I am very intrigued by the Fluke8505A multimeter and would like to know what you think. I have seen some documentation and the resolution of 7.5 digits in the 10V range is certainly interesting. I have an HP34401A and a 7.5 digit Prema. I have seen that the HP34401A is not that great in AC. I'd like to play with the Fluke 8505A but I'd like some opinions from you. thank you Luciano Luciano P. S. Paramithiotti timeok@timeok.it www.timeok.it Da "volt-nuts" volt-nuts-bounces@lists.febo.com A volt-nuts@lists.febo.com Cc Data Wed, 22 Jul 2020 12:00:02 -0400 Oggetto volt-nuts Digest, Vol 128, Issue 9 Send volt-nuts mailing list submissions to volt-nuts@lists.febo.com To subscribe or unsubscribe via the World Wide Web, visit http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com or, via email, send a message with subject or body 'help' to volt-nuts-request@lists.febo.com You can reach the person managing the list at volt-nuts-owner@lists.febo.com When replying, please edit your Subject line so it is more specific than "Re: Contents of volt-nuts digest..." Today's Topics: 1. Re: Keithley 417 electrometer upgrade to solid-state (ed breya) 2. Re: Keithley 417 electrometer upgrade to solid-state (Joel Setton) ---------------------------------------------------------------------- Message: 1 Date: Tue, 21 Jul 2020 19:38:59 -0700 From: ed breya <eb@telight.com> To: volt-nuts@lists.febo.com Subject: Re: [volt-nuts] Keithley 417 electrometer upgrade to solid-state Message-ID: <5F17A6C3.60302@telight.com> Content-Type: text/plain; charset=utf-8; format=flowed Update. I think I'm done with the overall new design, except for fine tuning and compensation, which will have to wait until I can build an extension cable to work on the plug-in out in the open. I did a lot of improvements mostly in the power supply, and added four new multiplier ranges, adjustable bias cancellation, and added a switch and new circuitry to run the remote-zero solenoid. It's looking pretty good now, burning in out in the garage. For ranging, I built a new switch with eight positions and a couple extra wafers for signal routing, extending the multiplier step sequence out to 0.03, 0.01, 0.003, and 0.001, providing maximum sensitivity of 1 fA FS. I originally planned to add two steps, after seeing how well the new IC-based front end was working, bias-current-wise. Searching my switch parts inventory, I could only find a compatible one with eight, without doing extra mechanical mods. So figured I'd go with it and have two left for experimental setups. The extra gain to go ten and a hundred times more is provided by a new meter drive circuit. I did some experiments with the original ranging feedback divider, which looked promising at 10X more, but made it very, very slow in response, without fixing up the overall compensation. So, I opted instead to change the meter sensitivity by 10X. In principle, this was no sweat, but the meter in this machine is 1 mA FS, so needs fairly low network resistance, making clamping and protecting it more complicated. It would require choosing between probably inadequate meter protection, excessive clamp leakage error, and not overloading the electrometer's output, which would cause instability on recovery from large excursions. This was solved by using a separate opamp follower circuit to buffer the original meter signal, and provide limiting to protect the meter movement - no more than 2-3 full-scales (mA) worth of drive current can be applied, under any condition. This is about the same as with the original meter setup. This worked out quite well, once I ran the meter circuit from separate supplies. When it was from the same ones the electrometer circuit used, it made a relaxation oscillator - the slight changes (maybe 10 mV) in the supplies during overload were enough to upset the front end, so it would just bang back and forth between extremes. Once I had committed to the opamp follower setup, the next natural thing was to consider getting 10X more still, by having a 1X/10X amplifier gain mode, and that's what I did. So, the first 10X shift is from the low resistance meter drive loop, providing 300 mV FS, which is a tenth of the original 3V. The next 10X comes from switching the follower's feedback for 10X voltage gain, while the meter is still set for 300 mV, so it reads 30 mV FS from the electrometer output. The other, original ranges are the same as before, with about 3 kohms of meter and resistor load. The range switch is set up to continue the original 0.3 and 0.1 multiplier feedback pattern, and route the meter to the original path or the new path for 10X, and activate the 10X opamp gain in the last two settings. It all worked out well, especially the new 30 fA and 10 fA ranges. With bias current looking like less than 1 fA most of the time, it's ten percent or less of the 10 fA scale, and with the new bias cancelling deal, it can be pretty well zeroed out, for short term operation. The 3 fA and 1 fA ranges are of course so sensitive that they're not really usable for external measurements without a lot of watching over and tweaking - it can go off-scale at any time from random events, but gradually drifts back. They are experimental for now, but very useful for making assessments and further improvements - I can really see what's going on now. The bias cancelling scheme is similar to one I made for an old K410A tube unit (now long gone), except that the tube ones tend to have only unidirectional grid current, while for this I needed bidirectional control. This was done with a ten turn pot driving the LEDs of two opto-couplers differentially, and connecting their E-B pins anti-parallel so their currents are opposite, and cancel when equal. The CTRs of the optos don't necessarily match, but with the right setup, there is a range of adjustment near the middle where there's no net current out, and at either extreme, a maximum of each polarity. This one is set up to deliver about +/- 1 uA. The voltage is set by the load resistance, in this case 5 kohms, so about +/- 5 mV out. The receiving end of this signal is the high megohm feedback resistor array, but only the highest two or three may need it. In the lower sensitivity ranges, the bias current is insignificant (even though it's still there). The small voltage signal is superimposed on the feedback signal voltage at each big resistor's end - the low-Z, feedback end - in accordance with the scaling needed. In this case, the +/- 5 mV is added at the E12 resistor, and that signal divided by ten is added at the E11 resistor, and so on. The divider is a 9k, 900, 100 ohm setup, but so far I have only connected the E12 and E11. The E10 circuitry would need a bit more hacking up of the board, so I skipped it for now but left the provision for it. This divider is insignificant compared to the huge feedback resistors, and with no current applied, the tiny feedback currents go through as usual. When correction current is applied, the result is a small voltage causing an excess small current in the resistor, that can cancel the input bias current. The E12 one, for example, will produce 1 fA/mV, which can cancel 1 fA at the input, if the polarity is right. So, the floating output of the opto rides on the feedback, and tweaks the bias according to the pot setting. A load trim resistor on the opto output sets the desired voltage range. I'm leaving it wide for now, but may eventually go narrower. Wide is good for experimenting like setting it at zero-center in the 3 and 1 fA ranges, where the needle bobs around a lot in both directions. Narrow is better for setpoint resolution, maybe after things are refined enough. It's also open-loop, of course, without the operator to turn the pot and adjust it accordingly - and slowly - not quite tedious, but very slow. Also, as the input bias drifts it needs readjustment. You can't do much about big, random hits, except wait for the needle to return. I'll have more to report after I see how it's actually running later. Ed ------------------------------ Message: 2 Date: Wed, 22 Jul 2020 17:42:53 +0200 From: Joel Setton <setton@free.fr> To: volt-nuts@lists.febo.com Subject: Re: [volt-nuts] Keithley 417 electrometer upgrade to solid-state Message-ID: <e275513b-9a0a-3696-a036-393a5118e7c5@free.fr> Content-Type: text/plain; charset=utf-8; format=flowed Ed, I can't believe you're working on 1fA full-scale ! This is impressive indeed and even though your message gives some very useful explanations, I can't wait to see your schematic and learn from it. Of course I'm staying tuned. Then, at the end of your project, we may have to change the name of this list to include the "amp-nuts" side of things !!! Joel ------------------------------ Subject: Digest Footer _______________________________________________ volt-nuts mailing list volt-nuts@lists.febo.com http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com ------------------------------ End of volt-nuts Digest, Vol 128, Issue 9 *****************************************