Microchip Technology DM164134 Data Sheet
© 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.
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
required acquisition time T
ACQ
. This calculation is
based on the following application system assumptions:
• C
HOLD
=
120
pF
• Rs
=
2.5 k
Ω
• 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
T
ACQ
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=
T
AMP
+ T
C
+ T
COFF
V
HOLD
=
(V
REF
– (V
REF
/2048)) • (1 – e
(-Tc/C
HOLD
(R
IC
+ R
SS
+ R
S
))
)
or
Tc
Tc
=
-(120 pF)(1 k
Ω + R
SS
+ R
S
) ln(1/2047)
T
ACQ
=
T
AMP
+ T
C
+ T
COFF
Temperature coefficient is only required for temperatures > 25
°C.
T
ACQ
=
2
μs + T
C
+ [(Temp – 25
°C)(0.05 μs/°C)]
T
C
=
-C
HOLD
(R
IC
+ R
SS
+ R
S
) ln(1/2047)
-120 pF (1 k
Ω + 7 kΩ + 2.5 kΩ) ln(0.0004885)
-120 pF (10.5 k
Ω) 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 20
voltage references must be less than 20
Ω 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.
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.