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I have M inputs. I want to connect m of them to Vdd and leave other nodes floating. I know that in real experiments the floating nodes can get any value between 0 and Vdd. I was wondering how HSPICE will consider those nodes. Will HSPICE automatically connects these nodes to 0/Vdd/...?

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  • \$\begingroup\$ Are these nodes connected to some components inside your circuit? \$\endgroup\$
    – The Photon
    Commented Nov 13, 2017 at 17:18
  • \$\begingroup\$ @ThePhoton, yes, with wires and resistors. \$\endgroup\$
    – Shannon
    Commented Nov 13, 2017 at 17:27
  • \$\begingroup\$ Then those components will determine what voltage the node gets when no input source is connected. If you share your circuit (hit Ctrl-M while editing your post) we can give you more helpful information. \$\endgroup\$
    – The Photon
    Commented Nov 13, 2017 at 17:29
  • \$\begingroup\$ @ThePhoton, yes, you are right. Thanks for your response. \$\endgroup\$
    – Shannon
    Commented Nov 13, 2017 at 17:48

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Hspice will generate an error:

Node 0 is always ground and floating nodes (a node from which there is no connection to ground through circuit elements) will generate an error.

Source: MIT Spice Tutorial

The solving matrix can't solve for an equation with a floating node. If you really need to simulate what this would be physically attach a really large resistor on the order of \$ 10^9\$ ohms (or the resistance of air or FR4) to your 'floating node'. Or you could couple them capacitively with a large resistor, physically there are not many situations where you would get a 'floating node' as air and other materials have resistance that need to be accounted for when dealing with very small currents.

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