High impedance
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In electronics, high impedance means that a point in a circuit (a node) has a relatively high impedance to other points in the circuit.
Digital electronics
High impedance (also known as hi-Z, tri-stated, or floating) is the united state of an output terminator which is not currently driven by the circuit. In digital circuits, it means that the signal is neither driven to a logical high nor low level - hence "tri-stated". Such a signal can be seen as an open circuit (or "floating" wire) because connecting it to a low impedance circuit will not affect that circuit; it will instead itself be pulled to the same voltage as the actively driven output. The combined input/output pins found on many ICs are actually tri-state capable outputs which have been internally connected to inputs. This is the basis for bus-systems in computers, among many other uses.
In digital circuits, a wire at high impedance may sometimes have a voltage that is around, or even lower than, the threshold for a digital 0. This can cause people to mistakenly read a hi-Z wire as a digital 0. To verify if a wire is at hi-Z, a large value pull-up resistor can be used to try to pull the wire to high and low voltage levels. A non-hi-Z signal should not be able to be pulled in such a way.
Analog electronics
In analog circuits a high impedance node is one that does not have any low impedance paths to any other nodes. Since the terms low and high depend on context to some extent, it is possible in principle for some high impedance nodes to be described as low impedance in one context, and high impedance in another.
High impedance nodes have higher thermal noise and are more prone to capacitive & inductive noise pick up. They are often difficult to probe as the impedance of a scope or meter can heavily affect the signal or voltage on the node. High impedance nodes are used because some transducers require a high impedance load, and useful in amplifiers since high impedance allows for large voltage gains without large power consumption.
See also
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