Input impedance of the STM32H735 AD converter

[kazutobi]

Hello everyone.

I am currently considering using the STM32H735 AD converter.

As shown in the figure, I have inserted a Schottky barrier diode and a 1kΩ resistor just before the ADC terminal pin for overvoltage protection.

I was curious about the input impedance of the ADC pin, so I read the specifications, and I was wondering if RAIN corresponds to the input impedance? I will be using a 12-bit ADC, so in this case, will the impedance be MAX
680Ω? If so, the signal level will drop, so I will have to reconsider the protection circuit.

・What is the input impedance of the ADC pin?

・If you are actually using this microcontroller, what type of protection circuit do you use?

I would appreciate your advice.

 

 

[AScha.3]

Hi,

>What is the input impedance of the ADC pin?

Its complex, not just a "simple" impedance: input is a analog switch, connecting some 6 pF to the pin, to sample the voltage. So its "impedance" depends on the sample rate, you let it run on.

see ds, for minimum source impedance, depending on speed.

So for max. speed, minimum sampling time -> 47 r  source impedance; 

For more relaxed timing, longer sample time setting, input can be in the kohm range; you can calculate it.

read AN2834 .   

https://www.st.com/resource/en/application_note/cd00211314-how-to-optimize-the-adc-accuracy-in-the-stm32-mcus-stmicroelectronics.pdf

>If you are actually using this microcontroller, what type of protection circuit do you use?

You could use something like that, just i prefer (if possible) to use an R2R opamp , working on GND-VDD supply ;

so opamp output limits always to 0...VDD , no extra circuit needed.

[kazutobi · Accepted solution]

Hello, AScha.3.
Thank you for your reply.
I now understand that the source impedance determines the upper limit of the signal frequency that can be input.

And thank you for the link to the application note. It helped me understand better.

I will reconsider this, including the possibility of using an R2R op amp.
Thank you very much.