MC68HC16Z1 Datasheet, PDF (205/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series

Freescale Semiconductor, Inc.
The current out of the pin (I_OUT) under negative stress is determined by the following
equation:
I_OUT = --^V----^S---^T-_R--^R---_S-^E--_T-^S--_R-^S--_E---–-_S---_S^V-----^B---^E---
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.
VDD
kR
EXTERNAL VOLTAGE
R
TO DEVICE
VSS
ADC NEG STRESS CONN
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.
M68HC16 Z SERIES
USER’S MANUAL
ANALOG-TO-DIGITAL CONVERTER
For More Information On This Product,
Go to: www.freescale.com
8-19