help - how to protect STM32 analog inputs ?

[mehmet.karakaya]

Posted on December 02, 2015 at 14:17

hello dear forum,

I need protect the analog inputs of my STM32 

becouse I measure the output of a current detector

and STM32 GND is the same with the GND of power circuit

and I face problems with analog inputs

how can I protect them ?

thank you

#stm32-analog

[megahercas6]

Posted on December 02, 2015 at 15:04

1) First, don't design any circuit, that would allow to source more than 5V into your STM32 ( nearly all pins are 5V tolerant)

2) You can use zener diode, at around 4-6V. Problem is, it will draw current if voltage is high. That will cause you problems at high speed;

3) Use pair of Schottky diodes, connected in series between power supply and ground. Also place zener diode, so rectification action would not cause power supply to go up in voltage. And connect your signal as well as STM32 pin in middle point, this will limit any voltage around 0.3V to your power supply ( with 3.3V it will be -0.3 to 3.6V)

4) Also you can use active clamping with comparator and mosfet, but this is not very common, but allow good clamping power, best of all)

[mehmet.karakaya]

Posted on December 03, 2015 at 12:14

hello thank you for answer

I did 1) and

3) ( I used shottky diodes to clamp to -0.3 V and + 3.6 V) already

they dont protect enough - the analog input was damaged after some time 

what do you suggest to use TVSupressor diodes ? which TVS diode shall I use 

what breakdown voltage would be good ?

[AvaTar]

Posted on December 03, 2015 at 12:36

>...what breakdown voltage would be good ?

 

That is an information you need to provide.

Either estimate or measure the voltages/energies you need to deflect from your MCU inputs, and then design your protection accordingly.

Especially inductive loads (coils/motors) can turn out to be a real hassle for digital circuitry. You will need supressor diodes or VDRs specified for the energy levels involved, zener diodes will burn quickly (or immediately) in this case.

In the worst case, consider galvanic isolation to handle the input signals.