SPMCK16Z4MPV20 datasheet(205/500 Pages) FREESCALE

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M68HC16 Z SERIES

ANALOG-TO-DIGITAL CONVERTER

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

APPENDIX A ELECTRICAL CHARACTERISTICS)

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/KN‹‹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.

OUT

STRESS

–

STRESS

-------------------------------------------

ADC NEG STRESS CONN

VDD

TO DEVICE

EXTERNAL VOLTAGE

VSS

Freescale Semiconductor, Inc.

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