Datasheet

PIC14000
DS40122B-page 64
Preliminary
 1996 Microchip Technology Inc.
8.8 A/D Speed,
Resolution and Capacitor
Selection
The conversion time for the A/D converter on the
PIC14000 can be calculated using the equation:
Conversion Time = (1/Fosc) x 2
N
Where Fosc is the oscillator frequency and N is the
number of bits of resolution desired.
Therefore at 4MHz, the conversion time for 16 bits is
16.384 msec. Conversely, it is 256 µ sec for 10 bits.
Choosing the correct ramp capacitor for the CDAC pin
is required to achieve the desired resolution, conver-
sion time and full scale input voltage. The equation for
selecting the ramp capacitor value is:
Capacitor = (conversion time in seconds) X
(current source output in amps) / (full scale in volts)
Table 8-6 provides example capacitor values for the
desired A/D resolution, conversion time, and full scale
voltage measurement.
TABLE 8-6: CDAC CAPACITOR SELECTION (EXAMPLES FOR FULL SCALE OF 3.5V AND 1.5V)
A/D
Resolution
(Bits)
Conversion Time
(Seconds)
Full Scale
(Volts)
A/D Current
Source Output
( µ amps)
Calculated
CDAC
Capacitor
(Farads)
CDAC Capacitor
Nearest Standard
Value
16 0.016384 3.5 24.75 1.17E-07 .1uF
14 0.004096 3.5 24.75 2.93E-08 .022uF
12 0.001024 3.5 24.75 7.31E-09 6800pF
16 0.016384 1.5 24.75 2.73E-07 0.22 µ F
14 0.004096 1.5 24.75 6.83E-08 68nF
12 0.001024 1.5 24.75 1.71E-08 15nF
Note: Assumes F
OSC
of 4 MHz.