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I'm just wondering if anyone knows what feedthrough capacitor is, and how it can affect our switch.

As far as I'm concerned, if we use FETs as switches, feedthrough capacitors are the one that connect the input with the output. ( it can have values around 1pf-10pf). If I'm right, what problems can cause these parasitic capacitances?

Thanks in advance.

p.s

When I say input tied to output, I'm refering about parasitic capacitances that connect the drain with the source.

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  • \$\begingroup\$ Problem: you don't get an isolated input to output at high frequencies when the switch is supposedly open circuit. That's what capacitors do. \$\endgroup\$
    – Andy aka
    Commented Jun 16, 2018 at 10:05
  • \$\begingroup\$ This means that at high frequencies the signal passes from input to output even though the device is turned off? \$\endgroup\$ Commented Jun 16, 2018 at 10:07
  • \$\begingroup\$ Yes of course. Nothing is perfect. \$\endgroup\$
    – Andy aka
    Commented Jun 16, 2018 at 10:13
  • \$\begingroup\$ Compare it to two antennas close together. DC does no pass but high frequencies do. Sometimes you can short the input to the switch to reduce the effect. Depends on what is driving it. \$\endgroup\$
    – Oldfart
    Commented Jun 16, 2018 at 10:53
  • \$\begingroup\$ See electronics.stackexchange.com/questions/354295/… for an example of what happens with feedthrough capacitance \$\endgroup\$
    – crj11
    Commented Jun 16, 2018 at 11:26

1 Answer 1

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The FETs (MOSFETS) used in analog multiplexors have two series capacitors:

1) source-bulk (we assume there is no channel --DC path)

2) bulk-drain

These two capacitors in series set the high-frequency behavior IF the bulk is allowed to move freely.

Holding the BULK quiet is partly your responsibility, based on how firmly, how strongly, you control the GND and the VDD pins on the silicon.

Note I said silicon.

Different packages will have difference inductance, and even different pinouts and even different routing patterns (in the case of BGA, etc).

View your high frequency analog mux model as

schematic

simulate this circuit – Schematic created using CircuitLab

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