Null voltmeter - Page 1
Forum section: electronics / general

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 28, 2016, 11:02:36 am
content: I'm starting a topic which might be interesting for volt nuts.I'm going to share some thoughts on null voltmeters as I own three units.For me there are a beautiful example how the engineers in the 60's managed to overcome the lack of high precision components.Repairs:In meters that use neon bulbs most common failure are the neon bulbsThey get metalized inside and their flash threshold voltage gets much higher. They start to flicker. The flickering can be seen off course however You should remember about the 'dark effect' in neon bulbs (dim the lights in the room while observing).Off course electrolytic capacitors and leaking batteries are a common issue.Fluke 845AB- difficult access to the components because of it's mechanical construction . Batteries inside can cause problems due to leaking electrolyte. Goes down to 1uV full scale.Fluke 845AR- easier to repair, no batteries inside. Goes down to 1uV full scale.For both Flukes, notice two 47nF capacitors between LO- Guard and between Guard- mains earth. If You connect directly a output of an OPAMP to LO terminal it is highly probable that it will start to oscillate at high frequency. I warned You!HP419A- very good design. Goes down to 3uV full scale. Seems to have lower noise than Fluke models. Watch out when disassembling and assembling because the screws have different lengths (about 3mm and 5mm ) and if You put the longer one in place where it shouldn't be you will have a short between LO terminal and mains earth (it will disturb floating measurements). The meter utilizes a Mercury battery as a floating voltage source, but You can use the meter without this feature as well.If the meter is operated without batteries, two Zener diodes should be soldered in place of the batteries to create a low impedance ground path.Keithley 155- I heard that it is a very good design. If there is any owner, please write an opinion.Let's also consider using modern good digital multimeters with the autozero function at 100mV range. What are the pros and cons?

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 28, 2016, 12:11:18 pm
content: With 1uV full scale you obtain about 2nV resolution.Makes me wonder, how cheaply can we obtain 2nV resolution with modern off the shelf equipment.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: February 28, 2016, 01:58:45 pm
content: The Keithley 155 is based on a solid-state chopper. Some parts are hard to track down. I have one functional and one non-functional Keithley. Another plus is that it can run from four 9-volt batteries. If you are in the market for one, try and find one with the external power pack. It allows the meter to run from mains. If not, an external ps will need to be attached so it is easier to replace the batteries as needed.Edwin Pettis pointed out to me that ESI used the HP and Keithley null meters in the 242's. The 801 came mostly with the HP but it appears on the last model (801C) it was replaced with the K155. It looks like Fluke's design may have been too big to fit.I also have the 845AR and 845AB. I recommend any model that will null after warming up. Any that are unstable most likely need new neon bulbs. A member of the volt-nuts group uploaded an LED mod for the 845 to replace the neons. It located on the ko4bb website .

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 28, 2016, 04:37:18 pm
content: QuoteNope. Usable resolution is something about 50nV due to the low frequency noise.

author: TiN
author_rank: Super Contributor
timestamp: February 28, 2016, 04:47:49 pm
content: I'm new to null-meters (was under false impression that K2002 or 3458A can replace nullmeter), but already have F845AB in the mail my way.Should be working, but we know how it goes.Reason why I hit that evil "bid" button, is the fact that nullmeter can run from battery, floating from rest of circuit, which none of long-scale DMMs are able to do. With proper setup, I'm hoping to very low leakages.And since I have Keithley 182-M with lowest 3mV range and battery powered front-end amp, would be interesting comparison.

author: pelule
author_rank: Frequent Contributor
timestamp: February 28, 2016, 05:46:04 pm
content: Fluke, HP/Agilent/Keysight amd Keithley stated in artikels, a modern DMM is able to replace a null-voltmeter.In many cases this is true - but not, if an ultra high impedance is required.Typical modern DMM have input impedance of >10G, but null-voltmeter (when correctly nulled) impedance is "infinite".I have two HP419A- The latest build one is fully operating (2 years ago I complete refurbised, replaced the accumulators, cleaned and made performance verification).- The oldest one is planned to replace the NEON-Chopper circuit with a moden chopper opamp circuitry.I use the HP419A mainly for measurements of isolation/GOhm resistances and for no-load measures of my standard-cells.BRPeLuLe

author: branadic
author_rank: Super Contributor
timestamp: February 28, 2016, 06:46:55 pm
content: There is this little circuit based on LTC1050 chopper as a DVM front-end available:http://www.tradeofic.com/Circuit/8273-NULL_DETECTOR.htmlI'm sure with LTC2057 something similar would be possible.

author: pelule
author_rank: Frequent Contributor
timestamp: February 28, 2016, 08:02:31 pm
content: Yes, that's a part of the circuit of the "mini-metrology-lab" from Conrad Hofmanhttp://conradhoffman.com/mini_metro_lab.htmlBRPeLuLe

author: acbern
author_rank: Frequent Contributor
timestamp: February 28, 2016, 09:36:47 pm
content: After trying many meters for use as null meters (keithely 155, 182, 2182, 2002, 34220, 3458A) I ended up with the Keithely 155 and the 3458A. They deliver (in my setups) the most stable results. The 3458A actully is my preferred one for ths purpose. Never tested the Flukes and 419, pretty old design and I heard not really reliable and also noisy (true or not).

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 10:24:12 pm
content: HiYou *can* use a ballistic galvanometer as a null instrument and get "infinite" leakage resistance to ground. By the same token, anything that is solely powered by 9V batteries and has only a + and - terminal to the outside world can do fine in that respect.Past that it becomes a question of what sort of calibration you need. The spot on the wall from the galvo will tell you high / low. It's not a real good way to come up with +3.25184 nV high. The galvo is only a zero current device at null, so not a big limit as normally used.Bob

author: Squantor
author_rank: Contributor
timestamp: March 01, 2016, 01:02:00 pm
content: What about the keithley 148/149?It is a nanovolt meter and uses cadmium based solder internally to keep the thermal EMF's low.It uses an interesting scheme to achieve its performance. It uses a vibrator to convert the DC into AC and feeds that into a shielded transformer. This transforms the impedance from Kohm to Megohm and feeds that to a low noise JFET. The rest resembles the chopped amplifier scheme used in the Fluke 845 and friends.I have the PM2434 and Fluke 845.The Fluke is being completely revised, all the resistors and electrolytics have been replaced, neons replaced by LED's and the Zeroing circuit is now based on 1.25V bandgap references. All those carbon composition resistors have all drifted high, varying from 10% to 30% sometimes. Quite a chore but hopefully worth it.

author: manganin
author_rank: Regular Contributor
timestamp: March 01, 2016, 08:42:25 pm
content: Quote from: Squantor on March 01, 2016, 01:02:00 pmFortunately there is no cadmium solder. The whole front end is made of pure copper with crimped connections.The performance of the 148 is quite impressive, even today. Especially the stability.I still have two of these in fine condition.

author: enut11
author_rank: Super Contributor
timestamp: April 02, 2016, 03:41:08 am
content: Quote from: doktor pyta on February 28, 2016, 11:02:36 amHiI am experimenting with a wheatstone bridge as a better way of calibrating a Hamon Divider. I am using 22K 0.2% low TC wirewound resistors and a 10v source and have tried the following methods of null detection:1) Yokogawa 7552 5.5 digit bench meter set to lowest DC range2) Conrad Hoffman's metrology chopper circuit feeding a 4.5 digit DMM set to 200mV3) Conrad Hoffman's metrology chopper circuit feeding an analogue 100uA moving coil meter4) Levell TM10 DC Microvoltmeter with a low range of 30uV.All seem to have limitations around the null point, mainly with widely varying readings when set to the most sensitive range.Maybe this is normal but I would appreciate feedback on your experience and possible solutions on how to achieve a more stable null detection.I might add that the bridge was built on veroboard and is currently not in a box. It is susceptible to "hand capacitance" near the null point.enut11

author: enut11
author_rank: Super Contributor
timestamp: April 04, 2016, 01:46:59 am
content: A bit more experimenting. I have solved the "hand capacitance" problem by rebuilding the bridge circuit.The original bridge was wired on veroboard using the parallel copper tracks.The new bridge was built on a board using "island" connectors and does not cause reading problems when I place my hand near the board.I can only assume that the copper tracks on the original bridge were responsible.enut11

author: lowimpedance
author_rank: Super Contributor
timestamp: April 05, 2016, 03:19:47 am
content: Quote from: manganin on March 01, 2016, 08:42:25 pmThe manual does mention Cadmium solder but this was for the DIY test cable/jig setup by the user and not for the instrument itself, and here is a pic of the front end goodness on a K 147 itself with the muMetal sheild removed for your veiwing pleasure. (salvaged 4 units that were destined for landfill a number of years back, had forgotten about them till this thread).The manual also states that no parts inside would need to be serviced/replaced except the mechanical chopper and the electrolytic caps !!.

author: zlymex
author_rank: Frequent Contributor
timestamp: April 05, 2016, 06:06:06 am
content: Good topic.There is this AVM-2000 ad advising to throw away 845A, 155 and 419A(I'm not related to this company in anyway)

author: mmagin
author_rank: Frequent Contributor
timestamp: April 20, 2016, 02:32:46 am
content: I got an HP 419A cheaply, and the 40 year old NiCd pack has leaked. I'm not so happy about how HP buried them between the case and inner shield. Besides the KOH electrolyte being corrosive to metals, it seems like certain plastics become brittle when exposed to it. Also it seems to get sticky, yuck.Anyway, I'm in the process of repairing it for fun, and most of the damaged stuff seems fairly straightforward (IEC connector, 115/230 volt switch, rear binding posts, dirty case pieces, maybe some trickery with the front binding posts), I'm left thinking about whether to replace the batteries (and if so, to maybe put them externally via some 3-pole non-shorting plug, I hate the original placement) or to just substitute a TLE2426 rail splitter for AC-only operation.I would also like to replace the 1.35V mercury cell (old one was dead) to restore it to full function. Any opinion on using a BR-2/3A lithium and a TPS709135DBVR (fixed 1.35V regulator) with appropriate MLCC caps (lower leakage than tantalum)? I know I could use an alkaline cell, but I'd prefer something with really long life (hence the BR lithium) since it's buried in there.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: April 20, 2016, 02:46:35 am
content: The Fluke 540B also used the mercury cell. Fluke eventually created a mod to do away with the mercury cell and replaced it with a lithium and a regulator pcb.Maybe this will be useful? You will need to tie a switch in to prevent the battery from discharging while it is not in use.http://www.ko4bb.com/manuals/97.76.44.130/Fluke%20540B%20Mercury%20Battery%20Rep.pdf

author: mmagin
author_rank: Frequent Contributor
timestamp: April 20, 2016, 03:07:14 am
content: Quote from: ManateeMafia on April 20, 2016, 02:46:35 amThanks.Yeah, the main reason for my approach is that the battery is 1.2 Ah and the quiescent current is around 1.2 uA at room temperature, meaning I could get away without a switchIt's only about 2% stability over temperature, but I'm not sure thats much worse than the garden variety mercury cell in practice.

author: amspire
author_rank: Super Contributor
timestamp: April 20, 2016, 03:15:53 am
content: I just had a look at your meter and I saw the battery is connected between polarity reversing switches, so you cannot easily separate the battery from the regulator unless you add an extra switch.Sounds like you plan is a good one.The battery will go flat in a million hours so you need to add a sticker to remind someone to change the battery in June in the year 2130.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 20, 2016, 04:08:48 am
content: Quote from: amspire on April 20, 2016, 03:15:53 amWhat got me thinking Panasonic BR series lithium is that the BR-series battery in my HP 3478A somehow lasted 30 years powering a static RAM. I'll be content if I get 10 years

author: sfox7076
author_rank: Newbie
timestamp: August 11, 2016, 03:06:47 am
content: Do any of you know anything about the Calibration Standards Corp. DC-100A? Cannot seem to find much info on it. There is one in this listing ( which has ended).http://m.ebay.com/itm/262522718669?_trksid=p2057872.m2749.l2649&_mwBanner=1

author: Vgkid
author_rank: Super Contributor
timestamp: August 11, 2016, 04:11:23 am
content: Looks like a differential voltmeter.

author: sfox7076
author_rank: Newbie
timestamp: August 11, 2016, 02:59:49 pm
content: You are correct. Sorry to sidetrack the thread.

author: VintageNut
author_rank: Frequent Contributor
timestamp: September 07, 2016, 02:25:05 am
content: With regard to the OP, I use a KE2000 as a null meter for measuring an Eppley Model 100 voltage cell.I have had the Eppley cell for about a year now. It required a week for the cell to stabilize. The cell degrades around 2 microvolts per month.The rig I use to measure the Eppley cell is a Fluke 731B, a General Resistance 7-digit Kelvin Varley Divider and the KE2000.I divide the 10V DC down to match the Eppley Model 100 cell. I measure the diff with the KE2000. I can see individual PPM changes when I adjust the KVD.The first stable measurement of the Eppley cell was 1.018757 on 7/26/2015. The latest measurement of the Eppley cell is 1.018731V on 6/11/2016. At the apparent degradation rate, the Eppley cell should not fall below 1.018V for another 30 years. At that point, I will probably not care about the Eppley cell.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 07, 2016, 05:29:49 am
content: The best range for Keith 155 is 1uV, 0.15uVp-p noise, 5 sec time constant, 1Meg input resistance.The best range for Fluke 845A is 1uV, 0.2uVp-p noise, 5 sec time constant, 1Meg input resistance.The best range for HP/AG 419A is 3uV, 0.3uVp-p noise, 3 sec time constant, 100k input resistance.Normally, the usable resolution cannot exceed the noise, so 0.15uV to 0.3uV is the limit for these null meters.If one DIY a simple null meter by chopper stablized opamps, the possible choice could be:- LTC2054, very low typ Ib at 1pA, but not so low noise at 0.6uVpp(0-1Hz)- LTC1052, very low typ Ib at 1pA, but not so low noise at 0.5uVpp(0-1Hz)- LTC2057, low typ Ib at 30pA, low noise at 0.2uVpp(0-10Hz)- ADA4528, not low typ Ib at 220pA, but very low noise at 0.1uVpp(0.01-10Hz)Obviously I'll rule out the first two because of the noise. Also the large Ib and current noise of ADA4528(10pApp) will limit the input resistance to 20k or so that I cannot tolerate neither.Therefore, I'll choose LTC2057 which has 1/4th of current noise density than ADA4528 allowing me to use 80k input resistor.AVM-2000 on the other hand is far more better(by spec) than all the above: 100nV best range, 0.4nVpp noise, 1M to 1G selectable input resistance.The use of Null meter for me could be:- compare two voltages, such as two cells(1.018V nominal), two 10Vs.- bridge balance detector.Another special use of null detector for me is high-ohm bridge where a very sensitive current is required. I'll use ADA4530-1.I've seen examples where ultra resolution is require such as transfer of JJVS where they use K2182A and EM N31(or alike) to measure the voltage difference.

author: TiN
author_rank: Super Contributor
timestamp: September 07, 2016, 05:51:14 am
content: There is also rainbow unicorn - preamp setup with Keithley 2001/2002 using variant of EM A10 (Keithley 1801).DCV Ranges of such setup are 20uV,200uV and 2mV. I'll be getting back tothis projectsoon.

author: guenthert
author_rank: Frequent Contributor
timestamp: September 08, 2016, 04:58:06 am
content: Quote from: zlymex on September 07, 2016, 05:29:49 amIs it? Why is that?Quote from: zlymex on September 07, 2016, 05:29:49 amUh, I don't quite follow. For many applications, e.g. buffering a reference voltage source to be used for a fast DAC, noise is an important limitation, but for a null meter? As long as it is random (and only then call we it 'noise', right?) it averages out, doesn't it?Quote from: zlymex on September 07, 2016, 05:29:49 amNow, I've never had the opportunity to use one myself, but afaiu, those are (the) typical uses of a null voltmeter. I'd think neither requires particular low noise in the amplifiers. Well, if one depends on the analogue (moving coil) meter, than I can see that excessive noise makes measurements difficult, but these days we connect an ADC and calculate the average, no?

author: zlymex
author_rank: Frequent Contributor
timestamp: September 08, 2016, 07:45:18 am
content: Quote from: guenthert on September 08, 2016, 04:58:06 amBecause it is a very low frequency noise of DC-10Hz, averaging takes long time say 10 seconds, and the 0.1Hz noise will interfere with it making the averaging even longer, and even lower frequency noise(0.01Hz, 1mHz, or lower) becomes troublesome.The good news is that there is virtually no 1/f noise for chopper stablized opamps, the noise density remains constant when frequency goes down to 0.1Hz, or even below 0.01Hz, making it a good choice to DIY a simple NULL meter.Two charts attached that can be regarded as the readout from oscilloscopes, one with 1/f noise and the other without.

author: d-smes
author_rank: Regular Contributor
timestamp: September 08, 2016, 10:57:59 am
content: Quote from: zlymex on September 08, 2016, 07:45:18 amThe upper chart of AD8676 looks to have about 100 nV pk-pk noise while the lower LT2057 has 200 nV pk-pk noise. Both have random "spikes" in voltage while only the upper plot appears to have shifts in the running average noise voltage (hard to tell with different vertical scale factors). Is that the 1/f noise characteristic to which you refer?Perhaps a related question- How can LT claim the noise voltage plot is representative of a DC - 10 Hz bandwidth with just a 10 second recording?

author: zlymex
author_rank: Frequent Contributor
timestamp: September 08, 2016, 04:43:08 pm
content: Quote from: d-smes on September 08, 2016, 10:57:59 amBroadly speaking, yes. It's getting late here so I'll say some conclusions first and will explain why tomorrow.The chart above for AD8676 is the noise for 0.1Hz to 10Hz bandwidth. If we modify the the bandwidth as 1mHz to 0.1Hz, or modify the the bandwidth as 0.01mHz to 1mHz, the chart will be the same(similar shape, same 100nVp-p). This can go on forever and theoretically these noise(of limitless bandwidth) will be added up and there is no boundary, and you cannot claim anything for bandwidth of DC-10Hz, that is the characteristic of 1/f noise.However, for LTC2057, Vp-p will be dropped(to 1/100th or 1/10th?) if bandwidth is 0.001Hz to 0.1Hz, and continue to drop for lower frequencies bandwidth.For the second question, for an opamp without 1/f noise, the Vp-p contribution from low frequency has a limit/boundary, the Vp-p noise for 0.1Hz-10Hz is not much smaller than 0.001Hz-10Hz which virtually the same as DC-10Hz.Edit: From wiki:https://en.wikipedia.org/wiki/Pink_noise'There is equal energy in all octaves (or similar log bundles) of frequency.'In another word, all frequency bandwidths of below carry equal energy:1-10Hz, 0.1-1Hz, 0.01-0.1Hz, 0.1m-0.01Hz, 0.01mHz-0.1mHz,,,,,, 0.01uHz-0.1uHz,,,,,Therefore, in theory, there is infinite amount of energy in DC-10Hz spectrum for 1/f noise source.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:33:20 am
content: I found out a chart that I recorded before which can get an intuitive view of the LF noise of one of my Fluke 845AB.

author: uncle_bob
author_rank: Unknown
timestamp: September 12, 2016, 01:25:16 pm
content: Quote from: zlymex on September 08, 2016, 04:43:08 pm..... but only if you have an infinite amount of time to watch the power meter ....One would guess that someday there will be a paper that changes the way we look at 1/F noise and "infinite energy". In a bit over a century of study, it holds up so far.Bob

author: zlymex
author_rank: Frequent Contributor
timestamp: September 12, 2016, 01:51:58 pm
content: Quote from: uncle_bob on September 12, 2016, 01:25:16 pmTrue. That is just another way of saying: averaging is no good for lowering 1/f noise no matter how long you are trying.

author: uncle_bob
author_rank: Unknown
timestamp: September 12, 2016, 02:02:56 pm
content: Quote from: zlymex on September 12, 2016, 01:51:58 pmHiWhich brings us right back to the start of this ... why 1/f noise is an issue you *do* need to worry about. Chopper front end ADC's are indeed a good idea to the extent they can suppress some of the 1/f noise issues.Bob

author: zlymex
author_rank: Frequent Contributor
timestamp: September 12, 2016, 02:13:43 pm
content: Some null meters(of existing and DIY) do show worrying 1/f noise.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 14, 2016, 02:06:18 pm
content: I've got a couple HP 419 meters and a Fluke 845. The 419s are good, but mine both need service. The 845 is my favorite, but watch out for leakage to ground. I think there's a leakage test in the manual involving high voltage. When I did it, a hard to reach piece of coax behind the front panel flashed over. I fixed it, but the leakage is still just out of spec. The mechanical design of that thing is, IMO, just plain stupid when it comes to service. I replaced my batteries with a big cap and zener diode, as they seemed to go bad on a regular basis. The null meter from the original MML articles worked well, but I just vastly prefer a center zero mechanical meter in this application. A couple of my bridges use eye tubes and as antiquated as they are, they make great null meters. Analog motion, but zero friction!

author: guenthert
author_rank: Frequent Contributor
timestamp: September 15, 2016, 06:12:06 am
content: Quote from: Conrad Hoffman on September 14, 2016, 02:06:18 pmI've seen magic eye tubes on radios (utterly mesmerizing), but never on test gear. You wouldn't happen to have a picture? Null meters benefit from battery operation, I'd think. How did they feed the heater current for those tubes? And don't use magic eye tubes a gate voltage in the order of ten volts? How was the signal so far amplified?

author: Squantor
author_rank: Contributor
timestamp: September 15, 2016, 07:25:28 am
content: Quote from: guenthert on September 15, 2016, 06:12:06 amI have a Wayne Kerr bridge that uses a magic eye as for nulling the bridge and the Philips GM6020 used two indicator lamps to show the null. Philips used a second moving coil meter as polarity indicator on many of their multimeters that you also can use as the Null indicator.Quote from: Conrad Hoffman on September 14, 2016, 02:06:18 pmAh that is a good reason to use those, I always wondered why they used a magic eye on the older bridges or millivoltmeters.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 15, 2016, 11:24:34 pm
content: Here you go- pictures of a GR bridge using the eye tube:http://conradhoffman.com/GR1611A.htm

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2016, 12:10:00 am
content: There are mechanical meters without a classical bearing and thus (static) friction to. So friction itself is not the problem with these, but response is slow and tends to show overshoot. You usually need to add dynamic friction to make is useful.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 16, 2019, 09:39:24 pm
content: Sorry for unearthing this thread, but maybe we can discuss the following question here:I often read the claim that a Nullmeter like K155 or Fluke 845AB have the property of an infinite input resistance when Udiff=0 at its input-posts.Since the datasheet of both devices mention input resistances of 1Megohm for the K155 and 10Megohm for the F845AB and one can see those resistors across input-posts in the schematic that claim is false.I understand that if Udiff=0 across say a wheatstone-bridge then theres no current flowing at all through the nullmeter and therefore it appears to have infinite impedance or as if it wasnt in the circuit at all, but thats no intrinsic property of the nullmeter at all.Therefore it should be possible to use any suitably high resolution, low noise and low bias current DMM as a nullmeter, as long as the bias-current from the meter doesnt introduce new errors due to high bridge-resistance (DMM-Mains-connected-common-mode-errors left aside here).Fluke mentions this in an appnote, but also states that Nullmeter-input-resistance is infinite at null, which i dont understand. ->http://assets.fluke.com/download/calibration/seminars/replacinganalognulldetectingmeters.pdfPage 30Can anyone point to my error in thought or is the appnote wrong in claiming "Input resistance infinite at null"?https://doc.xdevs.com/doc/Keithley/155/29031D%28Model55%29.pdfK155 Manualhttp://ftb.ko4bb.com/manuals/89.12.168.225/Keithley_155_Schematics.pdfhttps://xdevs.com/doc/Fluke/845AB/fluke_845a_ab_sm.pdfPage 48

author: TiN
author_rank: Super Contributor
timestamp: March 16, 2019, 10:00:55 pm
content: Perhaps that's due to no current flowing into floated nullmeter when there is no potential difference between the inputs = zero current gives infinite calculated resistance. That does not account for leakages, obviously.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 16, 2019, 10:20:31 pm
content: Interestingly the HP419A actually claims the infinite input impedance is achieved at null, because it uses a special battery-driven supply to completely compensate input current.Since K155 und Fluke 845AB dont claim (or i didnt see it in the manuals) to achieve infinite impedance at null and its visible that theres always an input divider at the binding posts.Ill assume the HP 419A (and maybe other types) is the basis for the claim that Nullmeters always have intrinsic infinite input impedance when nulled.http://bama.edebris.com/download/hp/419a/HP%20419A%20DC%20Null%20Voltmeter.pdfHP419A manual, Page 6 specs

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2019, 10:34:45 pm
content: The HP419 uses a LDR type optical chopper. So it is not even really high impedance, though possible a relatively low bias.Part of the confusion is due to different definitions for the input resistance. One can use a differential resistance (dU/dI) and an absolute resistance U/I that can depend on where to have the zero point of the voltage.The infinite resistance might be seen to describe zero bias or no disturbance. This is more something from the times of passive analog meters.The differential resistance of the null-meter is still the same, no mater what voltage is measured.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 17, 2019, 03:53:29 am
content: Quote from: Echo88 on March 16, 2019, 09:39:24 pmNo need. It's my favorite topic, I got a little obsessed with these devices.Quote from: Echo88 on March 16, 2019, 09:39:24 pmThat's bit sloppy terminology or marketing-speak. What is meant is that, when null'ed, there is no loading of the source, i.e. no input current into the null-voltmeter. Of course, the input current isn't exactly 0, but very, very small (low pA or high fA range, compare with the input current in the high pA range of modern long scale DMMs -- and that's average; their "pump out" current is pulsed and peaks in the nA range). For old galvanometers it's obvious why there was no current at the null point (if I'm not mistaken, mirror based galvanometers had a sensitivity in the low nA range). All modern null meter use however an amplifier and hence there's always some small input current (and for choppers this nasty pump-out current). One can either put a lot of effort into minimizing such or - as HP and others did - add a burden voltage generator [bucking voltage is the proper term]. That burden  voltage is manually adjusted with the input shorted so, that the current across the input resistor matches and opposes the input current of the amplifier -> presto, no load of the DUT.Away from null (|Udiff| > 0) the input current would be |Udiff/Ri| > 0, but since very small voltages are typically measured with a null voltmeter, only small currents flow. Of course, the smaller the input resistance, the greater the error. Unfortunately one cannot make the input resistance arbitrary large, as it is a noise source, which ultimately limits the sensitivity of the meter.No (or very little) loading used to be important when using standard cells, as they had a fairly high output resistance and held only a small charge. It's still of interest when using a bridge to compare very high value resistors or when using a voltage divider (including the still current Fluke 752A) to determine a voltage with high precision ("potentiometer method").

author: guenthert
author_rank: Frequent Contributor
timestamp: March 17, 2019, 04:04:46 am
content: Quote from: Echo88 on March 16, 2019, 09:39:24 pmOne essential feature of null-meters is the very high (iirc Fluke claimed 1TOhm for the 845A) resistance to ground. That's at least an order of magnitude higher than any long scale DMMs I'm aware of (there are digital electrometers with high impedance to ground, but they are typically not very sensitive) and could be of importance when measuring (or comparing) sources of high potential above ground with large output resistance, e.g. a bridge for large value resistors.

author: mikron
author_rank: Contributor
timestamp: March 17, 2019, 06:44:06 am
content: Quote from: TiN on March 16, 2019, 10:00:55 pmFollowing this kind of logic even the resistance of a 10 Ohm resistor rises to infinity whenever it is out of circuit.

author: jfphp
author_rank: Regular Contributor
timestamp: March 17, 2019, 10:07:40 am
content: The AVM 2000 is the only modern null voltmeter but it is very sensitive to static discharge and cannot withstand 50V like the 419A on the most sensitive ranges. An interesting approach would be to copy the very simple schematic diagram of the null meter incorporated into the Valhalla 2720GS (a brilliant but largely unknown 8 1/2 digit DC calibrator).I guess, I have seen on the net an upgrade of the HP 419A with a solid state chopper : any idea ?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2019, 10:19:38 am
content: Quote from: guenthert on March 17, 2019, 03:53:29 amThe input resistance of an amplifier is not directly producing noise. Good amplifiers have a noise level that is lower than the noise of a resistor as high as the input impedance. However there is still a relation that high input impedance amplifiers have high noise. Otherwise one could just add more in parallel and reduce the noise without a negative effect (except cost and power).A different point is the resistance used in the protection at the input - this adds to the noise. At the very lowest noise level this can be a reason not to have so much protection.No all modern null meters use an amplifier at the input. The most sensitive ones are superconducting ones that measure the current (down to the sub pA level) through a superconducting coil from it's magnetic field. These are thus null meters with a crazy low input impedance - just a few µHs and 0 Ohms, and they are crazy sensitive, like nV full scale and down well below pV.So there is likely not just one ideal nullmeter that fits all tasks - it is still a balance between low bias or low noise.

author: ap
author_rank: Frequent Contributor
timestamp: March 17, 2019, 11:34:44 am
content: The AVM2000 seems to be discontinued. The 3458A, at least with high impedance sources such as the Fluke 752A, is not a solution, I did check with one of my 3458As and the bias current is just too high. Would be interesting to see how a 8508A behaves. So seems the only items still available are the EM Electronics one. Other than that, older meters such as the K155.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 17, 2019, 12:05:38 pm
content: https://doc.xdevs.com/docs/Valhalla/2720GS/2720GS%20VOL.%202%20SCHEMATICS.pdfPage22 Nullmeter@jfphp: Would be a pleasure if you could share more infos/pictures about your Valhalla 2720GS, since the forum and generally the internet is lacking that kind of info of this very interesting device. AFAIK only the user Kosmic has another 2720GS.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 17, 2019, 12:39:14 pm
content: I have been using the Keithely 155 Null Detector for some time now and it gives repeatedly good results.It is still on the first et of new batteries, after I repaired it here:https://www.eevblog.com/forum/repair/keithley-155-null-detector-repair-and-restauration-attempt/

author: Kosmic
author_rank: Super Contributor
timestamp: March 17, 2019, 12:54:41 pm
content: Quote from: Echo88 on March 17, 2019, 12:05:38 pmWas about to start a thread about my repair progress. With full disassembly picturesShould be able to find some time next week to post something.I also have a Fluke 845ab in the repair queue, so I might be able to do a comparison at some point.

author: Echo88
author_rank: Frequent Contributor
timestamp: March 17, 2019, 02:44:17 pm
content: I was indeed thinking about asking you for potential teardown-photos and infos Kosmic.Such a thread about the Valhalla 2720GS would be much appreciated by all members here i think.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2019, 03:03:26 pm
content: The Nullmeter part from the Valhalla looks really simple: Mainly just an AZ OP with switchable gain. So this one could have a bias current in the 3-10 pA range, so maybe a little better than a good DMM, but not much.A null-meter of that class could be a relatively simple DIY project, maybe even dead bug or on a raster board (The LTC1052/ICL7652 is available in DIP). As a very small circuit, battery powered operation is very practical. I would even consider adding bias current compensation, so one should be able to manually adjust the bias to the low/sub pA range.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 17, 2019, 03:27:05 pm
content: There was some discussion in the 'TEA' group about the HP 419A starting around post #25975. I posted this with a couple of photos of my replacement batteries in post #25993."Like beanflying in post #25975, I had to do a lot of cleaning and replace the leaky and dead battery packs in my HP 419A. I replaced the original batteries with packs that were designed as phone replacement batteries and they fit with little difficulty. The new batteries have been in the HP 419A for a couple of years on what was fast charge for the original smaller batteries but is trickle charge on the new larger ones. "

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 17, 2019, 05:39:22 pm
content: I've got a couple 419s and it's probably time for new battery packs again. One is a bit older than the other and I remember it having some mysterious issue that turned out to be corrosion on the front panel where the input jacks go through. It was causing leakage. Removed the jacks, cleaned everything up, and no more problem. They're really quite a good meter with good ergonomics.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 17, 2019, 07:07:47 pm
content: Quote from: Kleinstein on March 17, 2019, 03:03:26 pmLikehttps://drive.google.com/open?id=1yzzUKX1S4_AR7HiQNTvz-Z_wjBTFUkpphttps://drive.google.com/open?id=1NALNYXngEHq8iBmX4gUHewEG81J5gXoT?(Well, yes, I'm not paid for my handiwork -- this is just to show that anyone can build a null meter

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 17, 2019, 09:35:34 pm
content: Quote from: Echo88 on March 16, 2019, 09:39:24 pmHi!We had exactly this discussion 2..3 years ago.My criticism about the 845a was, that its bias current was NOT specified, instead the false expression of 'infinite' input resistance @ very small input voltages was chosen.It's obvious, that <1uV over 1MOhm creates <1pA, which gives virtually very high impedance, when comparing 10V sources, for example. 10V/1pA gives 1013Ohm, zero difference infinite resistance, virtually.A really low bias current is the important feature, and another owner really measured that bias of the 845a to be lower than 30fA, due to galvanic separation by this optical input chopper.As this had been proven, I also bought an 845A, because my 3458A had a few pA of bias, really influencing the trimming/calibration of instruments like the 752A, or the 5450A, or any other instrument relying on the Wheatstone Bridge alignment method.Therefore the Fluke app note that long range DMM can replace these old null voltmeter, is simply wrong. Even the 8508A has pA of bias. The trimming accuracy of these old bridges might be just sufficient using a DMM, but it's really no fun, as you really notice the limitations by their bias currents.I could also test my 845A for fA bias AND leakage to case, so the alignment of my DIY Hammon divider now is much better and easier than using the 3458A .The hp419 might also have low bias, due to optical chopper, but that has to be measured.I have no idea about the other instruments.It's clear, that all the chopper ICs have much too high bias, also. Mostly, low bias currents and low noise are contrary requirements.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 17, 2019, 10:04:33 pm
content: Does anyone looked into the input current over time on the 8508A?Perhaps, it is much better than the 3458A, because it is not doing auto zero. But if one measures zero there shouldn't be much charge due to auto zero.

author: 1audio
author_rank: Frequent Contributor
timestamp: March 18, 2019, 12:40:40 am
content: How do you measure the input bias current of a meter when its that low? You need to separate the bias from leakage current which may be difficult. Would it me meaningful to measure the voltage across an open input?Traditionally a sensitive galvanometer would be used as a null detector. Internal resistance in the 10's of ohms BUT when the voltage on both sides is the same no current and effectively infinite impedance. Switching to an electronic indicator should be easy if you can keep that really high leakage requirement. However a null detector is all about the null. Once you go off null the infinite is no longer infinite.I have been playing with one of these:https://quantasylum.com/products/qa350-microvolt-dc-volt-meteras a nullmeter. I checked its isolation between the USB (its power source and display host) and the circuit when I got it on my HP 4329 and I got something like 5 X 10E14 Ohms, essentially the limits of the instrument at 100V. I did not want to go further then.Pluses- 24 bit at 5V FS or 50V FS. Seems pretty stable. Running off of a PC is nice if you have enough isolation. It seems the resistive isolation is really high and the capacitive is in the pF. Software is all open source so you are free to hack away.Minuses- Input is a BNC connector (not good when you consider thermals( I added a pair of banana jacks). Input impedance is not that high at 1 MegOhm. Software does have a bug in the logging, it locks up at about 8000 records. Software's slowest logging is 2.5/second so a long session may have a lot of records.Odd- is that its converting the binary output of the ADC to decimal so at the lowest steps they are a little odd with skipped decimal steps. The software display is very similar to a DVM and it graphs so you can get an immediate display of fluctuations. Attached is a plot measuring my 732A. I believe most of what is shown is internal noise on the 732A.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2019, 01:21:53 am
content: Quote from: ArthurDent on March 17, 2019, 03:27:05 pmActually the 419A is back on the bench this morning fitting in a floating 1.35V Mercury Battery Replacement board, the main packs have already been done. Also in the list for refurb is a Fluke 895A and also the HP 740B and sort out the Null meter section on my 335D.I don't have a Null meter problem, I don't have a Null Meter problem .....

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 18, 2019, 08:19:05 am
content: Quote from: guenthert on March 17, 2019, 07:07:47 pmCan you show your scheme?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2019, 05:24:21 pm
content: Quote from: 1audio on March 18, 2019, 12:40:40 amFor measuring the bias / input current there are 2 common methods: one is measuring the zero voltage with 2 different resistors at the input, usually a short (or near short) and something like 10 M or 1 M. If the meters own input impedance is relatively low (e.g. 10 M) one can also use the open circuit voltage.The other method is using a small, low DA capacitor (e.g. 1 nF or whatever is suitable for the current range) and measure the voltage drift rate. With a 1 nF cap 1 pA of current give about 1 mV/second. As one usually only need the rough size of the current even a manual reading is often suitable. Not so relevant to a null meter, but with a DMM this method also allows to get the input current at a different voltage.

author: Bill158
author_rank: Regular Contributor
timestamp: March 18, 2019, 06:58:10 pm
content: Quote from: 1audio on March 18, 2019, 12:40:40 amThere is an interesting App note from Fluke about this subject of correcting for input bias current when using a DVM/DMM for a null meter. See attachment.The F845A takes care of it's input bias current by having you turn the "OPR/ZERO" switch to "ZERO" before making a measurement like balancing the bridge adjustments in the F752A or the F720A. If you look at the schematic of the F845A you can see that the 1 meg resistor is in the circuit so doing a zero null takes out any offsets from the measurement when in OPR.I have used the method in the App note and have made excellent adjustments to the F752A and then compared the adjustments using a F845A. I use a HP 3458A. I find that the 3458A has around 8 pa of input bias current, at least mine does. I guess the biggest issue here is that your DVM/DMM has an extremely high isolation resistance from both the high and low inputs to either the guard or ground depending upon how the guard is connected.Bill

author: guenthert
author_rank: Frequent Contributor
timestamp: March 19, 2019, 04:05:07 am
content: Quote from: MegaVolt on March 18, 2019, 08:19:05 amUh, oh. I was thinking to put it in a project page (it's rather a beginner's project than belonging in the metrology section) or add it to my new project: the Null Meter, Mark II (now digitalIt's in Dave CAD format and I really need to look at the device to verify it, but unfortunately, it's in a storage facility half way around the globe.There isn't much to it though. I used a LF411 (not the best choice, but what I had laying around) as trans-conductance amplifier driving the meter for the mV range. Making sure the MCM wasn't over-driven while simultaneously minimizing gain error was one of the greater challenges (one of the requirements was a 2V range, as this null meter ought to do double duty display for my Keithley 617 analog output -- that turned out to be a questionable idea, as it made input protection considerably more difficult). For the uV range I used good-ol LTC1050 (I wanted to use a OP27 with a discrete matched FET pair as input buffer, but I couldn't get my hands on a reasonably priced one) as fix gain (1000x) pre-amplifier. For the burden  voltage generator I used a single AAA cell and had a good look at the schematics of the HP419A.Noise I did combat with heavy handed low pass filtering. I went back and forth changing the capacitors as the first version was overdampened (response time around 13s to 90%) taxing my patience. Overall the performance is just so-so (1 and 2uV ranges are unusable, 4uV is ok-ish, but the LF411 drifts too much; resistance input to ground about 250GOhm as it is battery powered and unlike commercial products provides no output), but was a great learning experience. It's really something to do to understand what not to do next time

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2019, 10:40:09 pm
content: There are different null meters. Some have lower voltage noise and other have lower bias / current noise. Which type of amplifier is best, depends on the signal source. For a simple, low cost version AZ OPs can be an option. One would still want some input filtering against spikes and EMI.Besides the AZ OPs just listed, I would consider the AD8628 as an intermediate between LTC2057 and LTC1052.The higher bias OPs may want a compensation for the bias current. However this does not help against current noise - so the use with high impedance sources is still limited.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 15, 2019, 10:51:51 pm
content: Quote from: 3roomlab on December 15, 2019, 10:29:36 pmWell, it might be the wrong way to look at things. When comparing the potential of two voltage sources with low output resistance, it is not needed and you might as well use any DMM or use a long scale DMM to measure those voltages directly. If however, you want to compare two voltage sources of which at least one has a 'high' output resistance, then the input bias current of your measurement instrument might make a significant difference.One of the remaining use cases of a traditional null meter is the calibration of Fluke's reference divider (752A). I believe its output resistance is about 40kOhm. An input bias of e.g. 10pA (not untypical for a long scale DMM) yields then an error of 0.4uV, while the calibration procedure requires a null detector which can determine balance to within (i.e. the total error required to be less than) +/-0.5uV ...

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2019, 10:03:41 am
content: One of the points where a null meter can shine is using battery power and thus essentially avoid coupling to mains ground. So the null meter can be really floating. Otherwise a null meter is more like a sensitive voltmeter (e.g. 100 nV range resolution), that does not need high accuracy or high speed. It is often more about having extra filtering / low speed to reduce the noise.If the signal source are low impedance (like 2 voltage references) one could as well use a normal mains powered DMM with sufficient resolution.If the signal source is high impedance (like a relatively high value divider) a classical mains powered DMM could be a problem and a low input current is needed.So the main need would be more with low input bias (e.g. < 10 pA, maybe less with adjustment). A higher input bias and lower noise version would be more like nV meter - still useful in some cases. When using AZ OPs the circuit could be essentially the same - just swap the input OP.Compared with a normal voltmeter one could save on having less protection, no need for a super stable reference and less gain accuracy. So it could be a relatively simple DIY project: protection + input amplifier + ADC chip + µC + LCD powered from batteries. An ADC with a little more resolution than the old analog scales could simplify gain switching as less gain settings are needed. So 1:10 steps should be ok - no more need for old days 1:3:10. With low voltages mechanical switches can become tricky and it could be easier to use CMOS switches to choose the gain. So at least for the low end part without a divider one could have some auto-range function.For low leakage one could consider dead bug style for the input part - so one may not even need a PCB.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: December 16, 2019, 10:40:26 am
content: The ADS1263 has an integrated chopper amplifier, PGA, input impedance of 1 GΩ and 8 nV RMS noise.You can play by purchasing these boards:http://www.yoctopuce.com/EN/products/usb-electrical-sensors/yocto-bridgeorhttp://www.yoctopuce.com/EN/products/usb-electrical-sensors/yocto-maxibridge

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2019, 06:35:24 pm
content: Quite some SD ADCs have zero drift inputs. However they usually have relatively high input current. Especially the ADC1263 is more like a high end ADC and likely needs a lot of attention to the board to get the full performance.I think it would be better to have a separate amplifier like the AD8628 ( about twice the noise of the LTC2057, but more like 1/10 the input current). So one could chose between low bias (e.g. max4238) and low noise (AD4528) or intermediates.For the ADC I would more think about a low end SD converter like MCP3421 or a slightly faster (and external clock to work in sync with the chopper) version if a discrete chopper is used.At least in theory a discrete chopper could reach a better noise / bias combination than the usual AZ OPs. However the discrete chopper would need more power and more circuit.

author: iMo
author_rank: Super Contributor
timestamp: December 16, 2019, 06:59:42 pm
content: Why do you need an ADC and an MCU and a display? With 1-2 opamps and 9V battery you may build a null voltmeter in an hour, the output could be almost any DMM, imho.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2019, 08:27:29 pm
content: Many of the cheaper DMMs sample the input only for a small part of the time and thus have a higher than needed input bandwidth. This would require a relatively slow filter between the amplifier and DMM to avoid extra noise. So amplifier + DMM is still possible. Instead of a full DMM one could also consider a small LCD type panel meter. The ADC+ µC/LCD part would be a little like a more flexible panel meter version and one can choose an ADC that does more continuous reading. Digital filtering in the µC can be of FIR type and thus get faster settling than analog filtering that normally is IIR type. Depending on how well one knows the µC, the effort for the ADC+µC is not that large.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 17, 2019, 01:50:59 am
content: There are many ways to skin the cat, but building a low noise, low drift, low offset amplifier isn't exactly trivial (which I only realized after I tried ;-)Taking apart a HP 419A is quite revealing. Despite its age (early 60s?) and compared to modern units like the AVM 2000 mere modest performance (p2p noise <.3uV, 100kOhm input resistance, unspecified input current), it is quite a sight to behold. The robustness, attention to detail and effort (gold (!) traces on the PCBs!) is quite impressive. Best of all, they were once fairly popular, but are long obsolete and with a little luck can be acquired for reasonable prizes on the secondary market (unlike the competing, a little younger, slightly better performing products from Fluke and Keithley). I was once of the opinion that everyone could build a Null meter (I mean, how difficult can that be?) -- after attempting it myself with rather mediocre results and seeing the HP 419A, I have now more respect for the subject.Someday I might attempt again to build one. Then I probably go the MCU with built-in ADC and external chopper route. That seems promising and comparatively cheap.

author: notfaded1
author_rank: Frequent Contributor
timestamp: December 17, 2019, 03:08:21 pm
content: I'm kinda giving up on my Fluke 825AR... I replaced the reference tube because I couldn't do the setup calibration process and the user guide said to replace the reference tube (which I did and it didn't help)... so I decided to buy an 845AR instead. The 845AR is in great shape but on lower ranges it doesn't stick on a reading and fluctuates back and forth the needle. I found some of the original neon tube types used in the 845AR the NE-2U neon tubes. I'm really into neon and finding the type isn't particularly easy because there are many types of NE-2 tubes and unless stated well. I'm not sure if the different NE-2 are interchangeable in the Fluke 845 really?I also in the meantime scored a dekavider RV622A with a rack built in 10V source including an additional divide by 100 passthrough. I was amazed because last night I hooked it up to my 34401A on 6 digits with the slower 100PLC. The divider was spot on all the lowest ranges including passed through the additional divide by 100. Only the largest first decade was off by a mV on a few numbers. I could dial up exactly on all but the first knob (still only off by 1 mV) any number on the 34401A. Attached some of the original pics... I'll take some of my own later and post or maybe make a video. I'm tempted to take apart the 10V source and see how it's built inside. I wasn't really expecting the 10V source to be so good and also the dekavider. When 3 or 4 of my voltage sources including my Fluke 343A all agree it gives me some confidence. The 343A is pretty spot on too except the 300V and I think either the 600 or 900 range are off by just a small amount. All the other decades on the 343A are spot on. I wonder if there's some relationship between the 300 and the 600 or 900 range? I mean where 100, 200, 400, 500, 700, and 800 are all fine. I'll do the test again and post the results... maybe someone here has an idea why only a couple high ranges are off just a little. It would be nice if it was just a cap that could be replaced. I do have a DE-5000 and was planning on testing all the caps but since it's already almost exact on 95%+ of the ranges I'm a little hesitant to just start tearing the 343A apart.Bill

author: Echo88
author_rank: Frequent Contributor
timestamp: December 17, 2019, 03:47:28 pm
content: Regarding the F845AR please read the following:https://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-(and-repairs)/msg2595273/#msg2595273(unless you already did...)

author: Echo88
author_rank: Frequent Contributor
timestamp: December 30, 2022, 02:13:42 pm
content: Due to the inherent difficulty of building a 8.5 digit DMM lets start with something more approachableAttached is:-my take on a modern nullmeter as general schematic with lots of remarks-a list of all (shown on respective manufacturer-website) single Autozero-OPs made by Analog, TI and some examples from Renesas/Microchip/Onsemi and two good JFET-OPs for comparison. The list isnt complete, may contain errors and unreasonable current noise datasheet values are colormarked. I used the current noise measurement results from user chuckb for 3 OPs and commented them in the current noise column.Inspirations did come from K155/F845AB/Datron 1045. I still need to read the AVM2000-document and see how they did it.In my case i wanted to go with a pure analog and battery powered design, avoiding mains-powering and any µCs and DCDC-converters so far.Want a graph or digital display? Plug the recorder output into your DMM6500/HPAK34465A or any other DMM/logging PC-interface.All based on the idea that a AZ-OP or a JFET-OP should be suitable as a modern replacement for the nice discrete chopper-implementations of yesteryear that is...Maybe someone already did current noise measurements on AZ-OPs apart from chuckb and wants to comment?Debunking datasheets in this regard is a nice sport.Hope for some nice discussions and hints for schematicerrors.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2022, 03:03:51 pm
content: I have looked a little at the current noise of AZ OP -amps.I got a crude measurement for the MCP6V51: around 200-250 fA/sqrt(Hz). This sounds reasonable: a little worse than the comparable, but more expensive LTC2057.I also had a look at the AD8628: the datasheet value is a bit optimistic, but the noise is still low (have to look up the old data).For the bias current there some effect of the input impedance / input capacitance on the input currents. With balanced capacitance one has a good chance to get better than the listed typical calues.The bias values from Microchip are using Bias = average input current and this way get to optimistic. One would need to use 1/2 offset + bias as a comparable value.The JFET OP-amps are not really a reasonable alternative, if there is no extra automatic/reasonable fast Zero function.The usual photovoltaic couplers provide very little power. This could be only enough for a super low power OP-amp.Probably better to use a small 2nd battery (e.g. 2 or 3 x AA) if needed for the analog output.As a problem the analog coupler is unipolar only - so not really suitable for a signal around zero. So one would need something else there.For a nullmeter with already a divider for most of the ranges I see little need for a higher supply and thus the bootstrapping. With battery supply it helps to keep the power consumption low.It is more that one may want a choice of the amplifier: low bias or low noise as separate versions.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 30, 2022, 03:49:12 pm
content: Yeah, the LMP2021-datasheet page 18 touches on the Cin vs. Ib-subject, i worry that different capacitive inputs would destroy the Ib-compensation-approach.Just need to add switchable input compensation capacitors and autozero/autobias-compensation-switches...I wanted to omit autozero/autobias-switches at least to get a continuous measurement device, where there are no selfcal-pauses during measurements if possible.The IL300 can be configured for bipolar output with a current offset, yet will still have relatively high current consumption.The suggested MAX409A has low enough quiescent current as a recorder buffer, but there might be better OP or ways.Using 2x AA-batteries is pragmatic and long lasting, might be the easiest way indeed.The bootstrapping is mostly used to keep the common mode stable and therefore hopefully the Ib (as done in the Datron 1045), but with varying Ib vs. Cin that is debatable.I assumed temp-compensated (e.g. with a thermally attached compensation transistor) JFET-OPs should also be usable, but i guess the inherent 1/f-noise would necessitate a periodical autozero-cycle?

author: Echo88
author_rank: Frequent Contributor
timestamp: December 30, 2022, 04:09:19 pm
content: Would about a chopped OPA140/matched JFET-difference amp?http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdfpage 8https://www.analog.com/media/en/technical-documentation/application-notes/an93f.pdfpage 10 i would have expected less than his stated 500pA...Also id reduce the voltage ranges to:1kV, 100V, 10V, 1V, 100mV, 10mV, 1mV, 100µV, 30µV, 10µV, 3µV, 1µV...12 instead of 19 ranges, with smaller steps for the interesting voltages.That increases the number of suitable rotary switches (whom also need to keep up with 1kV, meh)Edit: Trying to make sense of the AVM2000-input section feels like im having a stroke, geez.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjhysXu4KH8AhUBKewKHRFWA1AQFnoECAsQAQ&url=https%3A%2F%2Fwww.eevblog.com%2Fforum%2Fmetrology%2Fwhat-makes-the-avm-2000-nullmeter-have-such-low-input-noise%2F%3Faction%3Ddlattach%3Battach%3D1024710&usg=AOvVaw2omcFQs96N4PyXlkHzRPGO

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2022, 05:21:34 pm
content: For the ranges I would consider spliting them to 2 switches. One for the coarse 1:10:100:... part and than a seprate 1:3 or even 1:2:5 switch later on. With digital read out one would not need the 1:3 steps this is more something for an analog meter movement.In the plan the capacitance at the input is quite large. This can cause a slow response if the source impedance is high. For a response that does not depend on the source impedance one would have less filtering at the input and than a later filtering stage (e.g. after the clamping to avoid long recovery times after an out of range signal.Besides the normal RC response the capacitors can also add to a slow response from dielectric absorption. This is especially the case after large excursions. Settling to less than 1 ppm of prior voltage can take quite some time - more than the classical value of some 14 RC.The balance of the input capacitance is mainly the question in the low capacitance range. So 10 pF and 20 pF can behave different, but 100 nF and 1 µF could behave similar (both are large).A discrete build chopper could be an option, and in some circuits the OPA140 could be a suitable AC amplifier, other circuit may want more like a difference amplifier instead of an op-amp. The amplifier part is usually not the difficult part - the hard part is more the switching, especially when a low bias is aimed for. Here the integrated switches in the AZ OP-amp can usually get better matching. One the other side the CMOS AZ amplifiers can't have large capacitors inside and the CMOS part tends to have more 1/f noise than JFETs. So the integrated chopper stabilized amplifiers often use a relatively high switching frequency, whilt a discrete build could use a lower chopping frequency. This can help a little with less current noise and give the option to use a higher capacitance but lower noise amplifier. Another advantage in a discrete build chopper is the possibility to separate heat sources from the critical section. I mainly see a discrete chopper something for the very low noise but higher bias range ( e.g. < 10 nV/sqrt(Hz)).The input section shown is more for a low bias version, not for a very low noise version (more like nV meter). The resistors for the protection alone give quite a bit noise. 140 K of series resistance gives some 48 nv/sqrt(Hz). A circuit for very low noise would need a different kind of protection - possibly to a lower maximum voltage.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 30, 2022, 06:34:29 pm
content: I like the idea of building a null meter (in fact, did so, even if not all that successfully in first attempt). Also I think it is an attainable goal (and perhaps relevant as there don't seem to be any made commercially anymore and eBay (asking) prices went through the roof).In any case, I think it's important to start with use case / requirements. The AVM-2000's specs sure are impressive, but would be rather a stretch to replicate, I'd think. Things which make it difficult are (of course) very high sensitivity (low noise), high impedance, isolated output (at least when staying all-analog), very high insulation to ground (battery only should get you easily to 100GOhm, beyond that it'll get tricky). I'd like to see the use case and error-analysis first before fixating the requirements. Is, e.g. a 1000V range truly useful today (we're not dealing with tubes anymore, are we?)? If only ever 30V max are applied, then input protection could be simpler.From my experience with building one and restoring (on-going) of a HP 419A, I came to appreciate the mechanical aspects. The 419A is built like a tank: double walled (inner cage on LO potential, outer on GRD), low thermal inputs (regrettably not isolated outputs), quality, sturdy switches. Their weak point, if one can call it that, is that their neon tube driven photocell chopper fails after a few decades (the neons wear out eventually, but the photo cells apparently become slow with age alone). The amplifier actually still works fine. The 419A is spec'ed for 0.3uV pk-pk noise (at 3s settling time to 95%), but I've seen (and read about) 0.1uV actual performance, which I think would still be a challenge when going with integrated OpAmps alone. Also it doesn't drift (unlike my self-made attempt) or rather the remaining, minuscule drift is lost in the noise.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 30, 2022, 06:52:37 pm
content: Quote from: Echo88 on December 30, 2022, 03:49:12 pmI don't follow. GND is on LO, so after the voltage divider common mode is minuscule and even poor OpAmp have CMRR of 100dB or more. Common mode is least concern in a null meter, which only needs to indicate nullaccurately. I wouldn't mind a gain error of a few percent. Ib better be close to zero (back to requirements / error analysis), so close that we can't measure it precisely and is then allowed to vary a few % as well.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 30, 2022, 07:03:01 pm
content: Jep, the used prices for nullmeters are absurd now, which makes it more motivating to DIY one.I consider the AVM2000 an interesting specimen, but nothing more so far.The nullmeter i like the most is the K155: solid state design, long battery life, elegant and simple frontpanel design.Specwise the DIY-variant should be at least as good as the K155 in every spec, especially noise and bias.Of course the necessary voltage range (1kV) nowadays is debatable, but usage cases like F752A need 1kV and the DIY-variant should at least be protected against 1kV-accidents and ESD.The mechanical guarded double wall construction, the high quality panelmeters and those sturdy rotary switches are really quite nice indeed...quite expensive when wanting to obtain them new though.I attached the unraveled schematic of the analog input board of the AVM2000-board btw, as it was of interested to me how they did it.Still dont know how often they switch the relays, they speak of matched pair relays for polarity switching...Difficult to believe (at least for me) that they achieve the same low bias/noise-specs with high impedance source, while not polluting the measurement with DCDC-noise.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 30, 2022, 07:17:17 pm
content: Quote from: Echo88 on December 30, 2022, 07:03:01 pmI got a 2nd 419A (with dying neons) as parts donor, which I plan (someday) to use as basis for a self-made one. Proper mechanics are far beyond my abilities.Quote from: Echo88 on December 30, 2022, 07:03:01 pmOn the interwebs someone complained about the acoustic noise of the relays, which made me think that they are used as low frequency chopper (but I might be completely wrong about that).The AVM-2000 is big on filtering afaiu, with settling times up to 100s (!), so I'd think the noise from the digitial parts and DCDC converter is filtered out.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 30, 2022, 07:20:24 pm
content: Quote from: Echo88 on December 30, 2022, 07:03:01 pmI don't think the K155 (or any other null meter) specifies the bias current, even though it is of interest. Anecdotally it is very low (less than 1pA).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2022, 07:42:01 pm
content: 300 nV_pp noise is about 50 nV RMS noise. A response of 3 s to 95% should about correspond to bandwidth of some 1 Hz and thus an input noise of some 50 nV/sqrt(Hz).Compared to the Fluke 845 the protection is quite a bit lower resitance (only some 16 K ohm of series resistance) and thus less noise possible.There are quite some AZ OP amps with a noise of less than 40 nV/sqrt(Hz) and not that much input bias. A small input bias could be compensated. A possible candidate in this range is the AD8628 (some 22 nv/sqrt(Hz)) and a chance to get a input current in the 5-10 pA range, maybe with an optional added capacitor.Getting a low bias may need selecting the AZ OP-amp or possibly a compensation through a high value resistor (usually more than the 47 M Ohm suggested in the plan).From the schematics the Keithley 155 has a compensation for the bias current with 1 Gohm and some -150 to +500 nA adjustment range. So the actual bias current seen to be in the -200 pA range and can be adjusted to maybe the +-5 pA range depending on the pot quality and stability of the bias.

author: Echo88
author_rank: Frequent Contributor
timestamp: December 31, 2022, 06:32:17 pm
content: Youre right guenthert, bootsstrapping seems unnecessary given the signal is +-1mV max and the other values such as CMRR/PSRR/gain accuracy arent that important for this application.Bias current is indeed not specced in any nullmeter.It appears to me that the whole AC/Normal Mode Rejection for mains frequency and above is mostly done in the chopper and not the input filter, is that correct?The K155 states 100dB Normal Mode Rejection Ratio at 50Hz and above, while LTSpice gives a attenuation factor of about 400 at 50Hz for the input filter: 24k + 1µ + 22k + 1µ + 22k +1µ.If the 50/60Hz-filter could be implemented after the main-OP one could decrease the input protection resistor further, allowing for lower noise results/more freedom in OP-choice, maybe with a nice high order sallen key filter as one in the AVM2000?I attached the changed schematic with a changelog and the suggested Sallen Key filter.Happy New Year btw.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 01, 2023, 12:41:10 am
content: Here are another two nullmeters, for those that are interested in the topic:https://nvhrbiblio.nl/schema/Philips_PM2434.pdfPM2434 Already JFET-based, but only 10µV as smallest range.Attached is the schematic of the Leeds & Northrup 9828 Null Detector.Somehow i cant find the pdf-manual right now, so the schematic has to suffice for now. The most "modern" nullmeter i know, as it already uses op-amps. I own one, but the meter is broken and i need to repair it one day.It only has 4 ranges: 2.5µV, 25µV, 250µV, 2.5mV...truly a nullmeter

author: guenthert
author_rank: Frequent Contributor
timestamp: January 01, 2023, 01:13:10 am
content: Quote from: Echo88 on January 01, 2023, 12:41:10 amBut the interesting stuff is still being done by discrete transistors and some transformer magic (wonder what the input impedance is). I see it being referenced in '74, so not much younger than the Keithley 155.A 2.5mV max range certainly makes input protection easier.Happy new year everyone!

author: Vgkid
author_rank: Super Contributor
timestamp: January 01, 2023, 04:33:22 am
content: Quote from: Echo88 on January 01, 2023, 12:41:10 amThanks. I own a modified version of a 9829 linear amplifier. It does work sometimes. It has been in storage for years , along with all of my gear.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 09:42:09 am
content: The L&N 9828 circuit looks rather low input impedance: the resistors in the protection are only 3x2.2 K and the capacitors are large.It is strange that they use a BJT based amplifier after the transformer. Normally the transformer would increase the voltage and impedance and with a high impedance and AC only FETs are usually better.There is some 6.2 M load to the AC amplifier that would transform back to the input.At least they have a compensation for residual input bias, but the resistor value is not readable - likely in the Gohm range.Another strange part is that the input capacitors seem to be electrolytic. For me this would be a big no go.The PM2434 has the JFETs at the recorder output, not at the input. The input is rather similar to the K155, with MOSFETs for the chopper and a BJT based amplifier. The output with rectifier and seprate polarity indicator is not that suitable for a null-meter.Today a moder AZ OP-amp should be a good enough substiture for the input, if the requitements are not extreme ( bias < 2 pA or noise < 6 nV/sqrt(Hz)).On chip they are likely quite good in compensating the switching spikes and charge injection.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 01, 2023, 06:30:15 pm
content: Quote from: Echo88 on December 31, 2022, 06:32:17 pmI'll need to have another close look at the K 155's schematic. The chopper eliminates offset and low frequency noise from the amplifier itself, but I'd think you want to filter "high" frequency (higher than chopper f. / 2) of the input signal before the chopper. The demoduator is followed by a low pass filter which is meant to effectively filter the chopper f. (and higher) out. To do that, the knee frequency will be considerably lower. The line frequency is filtered then as side effect methinks.As a side note: HP's 419A uses a two stage RLC low pass filter before the chopper. I couldn't find the specifications for the coils, but given that they're housed in a case like the one used for the neon transformer, I'd think they are rather large.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 01, 2023, 06:40:03 pm
content: Quote from: Kleinstein on January 01, 2023, 09:42:09 amI read R106 as 6*10^9Ohm 20%.Quote from: Kleinstein on January 01, 2023, 09:42:09 amPerhaps they figure that at 2.5mV max, leak current through an electrolytic C. rated at 20V will be negligible (and only contributes to gain error anyhow).

author: Echo88
author_rank: Frequent Contributor
timestamp: January 01, 2023, 07:15:02 pm
content: Regarding the 9828: The input caps are 8.2µF tantalum and the Ibias-compenation resistor is 5GR.What are you thinking about the proposed active 4th order filter behind the main-OP to get the required NMRR at 50/60Hz while being able to use lower input resistance (at the expense of more quiescent current for the necessary OPs)?The HP419A (~16kR input resistor) btw. only states max50Vin overvoltage on the most sensitive ranges, while K155 (24k input resistor) allows "momentary" overvoltage up to 1.2kV and the F845 (150k input resistor) 1.1kV apparently indefinitely.I made some further changes to the schematic regarding the rotary operation switch and the battery charge level measurement.As the nullmeter should be buildable by everyone i reduced the operating mode rotary switch poles x positions (like it did with the range switch, from 19 -> 12positions), to significantly reduce price and make it easier to obtain such switches from a wider range of manufacturers/market places.It would have been nice to keep the elegant 2x rotary switch K155-frontpanel-design, but when i see on mouser that there are maybe some 20 fitting 8x5 rotary switches starting at 60$ then it would reduce the willingness of people to build the nullmeter.Let me know what you think.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 01, 2023, 08:28:15 pm
content: The idea with a 4 th order filter behind the main amplifier makes some sense. However one would still need enough headroom. A +-5 V for the main amplifier (e.g. a LTC2057) would help somewhat here. One may still want also some gain (e.g. a factor of 10, maybe even 100) after the filter. This would also help a little to not have that much gain (up to 1 million) in 1 stage.The old null-meters also have some low pass filtering and DC gain after the chopper part. The should also be the main low pass filter. It does not look like a proper active higher order filter though. More like 1st order for the main part. The overall filter-function / response is hard to see (may want simulation) as it involves the chopper part and gain there.´A little more filtering at the input could still make sense to keep the amplitude of mains hum small, not to easily drive the main amplifier to limiting.Unless AC is wanted I think there could be more filtering at the input, maybe some 100 nF and also filtering before the MOSFETs to add some ESD protection.Very low resistance at the input would only be needed for a very low noise version. Unless really needed for the noise, more resistance in the protection is OK and helps.The very low noise version would not be ideal for high impedance signals and thus does not need low capacitance.The input protection in the lower version still looks a bit odd: claming to ground is still way better than clamping to the supply. The voltage is small anyway and limiting the voltage at the capacitors helps to speed up recovery from overloadThe input filter in the K155 is more Sallen key like. This reduces the effective input capacitance. The first to capacitors are not towards ground, but towards the Feedback signal which approximates a buffered input.For protection one may want extra clamping diodes in the path too.Electrolytic capacitors have not just leakage, but also a leakage like current from dielectric absorbtion. The DA is usually assumed to be linear and thus also a problem with small voltages. In overlaod the voltage at the capacitors can be in the 1 V range and recovery after that to a low current could take several days.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 03, 2023, 12:07:18 am
content: I integrated the suggested changes, especially splitting the gain between 2 OPs makes sense regarding GBW and mains hum saturation.Why did you want to set the Sallen Key filter between the gain-OPs and not behind them?I did some calculation based on the usage of the nullmeter for F752A and F720A as worst DUTs (respective output resistance).Seems so far the thermal noise of the 70kR would dominate the calculation.The AD8628 looks suitable for this task.Are there any other suggested nullmeter use cases apart from resistance dividers and comparing voltage references, where an adapted nullmeter with lower noise could be more suitable?I commented everything in the schematic.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2023, 10:48:52 am
content: For the input protection I would consider to have the diodes for clamping a bit further down the line. Instead of 2 JFETs one could use a low leakage diode ( BAV199) they are cheap, small and may withstand a higher current.The capacitors before the diodes should be small with a high voltage rating. Class Y rated capacitors (usually 4.7 nF) could be suitable. Otherwise PP type capacitors that are oftenavailable with good voltage ratings. The main capacitance should be at or after the diodes. A MOV in parallel to the diodes makes no sense - it would never engage. If at all a MOV would be at the 2nd last stage to protect the capacitor.The filter between the gain stages helps to make the circuit more resilient to hum. With the filter after the gain, hum could drive the amplifier into saturation as there is not much headroom.This is especially a point with not that much filtering (smaller capacitors) at the input.Not the main use, but as a good meter for low voltages one may also use the null-meter for other small voltages, like thermocouples. The AD8628 should still be good enough for this. For really small voltages there are special nV meters, but these usually come with a higher input bias.The AD8628 has a max supply of some 5 V.I have done a crude test on the noise current an got something on the order (not very accurate though) of 50 fA/sqrt(Hz).The first amplifier stage should have some capacitance in parallel to the FB resistor. This can be part of the filtering and as a minimum be an option to effect the input bias.Chances are one could get away without the bias current compensation with the AD8628 - still good to have the option.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 06, 2023, 03:41:16 pm
content: Quote from: guenthert on January 01, 2023, 01:13:10 amInteresting: Traditional transformer-coupled chopper amplifier but with FET switches in front end vs. mechanical chopper. What do the two coupling transformers look like? And what are their specs: DC resistance & inductance of each winding, turns ratio?Also what is chopping frequency? Mechanical choppers operated at 50, 60, 83, or 400Hz.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 07, 2023, 03:31:47 pm
content: Full ack on the last post Kleinstein and now i get why you mentioned a MOV: the symbol that is used in front of the main OP is a MOV-symbol, i intended to use it as a symbol for a thermal jumper but forgot to add the Theta (dont know if theres a more fitting one)We´ll see in the end if they aid the project.Heres the changelog for the current version, that is attached:Low leakage diodes are now BAV199, typical leakage with <1pA is good enough, see:https://www.eevblog.com/forum/projects/forward-leakage-of-a-diode/msg1287718/#msg1287718Added LDO-regulatorsDetailed the battery indicator section, taken from the Datron 4910. Current consumption with suitableLEDs is marginal compared to the OP quiescent current. Only implemented for pos battery, should be goodenough as an indicator supposing nullmeter quiescent current draw is sufficiently symmetrical during operationand recorderoutput draws current from positive supply, making it the fastest draining battery and therefore the most reasonableto choose for "battery low"-warning.Improved recorder-section, should now be bipolar input capable and draw absolutely neglegible current, seehttps://www.elektronikpraxis.de/galvanisch-getrennte-emi-freie-messschaltungen-a-1102943/Before i assumed the IL300 needs mA LED-drive current and V->f-converters that i found also needed at least 1.5mAA 6V-lead acid battery enables 2.5V and 5V supplyvoltage, as the LDO-dropoutvoltage and deep discharge voltageof the battery is fitting for the purpose. With 1.3Ah they should last 13d at expected 4mA max current draw.What still needs to be done:Finalizing all component values and producing a BOMChecking the correct function of the IL-300-sectionI found a few nice cases from Hammond and suitable rotary switches, gotta do the mechanical planning now.If someone has bipolar capable very low quiescent current (<1mA) V->f and f->V-converters im also interested.Im wary of digital isolators (capacitive and inductive) and similar stuff as a solution, since id like not to worry about the resulting EMI.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 07, 2023, 05:18:53 pm
content: If all amplifier parts get away with a +-2.5 V supply (or maybe a little less) one could opt. for a single battery and virtual ground. So only one 5 V regulator and than a OP-amp to buffer half of that as a virtual ground.The IL300 part should get away with a less expensive amplifiers. The signal is +-1 V range and I don't think one would need a 5 digit dynamic for the output. Some offset can be compensated relatively easy.For high resolution use one may want a little more current there. It is a compromise and still relative easy to change later if needed.For the output side It may help to have only 3x1.5 V (or oven just 2x1.5) so that 5 V range amplifiers are possible.The 10 K for the input protection are relatively small compared to the 2x22 K in the filter. For ESD spikes it would help if the 10 K can withstand higher pulses too. So the combination looks a little odd. I would try to get away with smaller capacitors, so that the input does not get too slow with a high impedance source.The LP filter is quite slow responding - it may help to have the option to bypass the filter, especially for the analog output or higher voltage ranges. The low speed is mainly needed with the low ranges to suppress hum and noise.

author: JohanH
author_rank: Frequent Contributor
timestamp: April 11, 2023, 02:33:39 pm
content: Quote from: Echo88 on January 07, 2023, 03:31:47 pmThis is interesting. I like the approach. Any status update?I'm building a KVD and already have the Hoffman Null detector adapter for a DVM. But it would be awesome with an analog display. I have to finish the KVD and some other projects first, but after that I will take a look at this. The meter and switches will be most expensive. I have some Elma switches, but they are only specified at 35V DC. There are also Chinese rotary switches specified for 150V DC. Could be a better alternative.As Kleinstein mentioned, a single battery would be nice, too. Which op amp are you using (planning to use?) in the latest revision (Main OP, low pass and Post OP)?

author: Echo88
author_rank: Frequent Contributor
timestamp: April 18, 2023, 08:02:21 pm
content: Hi Johan,im not quite finished with it. But so far it works nicely apart from 3 small bugs, and needs some cosmetic changes in the mechanical pcbs and i'll exchange the knobs for more fitting ones.Noise is about 150nVpp with the used AD8628 which are used for the complete analog path for simplicity.It differs slightly from the schematic in the last recent post:-only one 12V-battery is used together with a railsplitter and a ultra low quiescent current DCDC-converter for the isolated output-with the used 7Ah battery the runtime is about a month at roughly the same case size as nullmeters like HP419A/AVM2000-a battery charge/protection circuit was addedThe panelmeter was taken from a damaged K155, but the circuit has provisions to also use normal (non +-) panelmeters.The idea for the binding post thermal baffle was taken from the AVM2000. It can be screwed on the case-backside when not used.

author: JohanH
author_rank: Frequent Contributor
timestamp: April 18, 2023, 08:34:08 pm
content: That's so awesome. I already found a null meter (with mirror scale) on ebay and ordered it. M265M 100 - 0 - 100 µA. Not as nice as yours, but will do.What devices did you connect to the guard post? I think in null meters such as Keithley 155 only potentiometer shields are connected. Not sure about that, though. Manual states that guard should be tied to circuit gnd (-) if not used (they do it on the rear terminal). And is your inner box connected to the chassis?

author: Echo88
author_rank: Frequent Contributor
timestamp: April 18, 2023, 08:58:25 pm
content: The guard is connected via two capacitors to outer case and negative input like in the K155, the inner case (isolated with standoffs) is also connected to guard. That should aid CMRR in my understanding, but i could be wrong.See for referencehttp://hparchive.com/Application_Notes/HP-AN-123.pdfGuard connection depends on the specific use case as lined out in the manuals of nullmeters like the K155.Specific measurements like CMRR, NMRR, noise, etc. to completely characterize the nullmeter werent done yet though.

author: ecclectrics
author_rank: Contributor
timestamp: July 21, 2024, 03:05:17 pm
content: Dear All,I understand that the topic is not very recent, but hopefully not completely inactive. Thanks to all of you for sharing the design and knowledge! Please excuse if my questions are somewhat elementary as I am still rather new to precision design.I am particularly interested in the bias compensation circuit shown in Echo88's schematic above (https://www.eevblog.com/forum/metrology/null-voltmeter/msg4623649/#msg4623649). I wasn't aware of this possibility, which might be useful also for other applications.I wonder how the adjustment will be made: First shorting the inputs together and trimming the output voltage to zero with the offset compensation and then leaving the inputs open and trimming the output voltage to zero with the bias compensation? Possibly the two adjustments are mutually interdependent an need to be repeated iteratively?And is the 10M resistor to GND in the lower ranges of the input divider still absolutely required even after the opamp gets its bias current through the compensation network? Or could a 10G input resistance like for the lower ranges of many precision DMMs be easily obtained? (Whether it would be desired for the present application is a different question of course.)Finally, what is the purpose of the 47k resistor to GND in the bias compensation network? To maintain a definite state if the pot wiper looses contact?Help is greatly appreciated!-)Roland

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2024, 04:00:05 pm
content: There should be essentially no interaction from the bias compensation on the zero offset. A repeated adjustment would be more to cope with offset drift.The 10 M resistors to ground at the input is not really needed at all. It would be OK to have an essentially floating input with only parasitic resistance (e.g. in the Tohms range). Many high impedance meters are made this way. The specs are something like > 10 Gohm, but the actual input resistance could well be > 100 G. How large the input resistance will be depends on the details of the build, like parasitic leakage and the amplifier chip. > 10 Gohm should be relatively easy.The 47 K to ground in the bias adjustment are there to divide down the voltage before the 1 Gohm resistor.

author: ecclectrics
author_rank: Contributor
timestamp: July 21, 2024, 04:46:52 pm
content: Thanks for your explanation Kleinstein!So no interaction between the adjustments, good. But the basic procedure is correct: open inputs for bias and shorted inputs for offset?Concerning the 47k resistor, I should think of Thevenin equivalent of the 3 1Meg's (see image), which would be very roughly around 470k, so with the 47k it gives roughly a divide by 10.Is there any advantage for doing it that way rather than simply using ~100k instead of 1M for the pot?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2024, 04:57:51 pm
content: There is a slight advantage with the resistor to ground. The 1:10 attenuation also applies to thermal EMF generated at the POT. I don't know if this gets relevant.The offset adjustment can still effect the bias trim, though not much (one should aim for the same reading for the short and in the bias step).

author: ecclectrics
author_rank: Contributor
timestamp: July 21, 2024, 05:25:56 pm
content: Ok, thermal EMF of the pot - what all needs to be taken into consideration!One other question I have in particular to you: in some other threads on the Null-Voltmeter from Conrad Hoffman (e.g. herehttps://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/msg2893432/#msg2893432) you suggested smaller values of the caps for the input filter, which were at 0.47µ there. Here they are with 1µ even bigger. I wonder what the pros or cons are for having smaller (i.e. 47n .. 100n) or larger (i.e. 0.47µ ... 1µ) caps in the input filter?I have actually cobbled together a version of the Hoffman circuit with some jellybean parts, which was a valuable lesson on 1/f noise. With the opamp I used it was several µV and severely limited the possibility to get the offset really low. Now the autozero opamps seem to have it in the order of several 100nV depending on the type, but is still an issue if I want to get the offset lower than that. So I think a lot of filtering will be necessary for me, and I am looking for the best way to do it without messing up something else.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2024, 05:43:43 pm
content: The capacitors at the input give a filter response, where the source impedance adds to the resistor in the circuit. With a high impedance source this can become sluggish and dielectric absorbtion can make it even more sluggish than the RC time constant suggests, if there was a much larger voltage before.With an analog display one may want the analog filter fuction, especially for the highest gain range. Thus the filter at the amplifier that changes with gain.For use with a digital display it would be better to do the final filtering digital with a more FIR type filter and thus a finite response time instead of exponential decay type. Digital filtering is also easy to make flexible.The filter capacitors also are part of the ESD protection to reduce the peak voltage in addition to the diodes.

author: Vtile
author_rank: Super Contributor
timestamp: August 08, 2024, 08:14:19 pm
content: Has the parasitic resistances ( cabling ) talked about. This is a subject most of the times overlooked when the null detection is spoken with wheatstone bridge example in academia and totally forgotten the wiring resistance from equation given in these examples (which is much more complex with wire resistances) this of course is mostly for bridge setups using null methods.

author: guenthert
author_rank: Frequent Contributor
timestamp: August 10, 2024, 08:00:44 am
content: Quote from: Vtile on August 08, 2024, 08:14:19 pmI presume you mean the leakage of the insulation of those cables, as the resistance of the conductor is very low (expect it to be less than 100mOhm) compared to the input resistance (100kOhm in case of HP419A, 1MOhm in Keithley 155 and Fluke 845A) and irrelevant at null.I'd think the leakage of the cabling is implicitly included in the leakage of the case. Many bridge circuits have the detector at ground potential in order to minimize leakage effects (however then requiring very good isolation of the generator, see e.g. ESI 801).

author: acstd90
author_rank: Newbie
timestamp: August 28, 2024, 05:21:10 am
content: Have tried the AVM. Maybe someone else may have had better luck than myself but they seem to have problems with Noise, Drift and the battery is always dead (self discharge)Mine holds down a spot Under the workbench.

author: acstd90
author_rank: Newbie
timestamp: August 28, 2024, 05:34:48 am
content: There is a paper written by Neil Faulkner on the Fluke website that discusses the errors associated with replacing the Fluke 845AB.Also in the October 2023 issue of Cal Lab magazine is an updated article written by David Martson (1A CAL GmbH) on the pitfalls of replacing the analog null meter with a DVM as a null meter for use with a Fluke 752A and calibrating a Fluke 5720A

author: e61_phil
author_rank: Frequent Contributor
timestamp: August 30, 2024, 01:27:33 pm
content: Quote from: acstd90 on August 28, 2024, 05:34:48 amHi,do you have the 1A Cal article? I also started a discussion with them, because they said, that they are using a Keithley 182, but in my experiments there where several problems by replacing the 845A simply by the Keithley 182.Best regardsPhilippEdit: found it

author: iMo
author_rank: Super Contributor
timestamp: March 09, 2025, 08:14:40 am
content: A repair with schematics:

author: CalibrationGuy
author_rank: Unknown
timestamp: March 09, 2025, 06:37:59 pm
content: I saw the 3458a as one solution earlier in the discussion. I use my 34420a for low level measurements instead. I like it better when you don't have a good null meter handy.TomG.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2025, 07:37:56 pm
content: The main point of the Fluke 845 and HP 419 is having low input bias and very low common mode current / leakage. It depends on the use case how critical this is - in many cases one should be able to work around with an addional reversal of the source and maybe also the meter.For the input current chances are the Fluke 845 and HP419 are better than the 3458 or 34420 - voltage noise wise the DMM should be better. The AZ switching spikes at the input may case additional issues with a very high impedance source.The AVM-2000 from the repair video is in many respects more like a DMM and may well have rather high input current, high current noise and also CM signal injection. So it does not offer the advantges of the old Nullmeters. So it will likely not work well with high impedance sources.