MMC2107 - Freescale Semiconductor

Freescale Semiconductor, Inc.
Queued Analog-to-Digital Converter (QADC)
Pin Connection Considerations
The current into the pin (I INJN or I INJP ) under negative or positive stress is
determined by these equations:
–( V – V )
Stress BE
I INJN
= ----------------------------------------------
R Stress
V – V – V Stress EB DDA
I INJP
= --------------------------------------------------------------
R Stress
nc...
Freescale Semiconductor, I
where:
V Stress = Adjustable voltage source
V EB = Parasitic PNP emitter/base voltage
V BE = Parasitic NPN base/emitter voltage
R Stress = Source impedance (10 K resistor in Figure 18-51 and
Figure 18-52 on stressed channel)
R Selected = Source impedance on channel selected for conversion
The current into (I In ) the neighboring pin is determined by the K N (current
coupling ratio) of the parasitic bipolar transistor (K N ‹‹ 1). The I In can be
expressed by this equation:
I In = – K N * I INJ
where:
I INJ is either I INJN or I INJP .
A method for minimizing the impact of stress conditions on the QADC is
to strategically allocate QADC inputs so that the lower accuracy inputs
are adjacent to the inputs most likely to see stress conditions.
Also, suitable source impedances should be selected to meet design
goals and minimize the effect of stress conditions.
18.11.6 Analog Input Considerations
The source impedance of the analog signal to be measured and any
intermediate filtering should be considered whether external
multiplexing is used or not. Figure 18-53 shows the connection of eight
typical analog signal sources to one QADC analog input pin through a
separate multiplexer chip. Also, an example of an analog signal source
connected directly to a QADC analog input channel is displayed.
MMC2107 – Rev. 2.0
Technical Data
MOTOROLA Queued Analog-to-Digital Converter (QADC) 495
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