Input impedance (Zin)represents the opposition to current flow at the input terminals of a circuit or device. It defines the electrical load that the device places on a connected external signal source.
Here, R is the resistance (real part), X is the reactance (imaginary part), and j is the imaginary unit. Because reactance varies with frequency, input impedance is often frequency-dependent.
Selecting the appropriate input impedance is critical for signal integrity and measurement accuracy. The ideal value depends largely on the frequency of the signal being handled.
High Input Impedance (Low Frequency):
For general-purpose or low-frequency measurements, devices typically utilize high input impedance (e.g., 1 MΩ). High impedance minimizes the "loading effect" on the circuit under test, ensuring that the connection of the instrument does not significantly drop the source voltage or alter the circuit's behavior.
Impedance Matching (High Frequency):
In high-frequency and high-speed applications, impedance matching is essential to prevent signal reflections. For example, matching a 50 Ω output source to a 50 Ω input (such as on an oscilloscope) ensures signal fidelity. Mismatched impedance in these scenarios causes reflections, leading to waveform distortion and measurement errors. Therefore, matching the impedance of all system components--including cables, terminations, and instruments--is required.
Understanding these characteristics allows engineers to select the correct configuration for stable operation and accurate data acquisition.