AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 32

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

Application Note
Multiplexer
Impedance
Parallel Channels
32
NOTE:
All the internal multiplexers are made identical for reasons of balancing
the impedances seen by the input divider. The impedance of the
multiplexers is not a simple resistive switch, but a complex transfer
characteristic from the MOS transistors used in the multiplexer.
Therefore, this impedance increases as the supply voltage decreases,
the voltage across the multiplexer increases, or the device temperature
increases. The actual impedance of each multiplexer is difficult to
precisely determine, but the typical impedance behavior of a pair of
multiplexers in series is shown in
modeled as a resistor of 800–1500
1200–2900
When the channel select bus is connected directly to the input of
comparator 2, the multiplexer impedance is not an issue since the
comparator’s input impedance is several orders of magnitude greater.
The only effect would be a minor increase in the charge time of the
sample capacitor. When the divided input is used, a compensation
multiplexer is added to the bottom of the divider to track the impedance
of the channel input multiplexer and keep the overall divider ratio at 0.5.
However, this dummy multiplexer at the bottom of the divider cannot
compensate for multiple input channels being on in parallel.
The divided input is only compensated for one channel multiplexer at a
time. If multiple channel multiplexers are on simultaneously, then the
divider ratio will increase slightly.
Therefore, the exact impedance of the multiplexers is only a
consideration when the user is operating with more than one multiplexer
on at a time or is passing current into one channel multiplexer and out of
another. A first order estimate can be made by taking the data given in
Figure 14
three for three channels in parallel, and so on.
The ability to parallel input channels can be used to sum currents as
shown in
fed into the channel select bus by PB1 and PB2. Their summed current
then flows out of PB3 to a voltage conversion resistor, R.
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Figure
and dividing by two for two channels in parallel, dividing by
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at a V
15. In this example, two current sources, I
DD
of 3 volts.
Figure
at a V
14. This impedance can be
DD
of 5 volts and
1
and I
MOTOROLA
AN1740
2
, are