AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 33

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

AN1740
MOTOROLA
If PB1 and PB2 are being fed from current sources and sufficient voltage
range is available, then the impedance of the MUX1 and MUX2 channel
multiplexers does not create a voltage drop relative to the internal
channel bus. The voltage on the internal channel bus with PB1, PB2,
and PB3 all on will be:
If the PB4 pin is connected to V
MUX4 turned on in its place. The voltage on the channel bus will now be:
Assuming that Z
between these two voltages, therefore, is the voltage on the external
conversion resistor, and the effects of the channel multiplexer’s
impedances are cancelled. With this setup, the currents through port
pins should be limited to less than 1 mA.
Freescale Semiconductor, Inc.
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0
Figure 14. Typical Channel Multiplexer Impedance
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MUX3
V
1
CHANNEL
V
V
DD
CHANNEL
is approximately equal to Z
= 3.0 V
INPUT VOLTAGE – VOLTS
= (I
= (I
2
SS
1
, then MUX3 can be turned off and
+ I
1
+ I
2
) x (R + Z
2
) x (Z
3
MUX4
V
DD
MUX
= 5.0 V
Analog Signal Multiplexers
MUX4
)
)
, the difference
4
Application Note
T
A
= 25 C
5
33