S32K116 ADC问题

[haifeng_jin]

hi  咨询个问题，如下图，选取S32K116的一个AD口采样电压，原来的电阻是300K和33K分压，

由于系统有静态电流的要求，在12V的ADC采样电路上一直有电流消耗。该条通路上的12V是常电，消耗为12V/333K=36uA。修改后的分压电阻（同比例放大）， 1M（R332）和110K（R333）。消耗的电流变为 12V/1110K=11uA，可以降低静态电流。

我想确认：采样电阻阻值同比例放大，对MCU的ADC采样效果是否存在影响

[lukaszadrapa]

Hi,

specified accuracy can be met only if source impedance of measured signal is 5K ohms or less. This information can be found in the datasheet.

The higher source impedance, the lower accuracy. You need to find a compromise between the accuracy and current consumption.

There's nice explanation in this old application note:

https://www.nxp.com/docs/en/application-note/AN2438.pdf 

See section Input Leakage Offset Error.

Regards,

Lukas