Thank you for all the replies, very very kind of you
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The 50Hz interference behaviour is quite strange: depending on the load, there is some kind of a voltage/current "threshold". If the voltage/current exceeds this "threshold" the output signal is fine, almost without any 50Hz interference. If the voltage/current on the load is below that variable threshold though, in the output there is a 50Hz square wave(probably a saturated 50Hz sine). I mean, it's not always present, just when the current/voltage on the load is very small(200/500mV or so across the load). Could this behaviour be an issue related to the MOS current buffer? If yes, a BJT/LT1010 current buffer can solve the issue?
Yes this circuit should have this large current sensitivity, because i don't know which kind of device will be tested. To be honest I was taking inspiration from this circuit and the Keithley 220, which seems that can handle a large current range.
Watching at the schematics (i don't know if i'm allowed to share them, but you can find them in the 220's manual) it seems that the highest resistor is always connected, as Cerebus suggested, but how can i be sure about the value of the resulting parallel resistor? i mean, yes there is the formula, but how is it reliable considering both resistor tollerances? probably the best way is measuring the actual value of the resistors, but how can i do it without a specialized and very sensitive ohmmeter(considering that there is a 1GOhm resistor)?
Also, there are practically no capacitor in parallel with the sense resistors in the Keithley's schematic. Would just one capacitor (330pF) suffice for the entire range of resistors or are necessary different capacitor for each resistor?
Yes, I'm using mechanical switching(these relays, which should have at least 1.5TOhm insulation resistance: DIP12-1A72-12L). I'm assuming that you are referring to dry switching to the phenomenon where the relay's contacts get oxidised. Am I right? If not, what do you mean by dry switching?
Also, in passing, if any of your switches are mechanical (including relays) there is a real risk of not pushing enough current through to overcome 'dry' contacts.
Do you suggest the use of some kind of bootstrapping for the relays' coil, like a capacitor in parallel to a the series resistor with the coil to get a momentary current kick?
Instinct says that you are going to need very different (incompatible?) switching mechanisms to tackle dry switching and stray leakage currents on the 1G range and the nasty contact resistance dependency of the 1R range. Do
In the Keithley's schematic there is something strange, like a hybrid relay-NJFET switching mechanism, but i don't quite understand how it works. What about JFET's parassitic capacitances, leakage current? How just one NJFET can allow bidirectional current flow? Sure they are bidirectional unlike BJTs, but should it be necessary a PJFET in parallel to all of those NJFET to handle negative signals? The 220's circuit is quite complex, especially the circuit related to U319, and i don't quite understand what is going on to be honest, so please sorry if i'm saying something stupid: my electronics knowledge is still quite limited
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