# HSPCIE Default Value for Float Nodes

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/...?

• Are these nodes connected to some components inside your circuit? – The Photon Nov 13 '17 at 17:18
• @ThePhoton, yes, with wires and resistors. – Shabnam Nov 13 '17 at 17:27
• 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. – The Photon Nov 13 '17 at 17:29
• @ThePhoton, yes, you are right. Thanks for your response. – Shabnam Nov 13 '17 at 17:48

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.

• No need to say thanks, just mark answered if you think it answered the question and upvote – Voltage Spike Nov 13 '17 at 17:59