effect on Rl value on current source : Low R Load versus high value R load

[Overspeed]

Hello

I have with more explanation than Rload value have a effect on the current source circuit as :

A voltage controled OP AMP current source
The twin JFET circuit as the italian low noise current source

I my case I need to measure low resistance value ( under 1 Ohm ) with a 4 wire setup so shall I take care of some parameters / calculation ? with a 10 mA source or greater as 20 mA

I link a schematic of the Italian low noise JFET circuit but they have tested value starting at 200 Ohms

I want to avoid if possible to built a complete circuit for worth results a JFET cost 25 Euro on Mouser

Regards
OS

[Kleinstein]

For a resistance measurement the circuit makes little sense, as the current is not long time stable and the voltage reference used is the floating battery.

The probably use the relatively high resistance to really look at the current noise, more for a shorter time scale. In principle the circuit should be not much effected by the load resistance as the current source part is floating. The main link to ground is the input current of the INA and thus it's input impedance. This should be no issue with the relatively large current, it's more a few pA the go the wrong way there.

A precise resistance measurement should have a resistor as refrence element and use the same voltage ref. to drive the current and at the ADC to measure the voltage over the DUT. A ratiometric measurement does not even need a stable current source / voltage reference. In a simple circuit the voltage over the reference resistor is used as the ref. voltage to the ADC. One may still need a buffer, as the ref. input of some ADC draw some input current.

[Overspeed]

Hello

Thanks

I have several possible solutions for 10 mA low noise stable source , solutions which are sorted by :

A simple precision resistor linked to a precision voltage source , not really stable on 10 mA , work on 1 mA as precision source don't provide enough current .
A Jfet transistor with a simple resistor control , not really stable in temperature
A Jfet transistor circuit with voltage control not really stable in temperature
A voltage controlled Op Amp , can be a solution as Voltage control can be from a true precision source ( available at reasonable cost with a precision resistor divider ) that adjustable work nice on 1 mA as the linked circuit ( from this forum )

On Github there is this solution also base on two circuits one from direct OP AMP Output the other by using two transistor to increase the current level

https://github.com/Strathclyde-EQOP/Vernier-Current-Driver?tab=readme-ov-file

the most complete files set with details explanation I have never see .

Regards
OS