Thank you for the review! I really appreciate the advice. Good catch on the protection diodes, I had switched a different pair to 1N4148 for that same reason but didn't propagate to the other usage.
To your observation "It looks more like the 100 nA is the resolution and not the actual lowest stable current" -- I think it's basically... both? I don't have much need right now to go down into the 10's of nA and hold stable down there. And I think being able to dial in the current to ~100nA stability thru the micro-amps range also meets my needs. (Better, even; if I can dial in 200uA +/- 1uA that would probably be fine.) Over 0--85C, leakage current in the DG455 switches is limited to +/-2.5nA for an open switch, 5nA for the closed one, so at any moment that means +/-12.5nA total leakage in the measurement network (plus another 5nA thru the on/off switch at the bottom, so 17.5nA total). Not *amazing*, but I think that will meet the design brief. (Even with low-leakage MMSD4148 protection diodes, they still potentially add more error than that, although hopefully they somewhat cancel each other out.)
The choice of R23/R24 and RV1/RV2 was dictated by a few considerations that fed back on one another...
- As you note, the bigger the resistors, the more voltage you lose at the top end. So I'd like R23/R24 to be small, all things considered.
- The low end of the range on RV1 / RV2 suffer from residual / end resistance. The bigger RV1/RV2, the less that end resistance matters. Bourns PDB12-series potentiometers have guaranteed <= 10 Ohm end resistance, whereas many other panel-mount pots have end resistance as a percentage of the range. 100k / 5k gives me a 20:1 coarse/fine spread which "feels nice" to use in testing, and 10 Ohm is "small" relative to both of these. Scaling down to 20k / 1k is an option, though.
- The design range I'm targeting tops out somewhere around 2--2.5mA. The 2k2 positive feedback resistor gives me that, in conjunction with the 100k pot.
- For better precision on the low end of the range, a larger positive feedback resistor would decrease the gain. 5x that takes me to ~10k, which means the "fine controls" would extend up to 500uA. Which seemed "reasonable" to me. I could use a 22k resistor for R23 and get better fine control thru 250uA which might be better, but now we are starting to really sacrifice voltage headroom.
- For that matter, I could use a 4-way mux instead of the 2-way switch and introduce more options. I just hadn't thought too hard about how far to push that control.
I hear you on the P/N crossover at the high end of the opamp range. OPA192 isn't terrible but the deviation is present. Since most of the circuits I build run on +/-12V, my plan was to just run this circuit on +/-15V, which is still within the range for the OPA192 and the REF200. That means that under pretty much all circumstances, there would be 3V of headroom, and the opamps would stay out of the crossover range.
I'm curious why you suggest TMUX4502; Ron, Ron flatness, and Ids leakage are all about 100x worse than DG455. (It is much cheaper though, at least!)
I will look up the 34401 schematic though and take a look at that setup. Thanks again for the pointers!