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?][1]

[ADC over-voltage input protection while keeping accuracy][2]

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][3]][3]

 - **Approach 4: Unity Gain opamp with internal Clamp**

Texas instruments have a product for this role exclusively. 
[OP698][4]

**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.
  [1]: http://electronics.stackexchange.com/questions/35807/how-would-i-design-a-protection-clipper-circuit-for-adc-input
  [2]: http://electronics.stackexchange.com/questions/170176/adc-over-voltage-input-protection-while-keeping-accuracy
  [3]: https://i.sstatic.net/u9XgO.png
  [4]: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiovYq4yO3NAhUE5xoKHe55CucQFgghMAA&url=http%3A%2F%2Fwww.ti.com%2Fproduct%2FOPA698&usg=AFQjCNFd7tRpneAj55aDBDDILVX1TmB9Ng