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Pull Down resistor
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David Hess:

--- Quote from: Wimberleytech on December 29, 2019, 07:14:41 pm ---I looked at the datasheet before answering.  The mux consists of a conventional CMOS transfer gate.  While no details are given about input protection the datasheet specifies HBM/MM/CDM performance which would indicate serious diode junctions of both flavors on all inputs.  Thus I suspect it unlikely that a truly floating node would bump up against a supply and cause crosstalk via power rails.  Crosstalk voltage levels between the rail potentials is whatever it will be regardless of whether the signal is floating or forced.
--- End quote ---

It is not crosstalk via the power pins.

The problem comes about with junction isolated IC processes where the substrate is connected to the negative supply.  Negative going signals are clamped by the diode to the negative supply but still inject carriers into the substrate.  That is how junction isolated CMOS processes can die from SCR latchup even though they have input protection diodes.

It is more of a problem with multiplexers because the substrate current can leak into other channels.  For this reason, some multiplexers are deliberately built on dielectric isolated processes making them immune to this problem.  JFET multiplexers may be immune also although I am not sure.  Another way I have occasionally seen used with very old junction isolated multiplexers is to bring the substrate connection out separately and require it to be tied to a more negative voltage.

Is it a real problem?  At high resolutions it sure is.  For 3.5 digits, leakage is unlikely to be an issue.
Wimberleytech:

--- Quote from: David Hess on December 31, 2019, 03:09:20 am ---

The problem comes about with junction isolated IC processes where the substrate is connected to the negative supply.  Negative going signals are clamped by the diode to the negative supply but still inject carriers into the substrate.  That is how junction isolated CMOS processes can die from SCR latchup even though they have input protection diodes.



--- End quote ---
A floating node (a high-impedance to any source) will not initiate latchup in a properly designed integrated circuit (junction-isolated or otherwise).
David Hess:

--- Quote from: Wimberleytech on December 31, 2019, 02:13:12 pm ---A floating node (a high-impedance to any source) will not initiate latchup in a properly designed integrated circuit (junction-isolated or otherwise).
--- End quote ---

That is right but an analog multiplexer is sensitive to other failure modes from substrate currents like non-destructive leakage between channels which a digital IC would ignore.
Wimberleytech:

--- Quote from: David Hess on December 31, 2019, 05:09:34 pm ---
--- Quote from: Wimberleytech on December 31, 2019, 02:13:12 pm ---A floating node (a high-impedance to any source) will not initiate latchup in a properly designed integrated circuit (junction-isolated or otherwise).
--- End quote ---

That is right but an analog multiplexer is sensitive to other failure modes from substrate currents like non-destructive leakage between channels which a digital IC would ignore.

--- End quote ---

For high-res analog circuits, I would never leave a floating node.  In general, I do not like any unknown node potential, but in this application it is just not an issue.  I spent my life designing precision analog/mixed-signal ICs--in that world you certainly worry about everything.
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