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.

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