Picoammeter Design Problems

[dreaquil]

A while back I posted asking about picoammeter design ideas and reference material.

I designed a board using an LPC662. It's a feedback ammeter (looks like a low pass filter)
Rf=10Gohm and C=47pF. For some reason my output is saturating positively. I've tried replacing the IC twice. The feedback resistor too. Place 45V and Ri=200k (which should make it saturate to ground pretty hard) but nothing worked. I'm supplying it from a 5V rail. BTW the non inverting input is actually at 4.096V to shift the signal up such that I can read it with a single supply. I then replaced it with its slightly lower performance cousin the LMC6462 and it worked fine! Any ideas?

[Niklas]

Input common mode range. Vdd - 2.x V at 5V supply. Both inputs must be kept within the range defined in the datasheet.

[dreaquil]

Lesson learnt! Thanks a lot.