AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 40

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

Application Note
Sample Offset
Voltage
40
NOTE:
switching to the temperature diode, where the HOLD and DHOLD must
be changed at once and the sample and hold function is not available.
When using the sample and hold capacitor, always turn off the HOLD
and DHOLD bits on a separate write to the AMUX register before making
any changes to the other channel multiplexers.
When comparator 2 is not being used to measure a voltage, it should
have its sample capacitor and input tied through the HOLD multiplexer
to the channel select bus which has been connected to V
the input on comparator 2 may drift with stray leakage currents and
cause unwanted output changes or flags.
As the input voltages get near the V
of the comparator’s own offset voltage can create a situation where an
external pin must go below V
handle this case, the VOFF bit can add a small positive offset voltage
(nominally 100 mV) to the voltage stored on the sample capacitor as
shown in
During the sampling of the input pin, the lower end of the sampling
capacitor is connected directly to the internal V
ends when both the HOLD and DHOLD bits are off. In this case, the
sampling capacitor has the input voltage with respect to V
lower end of the sampling capacitor is raised about 100 mV above V
Now any input offset in the comparator will be added to or subtracted
from the nominal 100 mV.
The VOFF bit is not active unless the OPT bit is selected as a mask
option. About 20 A of I
the general analog bias source must be powered up. This can be done
by setting either the ISEN, CP1E, or the CP2E bits in the ACR.
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure
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18.
DD
current is drawn when the VOFF bit is set, and
SS
in order to flip the comparator output. To
SS
level, the magnitude and phase
SS
line. The sampling
SS
SS
. Otherwise,
, and the
MOTOROLA
AN1740
SS
.