AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 36

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

Application Note
Support Circuitry
Input Divider
36
A number of specific support circuits provide additional capabilities or
options as described in the next subsections.
The primary purpose of the input divider is to extend the common mode
range to allow measurement of pin voltages up to the V
This divider can be switched in between the channel select bus and the
comparator input bus by setting the DHOLD bit and clearing the HOLD
bit in the AMUX register. The input divider can be used only on the
channel select bus signals and cannot be used in conjunction with the
temperature diode. The divider is created with two identical resistances,
each of a nominal 60 k . This provides a divider ratio of 0.5. The
matching of these resistors is within 1%, which results in the divider
ratio varying from 0.49 to 0.51 between different devices.
The input impedance of the comparator is more than 1 M , so the
divider becomes a significant input impedance load when switched into
the signal path. At DC, this input impedance looks like 120 k , but at
frequencies above 10 kHz the 10 pF sample capacitor starts to bypass
the lower leg of the divider and both the input impedance degrades and
the divider ratio starts to drop. Neglecting the parasitic capacitances in
the multiplex circuitry, the input impedance can be roughly modeled as
shown in
Figure
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For More Information On This Product,
9.
Figure 17
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R
Figure 17. Divider Equivalent Circuit
MUX
f
c
with typical input impedance as shown previously in
800–2900
MUX
60 k
60 k
R
R
1
2
DHOLD
HOLD
8–13 pF
C
DD
supply level.
MOTOROLA
AN1740