PIC18F2331 Datasheet, PDF (257/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
20.4 A/D Voltage References
If external voltage references are used instead of the
internal AVDD and AVSS sources, the source
impedance of the VREF+ and VREF- voltage sources
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.
Note:
When using external references, the
source impedance of the external voltage
references must be less than 75Ω 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.
20.5 Selecting and Configuring
Automatic Acquisition Time
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 of conversion. This occurs when the
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 begins a conversion. Since the
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.
20.6 Selecting the A/D Conversion
Clock
The A/D conversion time per bit is defined as TAD. The
A/D conversion requires 12 TAD per 10-bit conversion.
The source of the A/D conversion clock is software
selectable. There are eight possible options for TAD:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC
• Internal RC Oscillator
• Internal RC Oscillator/4
For correct A/D conversions, the A/D conversion clock
(TAD) must be as short as possible, but greater than the
minimum TAD (approximately 416 µs, see parameter
130 for more information).
Table 20-2 shows the resultant TAD times derived from
the device operating frequencies and the A/D clock
source selected.
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 255