Thanks Iffyg again for the simulation !

Quite impresive for me, even withouth the LM334.
So, if a (digital?) potentiometer is put in R7 it can can generate the needed scale ranges without the "ugly" resistors?

Another reason this circuit caught my eye was that in this kind of small current measurements theres is a big resistor wich produces a lot of noise. This circuit has no big resistors anywhere, so i supose that contributes to less noise. Does somebody know which noise problems coukd affect this circuit?

 

Thanks Iffyg again for the simulation !

Quite impresive for me, even withouth the LM334.
So, if a (digital?) potentiometer is put in R7 it can can generate the needed scale ranges without the "ugly" resistors?

Another reason this circuit caught my eye was that in this kind of small current measurements theres is a big resistor wich produces a lot of noise. This circuit has no big resistors anywhere, so i supose that contributes to less noise. Does somebody know which noise problems coukd affect this circuit?

The main keypoint of the circuit is the voltage Vx at the base of Q2 which is given by the relationship:
Vx=Vth**ln(K), Vth is the thermal potential (kT/q), K is the amplification factor for a given range (e.g K=100 for 1µA fullscale)
Let's say Vth = 26.04 mV (which correspond to a ambient and stable temperature of 29°C for the circuit) => the exact Vx voltage needed would be 179.88 mV for K=100 (100nA range). I'm not sure that this voltage can be set exactly at this value using a digital potentiometer. I guess it would also be difficult with a D/A converter.
Of course you may approximate the needed Vx as 60 mV for K=10, 120 mV for K=100, 180 mV for K=1000...up to 360mV for K=10^6 (ie 60 mV steps).

Another keypoint if you were to build this circuit is to be sure that only one trimming (at 1µA as given by the datasheet) is sufficient and how to calibrate the more sensitive range (unless you have access to a precision current source or a well calibrated pA-meter).

Morever you may have noticed in the datasheet the warning about the PCB layout (guard ring around the inputs of LT1012) and leakage currents.
As far as I know the LT1012 is a low noise op amp at low frequencies, the noise could be cut off by the integrating cap and the inertia of a galvanometer will get rid of high frequency noise
If I were to build this circuit, I would separate every Vx with its one trimming pot to set the exact Vx for each range at exact room temp and would rely on my 6 digits 1/2 DMM Keysight 34461A for testing the 100pA range (100mV DC supply in series with a 1% 1GΩ resistor inside a shielded and grounded metal box)

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