circuit design - Current splitter/divider in nA/pA range possibilities

My objective is to realize a current splitter which could split current from a wire A in the range of pA, nA equally to wire B,C.

I was initially sure what this type of circuit would be called, but google redirected me to Current Conveyors page - https://en.wikipedia.org/wiki/Current_conveyor. (please correct if I am wrong here).

A CC-1 circuit is shown below:

I could correlate (X=A,Y=B,Z=C). However, I am not sure if it would support low currents (pA,nA).

• Voltage Range: (0-3.3V)



• Frequency Range: 1-100 kHz.


• Loads: Current sensing amplifiers, each on B and C. (Assume Input is a gate capacitor of MOS and hence high impedance)


• Reason on why I am doing this: To compare results of 2 current sensing amplifiers.

Any hint/guide on above will be helpful.

Answer

Here is a cheap approach, but you need access to a foundary with long-channel FETs. The purpose of using a foundary is the access to layout of matched (interdigitation of alternative strips) FETa and FETb, for matching during implanting and, later, thermal matching during operation.

^{simulate this circuit – Schematic created using CircuitLab}

How fast can a current splitter function? Or, how much charge is needed?

Assume the FETs are 1U by 1,000U each. Area of 2 FETs is 2,000 (micron)^2 which is about 4 picoFarads, in 0.6 micron process (140 Angstrom oxide). To change the voltage by 0.005 volts requires

Q = C * V

dQ = C * dV [assuming C is constant, and some FETS do vary near Vt]

dQ(coulombs) = 4pF * 0.005 volts = 0.02 picoCoulombs

At 1pA, this allows about 1pA/0.02pC or maybe 50 cycle-per-second operation.

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So what to do? buy tiny matched same-die JFETs, tweak the gate up or down by a few milliVolts on one of the FETS, so the current splitting is accurate.