docs: def V_total(self):
          '''
          Compute the input voltage (i.e., ``V1``) based on the measured
          high-voltage feedback values for ``V2``, using the high-voltage
          transfer function.
  
          See also
          --------
          :meth:`V_actuation` for diagram with ``V1`` and ``V2`` labelled.
          '''
          ind = mlab.find(self.hv_resistor >= 0)
          V1 = np.empty(self.hv_resistor.shape)
          V1.fill(np.nan)
          V1[ind] = compute_from_transfer_function(self.calibration.hw_version
                                                   .major, 'V1',
                                                   V2=self.V_hv[ind], R1=10e6,
                                                   R2=self.calibration.R_hv
                                                   [self.hv_resistor[ind]],
                                                   C2=self.calibration.C_hv
                                                   [self.hv_resistor[ind]],
                                                   f=self.frequency)
          # convert to masked array
          V1 = np.ma.masked_invalid(pd.Series(V1, pd.to_datetime(self.time, unit='s')
              ).interpolate(method='time').values)
          V1.fill_value = np.nan
          V1.data[V1.mask] = V1.fill_value
          return V1