Hi Ed. Yes, please continue with your very informative discussions on this subject matter. Helps me a lot to come to grips with what is important to look at if I want to improve some of my old 20th century gear! Lou
On Sat, 18 Jul. 2020, 5:30 am ed breya, <[email protected]> wrote: > Joel, I too have a K616. When I get a chance, I will pull it down and > look at the bias current. I also have a K610CR, and a K602, which I > looked at last week. These all use the same front-end module based on > P-channel MOSFETs with unprotected gates, differentially clamped (gate > to gate) with external diodes or transistor junctions. And, they all > have the same 5 fA bias spec. > > Of all the MOSFETs from the old days, I know of only one type so far - > the 3N163, I think - that has the unprotected gate. The early MOSFETs > (called IGFETs back then) needed special anti-static storage and > handling, then later models added built-in Zener gate protection for > more durability. This is what spoiled them for ultra-low current > applications. > > So, I'm pretty sure Keithley used selected and matched 3N163s or an > equivalent, for the inputs, and selected small-signal bipolar > transistors for the gate clamping B-C junctions. > > My 610CR fired up OK after many idle years, and settles down to right > around 5 fA, so OK there, but is very jumpy and sensitive to every > mechanical effect like tapping the controls and cabinet. I need to do a > thorough cleaning and Deoxit treatment on all the controls, and tighten > things up mechanically. I also found either a packaging design flaw, or > that there may be a missing shield part under the input section at the > front panel. There is a huge open area around the input connector at the > bottom, where it's exposed to the bottom cabinet cover. The cover edge > is just placed in a slot in the front panel, and has a couple clips > retaining the back. The slightest vibration or shifting position causes > huge jumps that would be gone if proper shielding was inside. I'm > assuming there's a missing part on mine, more likely than Keithley > missing this vital issue - either way I need to make an inner shield > part for it eventually. > > The 602 also fired up OK, after changing a couple of its six 9-volt > batteries. It settles to around 1 fA, and is not very jumpy, but the > zero drifts quite a bit. I think this is mostly due to the battery > voltage dropping during use (there is no regulation), and maybe would be > much better with all fresh batteries. BTW I modified this unit years ago > to replace the big old-style C-Zn block batteries with modern 9V ones, > and eliminated the Hg cell, by using a 1.22 V IC regulator and different > scaling resistors to get the 1 V reference for ohms mode. > > These models both are subject to rather large voltage offset drift, so > need quite a bit of tweaking to keep everything good, even though the > bias currents are OK. It's interesting that there are three levels of > zero adjust - H, M, L - to accommodate some amount of MOSFET variation > and operating temperature range. But, that's what it took back then, and > it's still pretty respectable, especially considering these are about > fifty years old. > > Anyway, regarding getting the bias current back to spec on a 616, a > thorough cleaning of all high-Z insulating structures should do it, but > don't forget to do the input MOSFETs. Their weak point is the package, > and possible gate leakage on the surface, from the leads to each other, > and to the can, which I think is the substrate and source, tied together > in this application. If you've tried every other possible thing but not > the MOSFETs, then I think it will work wonders. Gain effects tend to be > from leakage on feedback elements, while bias current and offsets tend > to be adjacent-node problems with DC present - even seemingly tiny > amounts. Also, since there's a digital voltmeter measurement behind all > this, part of getting things zeroed may include what's going on in there > too. I don't know offhand. > > Speaking of device packages and leakage and such, if there's enough > interest, I can tell my "AD542 story" from decades ago, which explains a > lot about this subject, and why I always pay lots of attention to it, > and how to avoid or improve it. And it would include a free rant about > how crappy device pinouts are for some purposes, and how the standard > advice in every high-Z data sheet and app note ignores the biggest problem. > > I'll report some more about the 417 project later, and maybe see what > the 616 looks like by then too. > > Ed > > > > > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there.
