I want to use AD536 as rms reader. I have no experience on this specific device. According to its datasheet ( page 1 line 21) it is well protected against input over-voltage. In fact, what was interesting for me in this device selection was this statement that means I don' t need input over-voltage protection circuit. But in page 3 of that datasheet and in the Absolute Maximum Ratings Table, the maximum input voltage is 25V peak. What I want to know is this 25 volt is the maximum voltage that device can measure but it can tolerate higher voltages ( for example 110 V) without damage or I need a over-voltage protection circuit also? Any help appreciated if anyone has experience on this device.
"absolute maximum" means exactly that -> "thou shalt not exceed these limits". This is set by material properties, process parameters and a host of other factors. Well protected against over-voltage means that if you hit the inputs with absolute maximum whilst running lesser rails voltage (say +/- 3.3 V) that it will be well behaved.
You almost certainly don't want to operate the device this way anyways as driving the inputs to saturation (even if it doesn't kill the device) will cause clipping of the waveform, which will cause distortion and subsequently much lower accuracy. You need to scale the the input appropriately to bring it within range of the inputs and use all that laser trimmed goodness.
But ... this device dates from 1977! It isn't even on Analog's web site. there are more modern devices to use.
The absolute maximum ratings have little to do with what the IC can provide accurate I/O relationships for. They should be strictly considered as conditions that if exceeded, the IC is no longer guaranteed to ever function correctly, ever again. So, no, you cannot put 110V into your IC.