Need information about ADC's external impedance

[paul-henri]

Posted on August 28, 2013 at 11:17

Hi everyones,

In the datasheet for STM32F407xx, in the electrical characteristics part (12-bit ADC characteristics), they deal with the Rain value. they define it like that : used to determine the maximum external impedance allowed for an error below 1/4 of LSB.

What do they mean by ''external impedance'', is it the input impedance of ADC ou is it the maximum output impedance of sensor (or anything else) that can be plug on this ADC ?

Thks.

#adc

[John F.]

Posted on August 28, 2013 at 12:10

The ADC input acts as a dynamic load on whatever is driving it. It takes ''gulps'' of current when sampling - there are ST Application Notes on the ADCs you can read.

Some devices / OpAmps have poor transient load response and their outputs change when suddenly loaded and take time to recover. Additionally, higher device / OpAmp output impedances will take longer to charge the ADC sampling capacitor.

Longer ADC sampling times will usually give best results. If you need short times, you may need to buffer the ADC input. Look at OpAmps designed for ADC buffering from Analog Devices / Linear Technology / Texas Instruments etc. (Look at their ADCs - they often advise suitable buffer ICs in the Data Sheets. It all depends on the performance you need.)