Here is the initial schematic of input section.I didn't included it in here because it was somehow wide.
C4 and C5 are added to give better characteristic in case of higher frequency division,Stray capacitance.(I just copy pasted the values from another schematic , I have no experience how do they work).Such is used in the Bitscope BT300 and in DSO MK3 .

Absolute maximum rating states +/-V for the common inputs of the Op-Amps where protection diodes clamp at +/-(V+1). I cheked with the Bitscope and some others. All of them used the same scheme in their inputs for over voltage input protection. If this is harmful for OP-Amps i'll have to use two of those Transzorbs in input. But all these things show a capcitance at higher frequencies which would not be desired (even these 1n4148s do)! seems like a loop :(

As a Non-inverting Amp ,AD844 could not accept more than +/- 1 V swing at its Non-Invertin input (stated in the datasheet) .When AD844 is used as an Inverting Amplifer it will show about 50 Ohm resistance in the Inverting input where this input typicaly draws 200 nA. Could i use AD848 as an inverting amplifier ? (I mean connect the CRO probe directly to the input)

It is mentioned in the data sheet that those seried resistors to the Op-AMPs supply pins could be replaced by Ferrite beads. I have BL02RN02 on my hand , could i use them here ? .

And the Chanell 1 position circuit is intended to move up/down the input signal within ADC input range.This maybe a total wrong one .(it is early morning here and i've been awake for all the night).

I'll be waiting for your suggestions and corrections.

Thanks in Advance

Farshid