Measuring currents between 100pA to 100uA
First, the direct answer. You need to connect your circuit between the lower end of the lower 1M resistor and ground. Any charge or discharge current in the cap also flows through both 1M resistors.

Now, about your other circuit.

R6/R7/R9 sets the reference current at about 1 uA.

R23 provides a 10 nA offset. Apparently the input current is expected to occasionally go negative. Alternatively, the designer wanted to guarantee that if the current goes to zero, the resulting output could be identified as that condition.

The R24/R25/R26/C9/C10/C11 provides a comprehensive low-pass filter for the input current. The varying capacitor values avoid problems with self-resonance at any particular frequency.

EDIT -

My direct answer referenced your resistor/capacitor diagram, and the result should look like







simulate this circuit &ndash; Schematic created using CircuitLab

You will note that the LOG114 should not be connected where you think it should. It is connected at "the lower end of the lower 1M resistor", just as I stated. 

And I agree that if you try to apply the low-pass filter to the top of the lower 1M resistor, and add a 50M resistor after that, you will get a lower current. But that is irrelevant, since you don't want to do it in the first place. Attempting to infer current by measuring the voltage across the lower 1M resistor is a supremely bad idea - at the very least, a 100 uA current will produce 100 volts, and I can't think of a good way to attempt to measure that.

You are attempting to adapt a circuit approach which is inappropriate to your problem.