PIC14000T-04/SS View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'PIC14000T-04/SS' PDF : 153 Pages View PDF
PIC14000
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 2N
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 µsec 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)
16
14
12
Conversion Time
(Seconds)
0.016384
0.004096
0.001024
Full Scale
(Volts)
3.5
3.5
3.5
A/D Current
Source Output
(µamps)
24.75
24.75
24.75
Calculated
CDAC
Capacitor
(Farads)
1.17E-07
2.93E-08
7.31E-09
CDAC Capacitor
Nearest Standard
Value
.1uF
.022uF
6800pF
16
0.016384
1.5
14
0.004096
1.5
12
0.001024
1.5
Note: Assumes FOSC of 4 MHz.
24.75
24.75
24.75
2.73E-07
6.83E-08
1.71E-08
0.22µF
68nF
15nF
DS40122B-page 64
Preliminary
© 1996 Microchip Technology Inc.
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