AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 34

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

Application Note
34
For the widest operating range, the currents and conversion resistor
should be selected such that the total voltage drop through two
multiplexers and the conversion resistor is less than the common mode
range for the comparators.
The multiplexer channels also can be connected to supply a source
voltage to an external device as shown in
be applied through three different resistors to PB1, PB2, or PB3; through
multiplexers MUX1, MUX2, and MUX3; and then out through MUX4 and
PB4 to an external thermistor. Each resistor can be selected for a given
temperature range of the thermistor. The resulting voltage can be
measured on the channel bus. Even though there is a multiplexer on
either side of the channel bus, there will be some effects from the
temperature variation in the multiplexer impedances. This effect can be
minimized at temperatures where the source resistor creates the same
voltage drop across both the thermistor and the source resistor.
V
Freescale Semiconductor, Inc.
R
= (I
For More Information On This Product,
1
+ I
R
2
I
1
) (R)
Go to: www.freescale.com
I
1
I
+ I
2
2
I
Figure 15. Current Summing Inputs
1
I
I
1
2
+ I
2
TCMP
TCAP
PB4
AN4
AN1
AN2
AN3
V
PB1
PB2
PB3
SS
CHANNEL SELECT BUS
MUX1
MUX2
MUX3
MUX4
OTHER CONNECTIONS TO PB1 THROUGH PB4
HAVE BEEN OMITTED FOR CLARITY
Figure
I
1
+ I
16. As shown, V
2
HOLD
V = (I
V = (I
1
+ I
MOTOROLA
2
1
) (R + Z
+ I
2
) (Z
DD
AN1740
MUX3
MUX4
can
OR
)
)