def METHOD_NAME(self, dac):
    span = self.d5a.span[dac]
    if span == D5a_module.range_2V_bi:
        validator = Numbers(-2 * self._gain, 2 * self._gain)
    elif span == D5a_module.range_4V_bi:
        validator = Numbers(-4 * self._gain, 4 * self._gain)
    elif span == D5a_module.range_4V_uni:
        validator = Numbers(0, 4 * self._gain)
    else:
        msg = 'The found DAC span of {} does not correspond to a known one'
        raise Exception(msg.format(span))
    return validator