highinputimpedance

High input impedance is a characteristic of a circuit or device indicating that the impedance presented to the signal source at its input is large compared with the source impedance. When input impedance is high, the device loads the source minimally, preserving the source voltage and signal integrity.

In practice, high input impedance is achieved through buffering or transistor inputs. A common approach is a unity-gain buffer or voltage follower built around an operational amplifier or a dedicated buffer, which can present input impedances ranging from megaohms to gigaohms. Devices using junction field-effect transistors (JFET) or complementary metal-oxide-semiconductor (CMOS) inputs tend to have particularly high input impedance.

Input impedance is specified as a magnitude and is often frequency dependent due to parasitic capacitances. At DC, it approximates the input resistance; for AC signals, the input impedance is the parallel combination of R_in and 1/(jωC_in). Typical laboratory instruments specify about 1 MΩ input resistance for oscilloscopes (with tens of picofarads of input capacitance), while probes in 10× mode can present effectively 10 MΩ. Some high-impedance instruments or sensors offer 100 MΩ or higher inputs.

High input impedance is advantageous in sensor interfaces, audio equipment, and measurement systems because it reduces loading errors and helps preserve frequency response. However, high impedance does not guarantee low noise or low distortion, and very high impedance nodes can be sensitive to voltage noise, stray capacitance, and electrostatic effects, which may require proper biasing, shielding, and protection. The concept remains central to impedance matching, impedance bridging, and the design of front-end amplifiers.