cc2640使用sensor controller中的ADC问题？

我在使用cc2640的adc的时候，通过sensor controller来控制adc。选择adc的参考电压为电池电压的时候，即配置：ADC_REF_VDDS_REL参数。再通过adc来采集电池电压：ADC_INPUT_VDDS。

发现这个读到的值不是0xFFF，而是0xF87，偏差有点大。这是为什么呢？

由于电池电压使用过程电压会变化，不能作为参考电压的。

换成内部参考电压试试！

测量电池电压需要电阻串联分压。

试过用内部参考电压，2640的内部参考电压为4.3v。用sensor controller ADC，有选择电源电压作为输入，为什么要串联电阻分压呢。我做过如下实验：

用两个电阻进行分下，其中一个电阻值已知，另外一个未知。当我用内部参考电压的时候，用ADC测量电池电压和已知电阻的电压，计算得到的未知电阻的结果波动很大；当用电池电压作为相对参考电压的时候，未知电阻的测量结果波动较小。

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;