Microchip Technology PIC16F716, Datasheet

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2003 Microchip Technology Inc.
Preliminary
DS41206A-page 51
PIC16F716
FIGURE 8-1: A/D BLOCK DIAGRAM
8.1 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 8-2. 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 10 k
Ω
. After the analog input channel is
selected (changed) this acquisition must be done
before the conversion can be started.
To calculate the minimum acquisition time, T
ACQ
, see
the PICmicro
®
Mid-Range Reference Manual,
(DS33023). This equation calculates the acquisition
time to within 1/2 LSb error. The 1/2 LSb error is the
maximum error allowed for the A/D to meet its specified
accuracy.
FIGURE 8-2: ANALOG INPUT MODEL
(Input voltage)
V
IN
V
REF
(Reference
voltage)
V
DD
PCFG2:PCFG0
CHS2:CHS0
000
or
010
or
110
or
111
001
or
011
or
101
RA3/AN3/V
REF
RA2/AN2
RA1/AN1
RA0/AN0
011
010
001
000
A/D
Converter
100
or
Note: When the conversion is started, the
holding capacitor is disconnected from the
input pin.
C
PIN
VA
Rs
ANx
5 pF
V
DD
V
T
= 0.6V
V
T
= 0.6V
I leakage
R
IC
≤
1k
Sampling
Switch
SS
R
SS
C
HOLD
= DAC capacitance
V
SS
6V
Sampling Switch
5V
4V
3V
2V
5 6 7 8 9 10 11
(k
Ω
)
V
DD
= 51.2 pF
± 500 nA
Legend: C
PIN
V
T
I leakage
R
IC
SS
C
HOLD
= input capacitance
= threshold voltage
= leakage current at the pin due to
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
various junctions