ADC over-voltage input protection while keeping accuracy?

i just want to get the idea for the input voltage protection for a fully differential ADC.

The analogue voltage inputs supposed to be 0-5v, but someone being careless could plug a higher voltage (up to probably around 20v) by accident and blow the ADC.

I have seen zener regulation/clamp circuits, but upon trying them realized that they of course produce a voltage drop. The voltage needs to be unaltered from the one the user puts in, so this does not work for me.

So, what I need is a way to protect my ADC inputs from over voltage as high as ~20VDC or more but still be able to accurately read a 0-5V nominal analogue signal without altering the voltage reading.

and my power supply for the board is 5V, so i only want to limit the input signal to 0-5V. yeah, of course the Voltage must be above 0.

besides, i have only found one overvoltage/undervoltage from TI , It is TPS2400 , but the range of the protection voltage is 3 to 6.9. it's not suitable for me.

so , is there any programmable VOP and UOP chips that i can use? the ADC that i use is ADS5272enter link description here

Any suggestions? Let me know! Thanks a bunch in advance!

i posted the Zener diode grafic see that , from -Vz to Vf it is not linear, and i thing it must have an effect on the input singal for ADC