PIC18F2331 MICROCHIP [Microchip Technology], PIC18F2331 Datasheet - Page 257

• Manufacturer Part Number: PIC18F2331
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D
• Manufacturer: MICROCHIP [Microchip Technology]

20.4
If external voltage references are used instead of the
internal AV
impedance of the V
must be considered. During acquisition, currents
supplied by these sources are insignificant. However,
during conversion, the A/D module sinks and sources
current through the reference sources.
In order to maintain the A/D accuracy, the voltage
reference source impedances should be kept low to
reduce voltage changes. These voltage changes occur
as reference currents flow through the reference
source impedance.
20.5
The ADCON2 register allows the user to select an
acquisition time that occurs each time an A/D conver-
sion is triggered.
When the GO/DONE bit is set, sampling is stopped and
a conversion begins. The user is responsible for
ensuring the required acquisition time has passed
between selecting the desired input channel and the
start
ACQT3:ACQT0 bits (ADCON2<6:3>) remain in their
Reset state (‘0000’).
If desired, the ACQT bits can be set to select a
programmable acquisition time for the A/D module.
When triggered, the A/D module continues to sample
the input for the selected acquisition time, then
automatically
acquisition time is programmed, there may be no need
to wait for an acquisition time between selecting a
channel and triggering the A/D. If an acquisition time is
programmed, there is nothing to indicate if the
acquisition time has ended, or if the conversion has
begun.
 2003 Microchip Technology Inc.
Note:
of
A/D Voltage References
Selecting and Configuring
Automatic Acquisition Time
When using external references, the
source impedance of the external voltage
references must be less than 75 in order
to achieve the specified ADC resolution. A
higher reference source impedance will
increase the ADC offset and gain errors.
Resistive voltage dividers will not provide
a low enough source impedance. To
ensure the best possible ADC perfor-
mance, external VREF inputs should be
buffered with an op-amp or other low
impedance circuit.
conversion.
DD
begins
and AV
REF
+ and V
a
This
SS
conversion.
sources, the source
REF
occurs
- voltage sources
when
Since
PIC18F2331/2431/4331/4431
Preliminary
the
the
20.6
The A/D conversion time per bit is defined as T
A/D conversion requires 12 T
The source of the A/D conversion clock is software
selectable. There are eight possible options for T
For correct A/D conversions, the A/D conversion clock
(T
minimum T
130 for more information).
Table 20-2 shows the resultant T
the device operating frequencies and the A/D clock
source selected.
AD
) must be as short as possible, but greater than the
• 2 T
• 4 T
• 8 T
• 16 T
• 32 T
• 64 T
• Internal RC Oscillator
• Internal RC Oscillator/4
Selecting the A/D Conversion
Clock
OSC
OSC
OSC
OSC
OSC
OSC
AD
(approximately 416 s, see parameter
AD
per 10-bit conversion.
AD
DS39616B-page 255
times derived from
AD
AD
. The
: