CC2640 ADC 外部基准电压如何连接

请教CC2640的ADC是否可以连接外部的基准电压？该连接到哪个引脚？

用户手册有说明，只能用内部的。

 参考下面代码：

/*!
 *  @brief  Specifies whether the internal reference of the ADC is sourced from the battery voltage or a fixed internal source.
 *
 *  - In practice, using the internal fixed voltage reference sets the upper range of the ADC to a fixed value. That value is 4.3V with
 *  input scaling enabled and ~1.4785V with input scaling disabled. In this mode, the output is a function of the input voltage multiplied
 *  by the resolution in alternatives (not bits) divided by the upper voltage range of the ADC. Output = Input (V) * 2^12 / (ADC range (V))
 *
 *  - Using VDDS as a reference scales the upper range of the ADC with the battery voltage. As the battery depletes and its voltage drops, so does
 *  the range of the ADC. This is helpful when measuring signals that are generated relative to the battery voltage. In this mode, the output is
 *  a function of the input voltage multiplied by the resolution in alternatives (not bits) divided by VDDS multiplied by a scaling factor derived
 *  from the input scaling. Output = Input (V) * 2^12 / (VDDS (V) * Scaling factor), where the scaling factor is ~1.4785/4.3 for input scaling
 *  disabled and 1 for input scaling enabled.
 *
 *  @note   The actual reference values are slightly different for each device and are higher than the values specified above. This gain is saved in
 *          the FCFG. The function ::ADC_convertRawToMicroVolts() must be used to derive actual voltage values. Do not attempt to compare raw values
 *          between devices or derive a voltage from them yourself. The results of doing so will only be approximately correct.
 *
 *  @warning    Even though the upper voltage range of the ADC is 4.3 volts in fixed mode with input scaling enabled, the input should never exceed
 *              VDDS as per the data sheet.
 */
typedef enum ADCCC26XX_Reference_Source {
    ADCCC26XX_FIXED_REFERENCE       = AUXADC_REF_FIXED,
    ADCCC26XX_VDDS_REFERENCE        = AUXADC_REF_VDDS_REL
} ADCCC26XX_Reference_Source;