Elimination of interfering voltage
For your convenience a translation:

The "polarity reversal method" is a method from measuring engineering for interference suppression in complex impedance measurements with a fixed measuring frequency.

In this method, three measurements are taken to determine the impedance of a DUT at measurement frequency:


Without source (without current injected in OP's case)
With source at measurement frequency (with current injected in OP's case)
With source measurement frequency 180° phase shifted (with negative current injected in OP's case)


Figure 1 shows the complex phasor diagram of the voltage measurement with injected current in a DUT using polarity reversal. Here is:


\$U_{S}\$ the voltage caused due to disturbance/interference/noise, measured when the source is switched off
\$U_1\$ the voltage measured with the source having 0° phase shift
\$U_{2}\$ the voltage measured with the source having 0° phase shift


The voltage \$U_ {M}\$ which is needed to determine the impedance (of the DUT) is calculated from the values ​​of the measured voltages according to the following equation:

$$U_{M}={\sqrt {{\frac {U_{1}^{2}+U_{2}^{2}}{2}}-U_{S}^{2}}}  $$

Now, you can calculate the impedance \$ \underline {Z} \$ using the equation:

$$ \underline{Z} = \frac {{\underline {U}}_{M}}{\underline {I}}$$

Important in applying the polarity reversal method is fulfilling the following criteria:


The disturbance/inteferance/noise may only occur at one frequency.
The disturbance/inteferance/noise must be stable