NXP Semiconductors MC68CK16Z1, User Manual

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M68HC16 Z SERIES ANALOG-TO-DIGITAL CONVERTER
USER’S MANUAL 8-19
The current out of the pin (I
OUT
) under negative stress is determined by the following
equation:
where:
V
STRESS
= Adjustable voltage source
V
BE
= Parasitic bipolar base/emitter voltage (refer to V
NEGCLAMP
in
APPENDIX A ELECTRICAL CHARACTERISTICS )
R
STRESS
= Source impedance (10K resistor in Figure 8-7 on stressed
channel)
The current into (I
IN
) the neighboring pin is determined by the 1/K
N
(Gain) of the para-
sitic bipolar transistor (1/K
N
‹‹1).
One way to minimize the impact of stress conditions on the ADC is to apply voltage
limiting circuits such as diodes to supply and ground. However, leakage from such cir-
cuits and the potential influence on the sampled voltage to be converted must be con-
sidered. Refer to
Figure 8-8 .
Figure 8-8 Voltage Limiting Diodes in a Negative Stress Circuit
Another method for minimizing the impact of stress conditions on the ADC is to stra-
tegically allocate ADC inputs so that the lower accuracy inputs are adjacent to the in-
puts most likely to see stress conditions.
Finally, suitable source impedances should be selected to meet design goals and min-
imize the effect of stress conditions.
8.8.5 Analog Input Considerations
The source impedance of the analog signal to be measured and any intermediate fil-
tering should be considered whether external multiplexing is used or not.
Figure 8-9
shows the connection of eight typical analog signal sources to one ADC analog input
pin through a separate multiplexer chip. Also, an example of an analog signal source
connected directly to a ADC analog input channel is displayed.
I
OUT
V
STRESS
V
BE
–
R
STRESS
------------------------------------------- =
ADC NEG STRESS CONN
VDD
R kR
TO DEVICE EXTERNAL VOLTAGE
VSS
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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