highimpedance

High impedance refers to a condition in which an electrical input or node presents a high impedance to the signal source, meaning it draws very little current and has little effect on the source’s voltage. It is a relative term; what counts as high depends on the source impedance and the circuit design. In contrast, a low-impedance node draws more current and loads the source more heavily.

Impedance combines resistance and reactance and is measured in ohms. A high-impedance input typically has an impedance in the megohm (MΩ) range or higher, and in some specialized sensors it can reach gigaohms. The higher the input impedance, the less the circuit loads the signal source, preserving voltage levels and frequency characteristics. High impedance is especially important for preserving small signals from sensors or probes.

Common applications include audio equipment inputs, sensor interfaces, and measurement instruments. For example, a high-impedance voltmeter or oscilloscope probe minimizes loading on a signal. In digital electronics, a high-impedance state, or Hi-Z, is used for tri-stated outputs on shared buses, allowing multiple devices to drive the line without contention.

Practical considerations include susceptibility to noise and leakage currents at very high impedance. Parasitic capacitance and stray charges can affect bandwidth and stability, requiring careful layout, guarding, and shielding. Instruments specify input impedance and capacitance to ensure proper operation, and designers sometimes balance high impedance with protection and noise-control measures.