electrometer front end for oscilloscope? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: May 15, 2025, 07:35:57 am
content: Does such a thing exist (someones project)? The idea is wide input voltage range, very low bias current, high input impedanceit could be as simple as a op-amp but there is the max-voltage problem. There is a few very good op amps.I imagined something with a gain knob, triaxial input, reasonable frequency response and a measure of accuracy.I thought there might be some clever circuits for this kind of thing. To extend the input voltage of one of those super low leakage op amps..Running it on like 20 9V batteries is acceptable for me! I don't know how feasible very high impedance voltage dividers are. You can compensate them with like teflon capacitors for frequency I think.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2025, 08:58:06 am
content: It is called aTektronix AM502 Differential Amplifier. It has a pair of jumpers to disable its 1 megohm input terminations making its input resistance 200 megohms or higher. Maximum input current is 50 picoamps, but the input JFET could be changed to lower it.Common mode input range is +/-5 volts which may seem low, however in electrometer mode there cannot be an input attenuator.ThePreamble DA1855has a 100 megohm input setting.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2025, 09:00:45 am
content: I have not seen such a project.What do you consider a useful bandwidth ?What would be acceptable input bias, impedance (R and C) and drift ?The reasonable choice would be a buffer amplifier with a bootstrapped supply. It depends on the range and scope, if one still needs an divider one the output. That divider would not need to be super high impedance, but would want frequency compensation, similar to a standard x10 probe, but could be lower resistance. Depending on the BW the cable could betricky - one would likely include it as part of the amplifier / probe.High BW and a bootstrapped supply can still be a bit tricky.Noise and drift from the input buffer would be less significant as it is relative to the high voltage. The dift of the scope would be more relevant due to the divider. I don't hink one would need much gain settings, more like a gain of 0.1 with DC coupling and maybe a gain of 1 with AC coupling. The main gain settings would be at the scope.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 15, 2025, 11:56:24 am
content: You might find manuals for the old Keithley electrometers. I seem to remember the circuit is quite clever and floats in a way I hadn't seen before. Your circuit will need its own range knob because I don't think you can do a circuit that works for a wide range and just relies on the scope knob.

author: coppercone2
author_rank: Super Contributor
timestamp: May 15, 2025, 08:30:37 pm
content: I have the remains of a old keithley electrometer that is what I was thinking something boot strapped or floating or whatever, but maybe with more modern semi conductors.Those circuits start getting 'funny' when the op-amp has something other then a voltage rail connected to the power pinsThat insane switch they have can be made into a only DCV switch with alot of rangesI don't know what useful bandwidth is. I think some where in the audio range? Like chopper amplifiers that were giving me grief I don't think chop more then 5KHz, though this was just the 'DC'. It was such a finicky bastard that I kept full scaling everything and thought it was broken until I connected it to a oscilloscope and saw it had a very narrow window where it did work, but it would zoom through it so fast that if you try to use the digits you would never see it. It worked but not like the app note.Honestly I don't know what the hell I will run into.At what frequency does it get completely absurd?Bias... well I can't say for sure other then if you connect a 34401A into it (in both modes) it would make the circuit go brezerk) but a keithley EM did not effect it. Same for drift.. I can't really give a number.I anticipate this being used to test something that has +-15V power supplies, but if its possible to get more I want it, because I am sure there is interesting things you can do with APEX amplifiers and stuff that runs on higher voltages.... doubt I could ever get it to work but I can tryI felt like there is alot of interesting things to experiment with in those types of circuits, but it sucks because they break if you try to measure half the nodes they have...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2025, 08:47:07 pm
content: The useful bandwidth is likely limited by the capacitance (as part of the circuit and the amplifier and maybe cables) and the source resistance. Depending on the DUT this could be rather low, but may be faster in cases where one could get away with a more normal probe too.For the circuit design it should be reasonable easy to get to the 1 MHz range. For a limited range (like +-2 V around be previous state) it would be the speed of the bootstrapped amplifier that sets the BW - so 1 MHz and a bit more should be easy (e.g. OPA140 has 11 MHz GBW).Because of the power loss with a high supply voltage one may want a lower current amplifier that is usually also a bit slower.One could get some slew rate limit (e.g. 0.1 to 1 V/µs) though, that could be a bit more limiting. The BS supply usually needs some slow down. The input capacitance may still be limiting - but that is the general issue with a high impedance source.

author: b177
author_rank: Contributor
timestamp: May 15, 2025, 09:02:43 pm
content: What about SR570?

author: coppercone2
author_rank: Super Contributor
timestamp: May 15, 2025, 09:27:26 pm
content: Hmm perhaps if I do some part time work with the bank I can afford that.Would the op-amps that are known for their ultra low leakage current be suitable for this? There is a handful I can think of off the top of my head. The OPA140 is 0.5pA typical, and 36Vhttps://www.analog.com/en/parametricsearch/13001really high voltage (140V) - 18 9V batterieshttps://www.analog.com/en/products/ltc6090.html(3pA typical, 50pA max)50V/40V/36Vhttps://www.analog.com/en/products/ada4097-1.html(typical bias is high though, I think its no better then a DMM?)https://www.analog.com/en/products/lt1464.html(0.4pA typical, 2pA max)https://www.analog.com/en/products/ad549.html(very low bias), 0.06pAThe 6090 voltage is so high that it might just be exactly what you would get if you modified the other op amps?I read some where on this forum searching that the typical value is something they measure in a lab, and the max is whatever they can get to measure cheaply on the assembly line as QC check for broke stuff.To how many volts can you increase the input with a modified circuit (within reasonable engineering effort)?I just got parts that have like.. . somewhat usable voltage ranges. There is those 5V opamps that have phenomenal specs. I guess the circuits would get much harder on those? But I don't have any idea how much you can boot strap something . If you can just pretty much raise it by a mile, would it not be better to start with the op-amps that have like femtoamper leakage currents that run on maybe 10V max?For instance this onehttps://www.analog.com/en/products/ada4530-1.htmlThe AD549 is right behind it, and it looks older and better package I think. With the metal can you can easily do the teflon standoff construction, it washes easily, etc. The TI chip is SMT.. I would prefer to work with a TH one

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2025, 08:17:43 am
content: One can bootstrap the supply to quite high voltages. It just gets a bit more effort for the BS part and the power consuption goes up. The simple form could still use 1 OP-amp (like the LTC6090 or OPA454) and a current source for bootstrapping. The OP-amp for the actual input can than be a 5 V type, though a bit higher voltage would allow higher slew rate at least for small steps.It is also possible to build the BS part from discrete transistors / FETs, but it can be a bit effort and supply current may be a bit higher.As the output signal is divided down ( like 1:10), the noise and drift of the input amplifier is not that critical. So the low bias amplifiers like LMC6081 would be OK for the input. I think the ADA4530 is a bit over the top.A slight difficulty with a bootstrapped amplifier is that they may may oscillate, especially with capacitive loading.For stability one usually needs to slow down the supply divers a little and have to find the right balance between stability (ringing) and slew rate limit (so that the BS supply can follow).One could start and test with a lower voltage (e.g. +-20 V supply) version.

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2025, 08:41:17 am
content: Is this a circuit that can work on a proto solder board? I have the best success with them and op-amps. I mean to test it for stability and stuff. i have the parts and alot of small teflon wires in different colors for connecting it ( i bought like 20 kinds)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2025, 09:10:31 am
content: One could use a proto board. No real need for teflon wire at the start.Some earlier discussion was here:https://www.eevblog.com/forum/projects/compensating-a-bootstrap-amplifier/though this is not for high voltage.It could also help to do a simulation, though this also needs some care: some of the OP-amp models may include a hidden ground reference.

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2025, 09:16:18 am
content: I don't even have normal wire and it pisses me off to solder it and have it burn when you are soldering it into holes and especially on prototype board you resolder it 10 times or hook something on it (finer wire) to solder it on to make a junction and if its PVC itt gets all messed up. And when I am done I desolder them, wick it, straiten it out with a wire straitener and have a box of tinned jumpers. If you take the time to straiten it and get some dividers it really works out nicely and the box gains valueI got like 100 feet of different solder braid for that K2001 project because I thought i was going to have to go through everything

author: David Hess
author_rank: Super Contributor
timestamp: May 16, 2025, 09:39:34 am
content: Quote from: coppercone2 on May 15, 2025, 09:27:26 pmThat will depend on your requirements. The OPA140 is excellent, but a CMOS input part will have lower input current noise, and potentially much lower input bias current.I have not tested any modern CMOS parts for this however. In the past I used the LMC6081 into the femtoamp range. Currently I am using a TLC2201 down to 10s of femotoamps or lower, but these are old parts.Something I want to try is trimming the input bias current of a bootstrapped CMOS input operational amplifier by changing its common mode voltage via the bootstrapping. Some datasheets show that this will work to zero the input bias current.Quote from: coppercone2 on May 16, 2025, 08:41:17 amWith care in the layout, sure. The sensitive non-inverting input likely needs to be air-wired for lowest leakage current.