def set_offset(chan_obj):
    """ Return a tuple of offset value and calibrate value. 

        Arguments:
            chan_obj (dict):    Dictionary containing channel information.
    """
    
    physical_range = chan_obj['physical_max'] - chan_obj['physical_min']
    digital_range = chan_obj['digital_max'] - chan_obj['digital_min']
    calibrate = physical_range / digital_range
    calibrated_dig_min = calibrate * chan_obj['digital_min']
    offset = chan_obj['physical_min'] - calibrated_dig_min
    return (offset, calibrate)