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M68HC16 Z SERIES
ANALOG-TO-DIGITAL CONVERTER
MOTOROLA
USER’S MANUAL
8-19
The current out of the pin (IOUT) under negative stress is determined by the following
equation:
where:
VSTRESS = Adjustable voltage source
VBE = Parasitic bipolar base/emitter voltage (refer to VNEGCLAMP in
RSTRESS = Source impedance (10K resistor in Figure 8-7 on stressed channel)
The current into (IIN) the neighboring pin is determined by the 1/KN (Gain) of the para-
sitic bipolar transistor (1/KN1).
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-
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