DAQ input protection (overvoltage & overcurrent) question

Hello Everyone
I´m designing a DAQ board as my thesis work.
I have several questions about input protection, I have searched and searched and I have found only 2 circuits, a crowbar and another one using 2 zenner and a PTC resetable fuse or a ordinary fuse.
My question is, if there is any other circuits that can protect inputs.
The measurement limits are +-20V with a maximum frequency of 100 kHz. At the input there will be a INA163 IA, and then the rest of the circuit. I have to protect these inputs to 230 Vac.
If someone knows any other method of doing this, please reply. I'm pending to the PTC+diodes.
Thank you

That seems a good approach, to keep it simple. About the resistors I don´t know yet if I´m going to use SMD type or the normal type, I have to see if there is enough space on the PCB. Probably if I choose the SMD type I will have to connect several in series as you said (230 V SMD resistors with those values are a bit hard to find, Ive checked farnell), but I will test it.
About the zenner, when choosing it, It should whitstands also 230V of direct applied voltage right?
I´ve attached a file with the circuit, as I pretend to measure diferential signals with the INA163, I think that this circuit is correct using your sugestion.
Now about the PTC, for the overcurrent protection, if I´m not wrong there are 3 parameters that must be taken into account:
- Holding current (I think this is the normal working current)
- Tripping current (At this value the PTC cuts the current going into the circuit)
- Operating voltage (I think I must choose the 230V version)
The holding current I´ve pointed it to 50mA with a tripping current of 100mA (I have no ideia if this is ok), the maximum current input of the INA163 is 10mA, the resistive divider will limit also the input current, but I would like to have some more protection.
Shoud I put one PTC in each signal branch (Vin+ and Vin -)?
Attachments:
circuit.png ‏17 KB

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