AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 56

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Early Conversion
A/D Fault
56
When the conversion time is long, the possibility exists for noise on
either the input signal or the PB0 ramping voltage to cause comparator
2 to trip the CPF2 flag earlier than would normally have occurred as
shown in
offset voltage is not as important, as the behavior can be seen in both
cases.
This problem is aggravated by long, slow-charging ramps where the
input signal may be near the ramping voltage for a longer period of time.
In general, it is best to use the fastest conversion time possible to reduce
the effects of noise on the input signal.
Another cause of an early conversion is to have some charge left on the
ramp capacitor at the start of the conversion. Always allocate enough
time to allow the ramp cap to fully discharge to V
is to allow at least 1/10 the charge time to discharge the ramp cap. If
the software process that is running between the start of each A/D
conversion is always long enough, then there does not need to be a
specific discharge time delay written into the software.
TOF/OCF
Freescale Semiconductor, Inc.
CPFR2
CMP2
CPF2
CHG
For More Information On This Product,
ICF
V
V
N
P
Figure
Go to: www.freescale.com
Figure 24. Early Conversion A/D Fault
24. In this case, the polarity of the comparator’s input
NOISE
SPIKE
SS
. A good rule of thumb
EXPECTED
TRIP
MODE 2 OR MODE 3 ONLY
MODE 2 OR MODE 3 ONLY
MOTOROLA
AN1740