PIC16C63A/65B/73B/74B
12.1
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 12-2. 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
), Figure 12-2. The source impedance affects the
offset voltage at the analog input (due to pin leakage
current).
The maximum recommended impedance for ana-
log sources is 10 kΩ.
After the analog input channel is
selected (changed), the acquisition time (T
ACQ
) must
pass before the conversion can be started.
To calculate the minimum acquisition time,
that 1/2 LSb error is used (512 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
For more information, see the PICmicro™ Mid-Range
MCU Family Reference Manual (DS33023). In general,
however, given a maximum source impedance of
10 kΩ and a worst case temperature of 100°C, T
ACQ
will be no more than 16
µsec.
FIGURE 12-2:
ANALOG INPUT MODEL
V
DD
V
T
= 0.6 V
R
IC
£ 1k
Sampling
Switch
SS
R
SS
C
HOLD
= DAC capacitance
= 51.2 pF
V
SS
Legend:
C
PIN
V
T
= input capacitance
= threshold voltage
V
DD
Rs
ANx
VA
C
PIN
5 pF
V
T
= 0.6 V
I leakage
± 500 nA
I leakage = leakage current at the pin due to
various junctions
R
IC
SS
C
HOLD
= interconnect resistance
= sampling switch
= sample/hold capacitance (from DAC)
6V
5V
4V
3V
2V
5 6 7 8 9 10 11
Sampling Switch
(kΩ)
EQUATION 12-1:
T
ACQ
=
ACQUISITION TIME
Amplifier Settling Time +
Hold Capacitor Charging Time +
Temperature Coefficient
T
AMP
+ T
C
+ T
COFF
T
AMP
= 5
µS
T
C
= - (51.2 pF)(1 kΩ + R
SS
+ R
S
) In(1/511)
T
COFF
= (Temp -25°C)(0.05
µS/°C)
=
DS30605C-page 82
2000 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
