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There will be 8 analog signals read by a microcontroller (STM32). I want to design a protection circuit for the analog inputs.

the analog input

The input can be 0-10 V or 4-20 mA controlled by the MCU using an inverter based on an NPN transistor.

When the pin is high (v2 = 3.3 V and v3 = 0V) the input is 4-20 mA. The op_amp sees the voltage across R2//R3.

When the pin is low (v2 = 0 and v3 = 3.3 V) the input is 0-10 V the output voltage is reduced to 0-3 V with the voltage divider (R6, R8).

R1 is used to limit the input current.

R2//R3 instead of one resistor to reduce the power rating.

I searched for a solution and I found this Over voltage and reverse voltage protection of multiplexed ADC with voltage follower.

I tried to use Matthew's suggestion but in the simulation I get 12.5 V with overvoltage input and -0.5 V in the case of reverse voltage. The problem here is that my op-amp allows only VSS-0.2 V to VDD+0.2 V.

I am looking for an explanation to how the back-to-back diodes work and any suggestion to fix this, or any other solution to protect my circuit instead of using 32 diodes or 12 two diodes in series.

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I suggest you use a bridge rectifier for reverse voltage protection and a zener clamp circuit to protect from over-voltage.

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  • \$\begingroup\$ The provided Link shows the right thing to do with the two diodes at the input... My idea would be to try that, and to reduce the input to the Opamp, add a voltage divider. \$\endgroup\$
    – schnedan
    Commented Sep 17, 2020 at 18:44
  • \$\begingroup\$ @schnedan the diodes only keep the input from exceeding Vcc which is fine only if the over voltage stays within the maximum rating. And the downside of your voltage divider approach is that you worsen the SNR in exchange for pnly a limited degree of protection. In my mind, your approach is not a very good solution. \$\endgroup\$
    – Sim Son
    Commented Sep 17, 2020 at 19:28
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    \$\begingroup\$ yes, but a bridge rectifier will even introduce some nonlinearity in the signal path... doesn't sound much better to me. Just to mention it, there are IC's which will fit in the application datasheets.maximintegrated.com/en/ds/MAX14626.pdf \$\endgroup\$
    – schnedan
    Commented Sep 17, 2020 at 20:18
  • \$\begingroup\$ @SimSon this means I need to add 4*8(8 analog input) + 16(zener clamps + rs)=46 components \$\endgroup\$
    – PCB-ABBS
    Commented Sep 18, 2020 at 9:44

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