I have two outputs of a voltage source that do ±10 V with 16 bit. This leads to a theoretical resolution of a bit step of 0.3 mV. My goal is to have more resolution by using a voltage divider of 100 on the second output and combining the two. This would give ±10 V with 3 μV theoretical resolution. This voltage will be used to charge up a capacitance <10pF. The adder will be nicely shielded and will be sitting at a temperature of 4 Kelvin.
The only problem is that I am seeking is adding the two voltages WITHOUT using active components, as transistors add noise and there is only a thermal budget of about 0.1 mW (less is better).
The whole passive network doesn't have to be perfect at all, though. The only object is to have more voltage resolution in the end.
I played around with resistors in LTspice but didn't succeed yet. I would be happy to get some new ideas.