Read RMS voltage samples from control board high-voltage feedback circuit.

def measure_board_rms(control_board, n_samples=10, sampling_ms=10,
                      delay_between_samples_ms=0):
    '''
    Read RMS voltage samples from control board high-voltage feedback circuit.
    '''
    try:
        results = control_board.measure_impedance(n_samples, sampling_ms,
                                                  delay_between_samples_ms,
                                                  True, True, [])
    except RuntimeError:
        # `RuntimeError` may be raised if, for example, current limit was
        # reached during measurement.  In such cases, return an empty frame.
        logger.warning('Error encountered during high-voltage RMS '
                       'measurement.', exc_info=True)
        data = pd.DataFrame(None, columns=['board measured V',
                                           'divider resistor index'])
    else:
        data = pd.DataFrame({'board measured V': results.V_hv})
        data['divider resistor index'] = results.hv_resistor
    return data