ADuM5241: What output impedance can we expect in Z state when Viso is unpowered?

Document created by analog-archivist Employee on Feb 23, 2016
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Truthtable in datasheet of ADuM524x Rev.A page 9 says that the output is in
"Z"-State, when Viso State is unpowered. We have a problem, that VOB is
influencing other functions in our design although pin should be "Z". How is
"Z" defined? What impedance of the output can we expect?

 

When there is no power, the high and low channels are both off...  so the
output buffer is off in both directions giving a high impedance.  
Unfortunately since there are ESD diode structures on I/O pins....If the
voltage at a pin is pulled up from an external source, then once the ESD diode
is forward biased, it will conduct and not look like a high impedence any
longer. So it's impedance curve will look like a diode to Viso or Vdd. The Z
really represents a floating output between the protection diodes.  It can not
be treated as a high impedance independent of applied voltage.

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