Transimpedance amplifier simulation acting weird
Try using MEG rather than M for the 50M\$\Omega\$ resistor and 10M\$\Omega\$ resistor.

Edit: since this is ADS that's not the problem. 

Check the op-amp DC output to make sure that it's not saturated, (and that the inputs are within the common-mode range for that part). At -12mV with a 0 to 5V supply the former seems to be an issue.