Other Parts Discussed in Thread: BUF634, IVC102,
We are using a LMC6001a op amp in an electrometer application for ion detection. Our lowest range is 1e-14 amp and go up from there We are using +/-5V power supplies which are bypassed with 0.1uF and 3.3uF tantalums. It is used as an inverting configuration and ii's output is buffered with a BUF634. Our full scale out put for each range is 1V. RF is 1Gig ohm paralleled with a 10pF capacitor. We are mounting the dip in a dip socket and have lifted the input pins up to float and make critical connection in the air instead of on a pcb with a guard ring.
Our problem is we are experiencing extremely long settling times on the very lowest range. To completely settle out it will take about 30 minutes to get to a static level. We know it is not associated with the bipolar power supplies as we have checked them very carefully. We also know it is not a thermal issue because if the amp completely settles out and then turn the amp off and then back off it will take the same amount of time to settle out again.
We have tried several more dip LMC6001A units and they all behave the same. We have not tried a metal can variant to see if that would be different. We have taken extreme caution to make sure the part is clean and have used tools to insert the part in place and therefore are confident we have not contaminated the part. This section of electronics in mounted in a copper shielded box.
We have followed the application note's suggestions but nothing there explains what we might be seeing.
We know we are pushing the limits of the 25 famps the op amp is rated for, but we are well above that limit.
We have spent a lot of time trying to diagnose this particular issue and are quite frankly stumped unless there is some thing unique to the op amp when it is operated at its extreme low currents.
Looking forward to comments from engineers that have worked at these levels for suggestion to try or let us know if we are chasing rainbows.
Craig Shull