2 2-Port Shunt-Through Measurement Theory

Figure 2-1 shows the 2-Port Shunt-Through circuit configuration to measure $Z_1^{}$ impedance. First of all, the S-parameter $S_{21}^{}$ is calculated to derive the $Z_1^{}$ impedance value. As per definition, $S_{21}^{}$ is:

Assuming that $R_0^{}$= 50 Ω and $S_{21}^{}$<< 1 (for very small impedance / resistances - $Z_1^{}$<< $R_0^{}$) when solving for $Z_1^{}$, the result is:

It is important to note that the measure of $S_{21}^{}$is influenced by the VNA performance such as the dynamic range, the signal-to-noise ratio and the effective number of bits.

As all grounds in a VNA refer to the same point, an additional common mode current is created and affects measurement accuracy resulting in measurement errors. A Semi-floating differential amplifier is then used as a ground isolator to minimize ground loop currents.

Figure 2-2 shows a 2-port shunt-through measurement setup. The 2-port shunt-through measurement uses a vector network analyzer (VNA) to compute impedances at very high frequency and a semi-floating differential amplifier to measure impedances down to 1 mΩ. Selecting a semi-floating differential amplifier with a large resistance to common mode current is paramount to improve the accuracy of the measurements especially at lower frequencies.