Datasheet

Table Of Contents
PIC18F87J72 FAMILY
DS39979A-page 278 Preliminary 2010 Microchip Technology Inc.
21.1 A/D Acquisition Requirements
For the A/D Converter to meet its specified accuracy,
the Charge Holding Capacitor (C
HOLD) must be
allowed to fully charge to the input channel voltage
level. The analog input model is shown in Figure 21-2.
The source impedance (R
S) and the internal sampling
switch (R
SS) impedance directly affect the time
required to charge the capacitor, C
HOLD. The sampling
switch (R
SS) impedance varies over the device voltage
(V
DD). 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. 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 21-1 may be used. This equation assumes
that 1/2 LSb error is used (1,024 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 21-3 shows the calculation of the minimum
required acquisition time, T
ACQ. This calculation is
based on the following application system
assumptions:
EQUATION 21-1: ACQUISITION TIME
EQUATION 21-2: A/D MINIMUM CHARGING TIME
EQUATION 21-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
Note: When the conversion is started, the
holding capacitor is disconnected from the
input pin.
CHOLD =25 pF
Rs = 2.5 k
Conversion Error 1/2 LSb
V
DD =3V Rss = 2 k
Temperature = 85C (system max.)
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=T
AMP + TC + TCOFF
V
HOLD = (VREF – (VREF/2048)) • (1 – e
(-TC/CHOLD(RIC + RSS + RS))
)
or
TC = -(CHOLD)(RIC + RSS + RS) ln(1/2048)
T
ACQ =TAMP + TC + TCOFF
TAMP =0.2 s
T
COFF = (Temp – 25C)(0.02 s/C)
(85C – 25C)(0.02 s/C)
1.2 s
Temperature coefficient is only required for temperatures > 25C. Below 25C, T
COFF = 0 ms.
T
C = -(CHOLD)(RIC + RSS + RS) ln(1/2048) s
-(25 pF) (1 k + 2 k + 2.5 k) ln(0.0004883) s
1.05 s
T
ACQ =0.2 s + 1 s + 1.2 s
2.4 s