Datasheet

PIC18F1230/1330
DS39758D-page 174  2009 Microchip Technology Inc.
16.1 Triggering A/D Conversions
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 14.14
“PWM Special Event Trigger”.
16.2 A/D Acquisition Requirements
For the A/D Converter to meet its specified accuracy,
the charge holding capacitor (C
HOLD) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 16-3. The
source impedance (R
S) and the internal sampling
switch (R
SS) impedance directly affect the time
required to charge the capacitor C
HOLD. The sampling
switch (R
SS) impedance varies over the device voltage
(V
DD). The source impedance affects the offset voltage
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
selected (changed), the channel must be sampled for
at least the minimum acquisition time before starting a
conversion.
To calculate the minimum acquisition time,
Equation 16-1 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 16-3 shows the calculation of the minimum
required acquisition time, T
ACQ. This calculation is
based on the following application system
assumptions:
C
HOLD =25 pF
Rs = 2.5 k
Conversion Error  1/2 LSb
V
DD =5V  RSS = 2 k
Temperature = 85C (system max.)
EQUATION 16-1: ACQUISITION TIME
EQUATION 16-2: A/D MINIMUM CHARGING TIME
EQUATION 16-3: CALCULATING THE MINIMUM REQUIRED ACQUISITION TIME
Note: When the conversion is started, the
holding capacitor is disconnected from the
input pin.
TACQ = Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
=T
AMP + TC + TCOFF
VHOLD = (VREF – (VREF/2048)) • (1 – e
(-TC/CHOLD(RIC + RSS + RS))
)
or
T
C = -(CHOLD)(RIC + RSS + RS) ln(1/2048)
T
ACQ =TAMP + TC + TCOFF
TAMP =0.2 s
T
COFF = (Temp – 25C)(0.02 s/C)
(85C – 25C)(0.02 s/C)
1.2 s
Temperature coefficient is only required for temperatures > 25C. Below 25C, T
COFF = 0 ms.
T
C = -(CHOLD)(RIC + RSS + RS) ln(1/2047)
-(25 pF) (1 k + 2 k + 2.5 k) ln(0.0004883)
1.05 s
T
ACQ =0.2 s + 1 s + 1.2 s
2.4 s