PIC18F1230 MICROCHIP [Microchip Technology], PIC18F1230 Datasheet - Page 170

• Manufacturer Part Number: PIC18F1230
• Description: 18/20/28-Pin, Enhanced Flash Microcontrollers with nanoWatt Technology, High-Performance PWM and A/D
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F1230/1330
15.1
The A/D conversion can be triggered by setting the GO/
DONE bit. This bit can either be set manually by the
programmer or by setting the SEVTEN bit of ADCON0.
When the SEVTEN bit is set, the Special Event Trigger
from the Power Control PWM module triggers the A/D
conversion. For more information, see Section 13.14
“PWM Special Event Trigger”.
15.2
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (C
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 15-3. The
source impedance (R
switch (R
required to charge the capacitor C
switch (R
(V
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 2.5 k . After the analog input channel is
EQUATION 15-1:
EQUATION 15-2:
EQUATION 15-3:
DS39758B-page 168
T
V
or
T
T
T
T
Temperature coefficient is only required for temperatures > 25 C. Below 25 C, T
T
T
ACQ
C
ACQ
AMP
COFF
C
ACQ
DD
HOLD
). The source impedance affects the offset voltage
=
=
Triggering A/D Conversions
A/D Acquisition Requirements
SS
=
=
=
=
=
SS
=
=
) impedance varies over the device voltage
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
T
) impedance directly affect the time
AMP
T
0.2 s
(Temp – 25 C)(0.02 s/ C)
(85 C – 25 C)(0.02 s/ C)
1.2 s
-(C
-(25 pF) (1 k + 2 k + 2.5 k ) ln(0.0004883)
1.05 s
0.2 s + 1 s + 1.2 s
2.4 s
(V
-(C
AMP
+ T
REF
HOLD
HOLD
+ T
C
– (V
ACQUISITION TIME
A/D MINIMUM CHARGING TIME
CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
S
+ T
)(R
C
)(R
) and the internal sampling
+ T
COFF
REF
IC
IC
+ R
COFF
/2048)) • (1 – e
+ R
HOLD
SS
SS
HOLD
+ R
+ R
) must be allowed
S
S
) ln(1/2047)
. The sampling
) ln(1/2048)
(-T
Advance Information
C
/C
HOLD
(R
IC
+ R
SS
+ R
S
selected (changed), the channel must be sampled for
at least the minimum acquisition time before starting a
conversion.
To
Equation 15-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Example 15-3 shows the calculation of the minimum
required acquisition time, T
based
assumptions:
C
Rs
Conversion Error
V
Temperature
))
DD
HOLD
)
Note:
calculate
on
COFF
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
the
= 0 ms.
the
=
=
=
=
following
minimum
© 2006 Microchip Technology Inc.
25 pF
2.5 k
1/2 LSb
5V
85 C (system max.)
ACQ
R
SS
. This calculation is
application
acquisition
= 2 k
system
time,