AD8428 : large differential input voltage at high gain

In my application I use the AD8428 with AD8610 JFET buffers connected directly at both input (supply voltage +/-12 V)  One of the JFET buffer is connected to a fixed reference of max (+/-)6 volt from a precise 18bit DAC (1ppm). The second buffer is connected to a high-precision high voltage compensated divider with 1/10000 voltage ratio, to monitor a high precision (+/-)60 kV d.c power supply (10-5 precision with less than 1V ripple).  The AD8428 is used to measure the differential signal between the reference and the high-voltage at the output, and this measure is used in fast analog feedback loop as error signal to regulates the high voltage with >250 kHz bandwith.  The ad8428 can do the the job very nicely, better than the AD8421/AD8422, but one of my concern is that differential input voltage can be as large as 6 v for a long period of time with one input at (+/-)6V (max) and the other at 0V (min).  For this reason i want to use some input protection resistors of about 1k at both input to limit current to less than 3mA. I would prefer to avoid diodes unless it is highly recommended in that case. 

It is not clear from the datasheet what is the SAFE level of Imax to avoid any cumulative effect of the input current over the life of the device. 

Some good pratical advices are much appreciated to get a long term safe operation of the ad8428 against large differential input voltage.  

Thank you. Thierry

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