error ≤ 1/2 LSb
•V
DD = 5V → Rss = 7 kΩ
• Temperature = 50°C (system max.)
•V
HOLD =0V @ time = 0
EQUATION 20-1: ACQUISITION TIME
EQUATION 20-2: A/D MINIMUM CHARGING TIME
EXAMPLE 20-1: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
TACQ = 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 kΩ + RSS + RS) ln(1/2047)
TACQ =TAMP + TC + TCOFF
Temperature coefficient is only required for temperatures > 25°C.
T
ACQ =2 μs + TC + [(Temp – 25°C)(0.05 μs/°C)]
T
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
T
ACQ =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 V
REF inputs should be buffered with
an operational amplifier or other low output impedance circuit.