Siggi,
You are correct.? The Ohms current source and protection that I developed has never been improved upon.? It has been incorporated in every instrument since the 3468A/78A.? ?It is robust, simple with low parts count and very close to an ideal current source as you may ever find.? The dymanic Norton/Thevenin output impedance is greater than 10^15 ohms and can be made even larger with appropriate choice of Op-Amp that bootstraps the output FET and a few other component changes.? Such an output impedance cannot be maintained when integrated into an instrument.? The equivalent output impedance is degraded by moving the current from the ohms converter thru relays and the input terminals of the instrument not to mention all the nodes that touch the PCB that add unintended parasitic shunt impedances.
Thank you all for getting me to think about these issues again.? This has inspired me to make a huge effort to finish my design tutorial so others can leverage off the years of blood, sweat, and tears I "suffered" to get to where I am today in designing instrument analog front ends.
Wish you all a productive new year in 2020
George Hnatiuk. PE
