I don't recall the values but at work there was some equipment that used them They were in part of an amplifier for a radiation detector similar to a very large geiger counter. It used a detector with a 4 foot long tube.
Yup. A couple of years back, I did a front end for a surface potential tool for semiconductor measurements that had switchable 1G and 50G feedback resistors.
There are lots of subtleties in a circuit like that--I had to bootstrap the coil of the latching relay, and switch it with another (nonlatching) relay. Also the 1G resistor had to be shorted out when the circuit was in the 50G mode, because otherwise its current noise would have gone straight through the capacitance of the open relay contacts (~0.2 pF) and trashed the measurement.
And, of course, you have to fix up the ridiculous RC rolloff in the second stage.
The good news about that is that the intrinsic SNR of the measurement isn't that good, so the RC can roll off a pretty long way before the second stage's noise becomes important. That's true of high-Z measurements in general.
The customer (Qcept Technologies) ran out of dough last summer and had to shut its doors with a bang. A pity--they made good stuff.
Been a long time since I used anything in the 100 Meg range or above, but I recall that there were a lot of recommended precautions. One was to clean the glass body very carefully (with alcohol, I think) to remove fingerprints which would otherwise trap dust, which would then allow humidity to cause a low-resistance shunt. You also want the body to be elevated above the board surface for the same reason. I also seem to recall something about teflon mounts/feedthroughs at the lead ends, but I can't recall exactly what that was supposed to do.
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Sure is. So is garden-variety IPA solvent. In high-Z circuits, I usually used guard traces top and bottom (with the solder mask removed) and a slot under the photodiode and the feedback resistor (and cap, if any). Being able to get solvent under the components helps a lot with the 1/f noise and stability.
The popcorn noise you get from a board washed with drugstore IPA has to be seen to be believed.
Isopropanol (IPA) has low surface tension, so it forms a film. It's also hygroscopic, so it becomes (after some evaporation) a water film full of any crud you can imagine. And when it dries, all the empty board space looks clean, because the dissolved stuff is deposited at the capillary-attractive sites you DON'T see. Water, by comparison, has higher surface tension, forms droplets you can remove with airflow.
Wash with IPA, rinse with distilled water, blow dry (with clean air) then bake dry at low temperature for an hour, if you really want to clean.