High impedance refers to the input impedance or the output impedance of electronic components, circuits or measuring devices. It states that the impedance of the input or output circuit exceeds a certain ohm value.
As a rule, one speaks of high-impedance, also called Hi-Z in audio technology, when the impedance exceeds a two- digit kilo-ohm value, i.e. is above 10 kilo-ohms. There is no upper limit to the high impedance. Values of 1 mega-ohm or 100 mega-ohms or higher are included.
High impedance has the advantage that low impedance circuits are not affected by upstream or downstream high impedance circuits. If an output circuit is low impedance and has an output impedance of 500 ohms, for example, and the downstream circuit is high impedance with 100 kilo-ohms input impedance, then the output impedance will not be significantly affected by the downstream input impedance. This is particularly important in measurement applications. Examples of high-impedance components and devices include field-effect transistors, the condenser microphone, CMOS technology, sample-and-hold circuits, and the oscilloscope.
Values below 10 kilo-ohms, especially values in the one- and two-digit ohm range are referred to as low impedance.