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I'm using the SN74CB3T3257 as a demux in a circuit I'm working on.

The relevant parts of the circuit is approximately this:

schematic

simulate this circuit – Schematic created using CircuitLab

What I want is if I disconnect the external circuit, OUT reads 0.00 V, and if the external circuit is intact, it will read ~VDD. I'll use this to drive a comparator. Keeping it low power is essential as we need to run it off batteries.

When the external circuit is connected it works as expected and reads ~ 0.99*VDD. But when i disconnect the external circuit, it still reads VDD.

I tried with some lower value resistors; everything behaves as expected if R2 is in the 10 - 50 kΩ range; after I disconnect, OUT goes to 0.00 V. But as R2 gets to > 100 kΩ, OUT doesn't fall anymore.

I'm not sure if there is a parameter in the datasheet I'm missing which explains this behavior, but I couldn't find anything. I tried multiple chips to confirm it wasn't a blown chip.

Any ideas why this behavior?

EDIT 1: Fixed schematic and included pin numbers.

EDIT 2: Updated schematic to show where out leads (into comparator). Note, OUT voltage doesn't change whether or not it is connected to the comparator.

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  • \$\begingroup\$ leakage current perhaps. Have you tried the resistor on the other side of the device. \$\endgroup\$ – Trevor_G Apr 18 '17 at 17:41
  • \$\begingroup\$ As in switching the positions of R1 and R2? I can't in this case, as I need to be able to connect R1 to different circuitry in mode 2. \$\endgroup\$ – Jim Apr 18 '17 at 17:48
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    \$\begingroup\$ Can I assume 2A1 and 2A2 are really 1B1 and 1B2? Maybe put the pin numbers because his could be a wiring problem. \$\endgroup\$ – owg60 Apr 18 '17 at 17:48
  • \$\begingroup\$ I mean attaching the 1M to 2A. If the input is floating the output may actually drive high... \$\endgroup\$ – Trevor_G Apr 18 '17 at 17:51
  • \$\begingroup\$ 2owg60 good catch, fixed. \$\endgroup\$ – Jim Apr 18 '17 at 17:57
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As mentioned by @ThePhoton, the critical spec here was the I/O leakage current spec (I_OZ). It was driving the output high even without an input signal when loaded with a 1 MΩ resistor.

By swapping out this demux with one which a much lower spec, everything worked as expected.

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