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PIC18F2525/2620/4525/4620
DS39626E-page 228
2008 Microchip Technology Inc.
19.1
A/D Acquisition Requirements
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (CHOLD) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 19-3. The
source impedance (RS) and the internal sampling
switch (RSS) impedance directly affect the time
required to charge the capacitor CHOLD. The sampling
switch (RSS) impedance varies over the device voltage
(VDD). The source impedance affects the offset voltage
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 2.5 k
Ω. After the analog input channel is
selected (changed), the channel must be sampled for
at least the minimum acquisition time before starting a
conversion.
To
calculate
the
minimum
acquisition
time,
Equation 19-1 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 19-3 shows the calculation of the minimum
required acquisition time TACQ. This calculation is
based
on
the
following
application
system
assumptions:
CHOLD
=
25 pF
Rs
=
2.5 k
Ω
Conversion Error
≤
1/2 LSb
VDD
=5V
→ Rss = 2 kΩ
Temperature
=
85
°C (system max.)
EQUATION 19-1:
ACQUISITION TIME
EQUATION 19-2:
A/D MINIMUM CHARGING TIME
EQUATION 19-3:
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
Note:
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
TACQ
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=TAMP + TC + TCOFF
VHOLD
=
(VREF – (VREF/2048)) (1 – e(-T
C
/CHOLD(RIC + RSS + RS)))
or
TC
=
-(CHOLD)(RIC + RSS + RS) ln(1/2048)
TACQ
=TAMP + TC + TCOFF
TAMP
=0.2
μs
TCOFF
=
(Temp – 25
°C)(0.02 μs/°C)
(85
°C – 25°C)(0.02 μs/°C)
1.2
μs
Temperature coefficient is only required for temperatures > 25
°C. Below 25°C, TCOFF = 0 ms.
TC
=
-(CHOLD)(RIC + RSS + RS) ln(1/2047)
-(25 pF) (1 k
Ω + 2 kΩ + 2.5 kΩ) ln(0.0004883)
1.05
μs
TACQ
=0.2
μs + 1 μs + 1.2 μs
2.4
μs
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