How Is Impedance Measured With Measurement Circuits?

Key Takeaways

• Impedance integrates resistive and reactive components, acting as a barrier to AC current flow, represented by Z = R + jX.

• Various methods such as bridge, resonant, I-V, RF I-V, network analysis, and auto-bridge offer different advantages, disadvantages, and frequency ranges.

• Tools like impedance meters, LCR meters, and impedance analyzers are essential for accurate impedance measurements.

Network analysis impedance measurement circuit, discussed further below. 

Impedance (Z) fundamentally represents the overall opposition a circuit offers to current, integrating both resistive (R) and reactive (X) components. Essentially, impedance acts as a barrier to the flow of current in an AC circuit. It is denoted by the equation Z = R + jX, where j symbolizes the imaginary unit. 

The impedance calculation involves dividing the circuit voltage by its current. Relevant elements exhibiting impedance include electrical networks, components, circuits, materials, and traces at high frequencies. Learn how impedance is measured using impedance measurement circuits and the relevant instruments associated with this process.

Impedance Measurement Circuits Summarized

How Is Impedance Measured?

Impedance is not a visible phenomenon, so it requires specialized measuring instruments for assessment. Tools that measure impedance include impedance meters, LCR meters, and impedance analyzers. Each of these tools has an internal impedance measurement circuitry that allows them to function. 

To determine impedance, at least two values must be measured, since impedance is a complex quantity. Many modern impedance measuring instruments assess both the real and imaginary parts of an impedance vector. Several methods are available for measuring impedance.

Impedance Measurement Circuits

Below, we’ve highlighted a variety of impedance measurements for different applications.

Bridge Method

This technique employs a bridge circuit to determine an unknown resistance. It involves adjusting the balance using a galvanometer. Despite its high accuracy (approximately 0.1%), it is not suitable for high-speed measurements. When no current flows through the detector (D), the unknown impedance (Zx) can be calculated based on the relationship with the other elements of the bridge. Different types of bridge circuits, utilizing combinations of inductance (L), capacitance (C), and resistance (R) components, are used for various applications.

Bridge method for impedance measurement

Resonant Method

By adjusting a tuning capacitor (C) to bring a circuit to resonance, the unknown impedance values of inductance (Lx) and resistance (Rx) can be determined using the test frequency, the value of C, and the quality factor (Q). The Q factor is directly measured with a voltmeter placed across the tuning capacitor. Due to the minimal loss in the measurement circuit, Q values as high as 300 can be measured. In addition to the direct connection method shown, both series and parallel connections can be used for a variety of impedance measurements.

Resonant Impedance Method

I-V Method

This method determines impedance by measuring the voltages across both a current detection resistor and the unknown impedance. It is also suitable for measuring grounded samples. As impedance increases, the method becomes more affected by the voltmeter's influence. The unknown impedance (Zx) is calculated from the measured voltage and current values. In practice, a low-loss transformer replaces the resistor to avoid issues caused by placing a low-value resistor in the circuit. However, this transformer limits the lower end of the applicable frequency range.

I-V Impedance Method

RF I-V Method

This technique is based on the same fundamental principle as the I-V method but allows for high-frequency impedance measurements. It uses an impedance-matched measurement circuit (50 Ω) and a precision coaxial test port to operate at higher frequencies. There are two arrangements for the voltmeter and current meter, designed for low and high-impedance measurements.

The impedance of the Device Under Test (DUT) is calculated from the measured voltage and current values. The current through the DUT is derived from the voltage measured across a known resistor (R). In practice, a low-loss transformer replaces the resistor to avoid issues caused by inserting a low-value resistor into the circuit, though this transformer limits the lower end of the frequency range. However, it is challenging to use this technique for wideband measurement because the frequency range is limited by the transformer's capabilities in the test head.

Network Analysis Method

The reflection coefficient is obtained by measuring the ratio of an incident signal to the reflected signal. A directional coupler or bridge detects the reflected signal, and a network analyzer supplies and measures the signals. Since this method measures reflection at the DUT, it is usable in the higher frequency range. 

Network Analysis Method

Automatically-Balanced Bridge Method

This method operates on the same fundamental principle as the bridge method and covers a broad frequency range from 1 mHz to 100 MHz, though it doesn't extend to very high frequencies. This technique is commonly used in many LCR meters.

In this method, the current Ix, which flows through the Device Under Test (DUT), balances with the current Ir flowing through the range resistor (Rr) via the I-V converter. The potential at the Low point is maintained at zero volts, creating a "virtual ground." The impedance of the DUT is then calculated using the voltage measured at the High terminal (Vx) and the voltage across Rr (Vr).

Auto-Balancing Bridge Impedance Method

Advanced Tools for Impedance Measurement

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PCB Impedance Calculator in Action

Analyzing Impedance With OrCAD X

OrCAD X integrated impedance analysis workflows provide a solution for identifying and resolving real signal integrity issues. Designers can easily filter and select critical nets for impedance analysis, with results displayed on a color-coded scale from red to blue, indicating impedance levels. This allows for quick navigation to problematic traces by clicking on data points in a table view.

For instance, if an impedance discontinuity is caused by a split in the plane, adjustments can be made directly in OrCAD X, and the analysis can be rerun to verify the fix. This streamlined process aids designers in swiftly resolving signal integrity problems and accelerating their designs to production.

Cross-Section Editor Impedance Parameters

 The cross-section editor within OrCAD X allows users to adjust parameters such as dielectric thickness to achieve desired impedance levels. These calculated values can then be applied to design rules within the Constraint Manager, where users can create and apply specific impedance rules to different layers and nets. 

Now that you know about different impedance measurement circuits and how impedance is measured, try out OrCAD X to see just how its impedance analysis tools can help you in your designs. Unlock precise and efficient impedance analysis with OrCAD X today!