Input impedance (Zin) is the opposition to current flow at the input terminals of a circuit or device. It represents the electrical load that the device imposes on an external source connected to it.
where R is the resistance (real part), X is the reactance (imaginary part), and j is the imaginary unit. The reactance can be either capacitive or inductive.
Input impedance typically exhibits frequency-dependent characteristics and serves as a critical design and selection parameter, particularly in high-frequency circuits and measurement instrumentation.
For example, in high-frequency applications, matching the 50 Ω input impedance of an oscilloscope to the 50 Ω output impedance of a signal generator minimizes signal reflections and ensures accurate measurements. An impedance mismatch causes signal reflections, which can lead to waveform distortion and measurement errors.
For many general-purpose or low-frequency measurements, an oscilloscope's input impedance is set to a high value, typically 1 MΩ. This high impedance minimizes the loading effect on the circuit under test, ensuring the measurement itself does not significantly alter the circuit's behavior.
Conversely, for high-frequency and high-speed signals, impedance matching (e.g., using a 50 Ω input) is critical. In these cases, matching the impedance of all system components--including cables, terminations, and instruments--is necessary to prevent signal reflections that cause distortion.
Proper understanding of input impedance and appropriate selection based on application requirements are essential for accurate signal measurement and stable system operation. The effects of impedance mismatch become increasingly pronounced in high-frequency circuits, making correct impedance matching absolutely critical.