Definition
The opposition that the input terminals of an electronic circuit or device present to a signal source. Input impedance is measured in ohms and combines resistance with reactance (the frequency-dependent opposition from capacitance and inductance). It determines how much current the circuit will draw from the source for a given input voltage, and how much of the source signal is actually transferred into the circuit.
Plain English
How hard it is for a signal to push into the input of a piece of electronic equipment. A high input impedance means the device draws very little from whatever is feeding it; a low input impedance means it draws more.
Context Anchor
Seen in aircraft electrical and avionics maintenance, especially when connecting radios, audio equipment, sensors, or test equipment.
Derivation
Impedance comes from the Latin impedire, meaning to hinder or get in the way. Input simply means the side where the signal comes in. So input impedance is literally how much the input side hinders the incoming signal.
Why Pilots Care
Proper matching prevents signal loss and ensures reliable communication and navigation performance.
Grounding Statement
If a connected device takes too much current from a signal, the signal can drop or change before the equipment can use it correctly.
Intuition Check
Input impedance is not just a label for the input connection. It describes how strongly the equipment electrically affects the signal coming into it.
Example Sentence 1
The voltmeter has a very high input impedance, so it reads the circuit's voltage without drawing enough current to disturb it.
Example Sentence 2
During preflight checks the technician verified the input impedance of the new audio panel.