Serial mode voltage
Parasitic phenomena perturbance affecting measurement accuracy - Rules for generators and multimeters

It's customary in metrology to call any parasitic voltage appearing at the operating terminals of a device the "series mode voltage". Figure 1 illustrates the case of a generator G with three parasitic sources s ₁ , s ₂ and s ₃ . These four sources do not produce signals of the same frequency. As a result, the output impedance ${\mathcal{Z}}_{\text{s}}\left(\omega \right)$ takes on a different value for each parasitic signal.