
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
16.2
A/D Acquisition Requirements
For the A/D Converter to meet its specified accuracy,
the charge holding capacitor (CHOLD) must be allowed
to fully charge to the input channel voltage level. The
source impedance (RS) and the internal sampling
switch (RSS) impedance directly affect the time
required to charge the capacitor CHOLD. The sampling
switch (RSS) impedance varies over the device voltage
(VDD). 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,
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.
required acquisition time, TACQ. This calculation is
based
on
the
following
application
system
assumptions:
CHOLD
=25 pF
Rs
=
2.5 k
Conversion Error
1/2 LSb
VDD
=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
=TAMP + TC + TCOFF
VHOLD
=
(VREF – (VREF/2048)) (1 – e(-T
C
/CHOLD(RIC + RSS + RS)))
or
TC
=
-(CHOLD)(RIC + RSS + RS) ln(1/2048)
TACQ
=TAMP + TC + TCOFF
TAMP
=0.2
s
TCOFF
=
(Temp – 25
C)(0.02 s/C)
(85
C – 25C)(0.02 s/C)
1.2
s
Temperature coefficient is only required for temperatures > 25
C. Below 25C, TCOFF = 0 ms.
TC
=
-(CHOLD)(RIC + RSS + RS) ln(1/2047)
-(25 pF) (1 k
+ 2 k + 2.5 k) ln(0.0004883)
1.05
s
TACQ
=0.2
s + 1 s + 1.2 s
2.4
s