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I have 8 analog input coming from various 5v and 3.3v sensors that have to be interfaced to a 3.3v micro-controller. The main system voltage is 24v-48v , i need to protect the ADC inputs against over-voltage from misuse , not ESD events. So if by mistake the 48v is connected instead of the sensor.

after looking into the following threads :

How would I design a protection clipper circuit for ADC input?

ADC over-voltage input protection while keeping accuracy

i tried the following :

  • Approach 1: Resistor and Zener

This approach is fine on digital pins but confirmed what people said in the posts about voltage drop and ADC error. I heard that using a shunt regulator like the TL431 instead could help but haven't tried it!?

  • Approach 2: Series Resistor and External clipping diodes

The approach works fine it have tries BAS70,BAT54 and similar , inorder to get lower drop i have found that using power (1A) schottky is better because they have much lower voltage drop at 1-25mA .

  • Approach 3: OPAMP, Resistor and diode

this approach i have found is not very common , and work fairly well , similar to approach 2 but more complicated.

Circuit

  • Approach 4: Unity Gain opamp with internal Clamp

Texas instruments have a product for this role exclusively. OP698

BUT: this lowest single supply is 5v not 3.3v , and the maximum clamping voltage is lower than the rail . and it doesnot tell how much is the maximum input voltage. and it takes PCB space , 4 resistors and 1 SO-08 IC per input.

Question :

At this point of search i got a bit frustrated and i think now that Approach 2 is the best in terms of robustness and space, please help and state any other approach i am missing ??
I am also interested in exotic solutions similar to automotive Input/ Output ICs that have multiple inputs and protection built in to interface the processors to harsh environment. similar to this MAX6816 used for digital signals.

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    \$\begingroup\$ I would use a combination of 1 and 2. Approach 2 relies on the 3.3 V supply staying 3.3 V, if you inject a lot of current it might become higher than 3.3 V, possibly damaging the microC as well. So I would use the diodes to clamp to ground and a Vcc which has a zener diode to prevent it from becoming too high. A 3.6 V zener on the 3.3 V supply might do the job. The TL431 is just a fancy zener diode, you don't need it here. \$\endgroup\$ – Bimpelrekkie Jul 12 '16 at 9:35
  • \$\begingroup\$ @FakeMoustache , sounds good , do you agree that using power schotkky diodes like CDBM1100 is better than small signal ones like BAT54?? (to get lower voltage drop at 1-25mA) ,, and does the mentioned zener on power supply have to be beefy , or just a normal zener since the resistor is series with clamping diodes will limit its power dissipation ?? \$\endgroup\$ – ElectronS Jul 12 '16 at 9:45
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    \$\begingroup\$ I don't think you need Schottky diodes, as long as the current is limited then a one diode drop will be OK. A normal 400 mW zener should be enough as again the current must be limited by a series resistor. The resistor has to do most of the work so to say. \$\endgroup\$ – Bimpelrekkie Jul 12 '16 at 9:49

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