Just finished swapping the relays to COTO 9002-05-10 and replacing the front end with ADA4530-1. Cleaned the sensitive part with ULSI grade IPA (overkill) and sonicated the air-wired opamp. I used the relay variant with magnetic shield, the shield is a exposed metal cover which turned out to be a annoyance when dead-bug soldering, some electrician tape had to be inserted to insulate it from the board. Applying epoxy to fix the relays on to the board is very helpful reducing the sensitivity to vibrations.
The performance looks really impressive after warm-up, with the bias current compensation circuit disabled, input connector covered by aluminum foil, <0.5fA bias current can be achieved. Did some simple noise characterization by simply reading from GPIB at raw speed (3.13 Hz). The current noise is still 1.75x above the thermal noise floor for the 100G
feedback resistor, through. Interestingly the excess noise does not appear as 1/f but a white noise, wondering what can be the source. There is also a popcorn noise, which I
don't think originated from ionizing radiation.
Another feature is it requires a really long warm-up to reach the 0.000pA reading. 2 hour is barely enough. I put a thermal couple on top of the preamp shield and saw the temperature raise from 22.4 degC up to 38.7 degC, the timescale of the temperature stabilizing roughly agrees with the time when the reading settled to zero. Maybe it's thermal EMF? The popcorn noise also seems to have a strong correlation with temperature.
Edit: 5fA at 100GOhm is 500uV, that's too high for typical thermal EMF, so there likely to be something else.