HP 34401A hacks and upgrades - Page 1
Forum section: electronics / general

author: cellularmitosis
author_rank: Unknown
timestamp: April 26, 2018, 12:48:59 am
content: I thought I'd start a thread for hacking on the 34401A, as a "hacked" 34401A seems to be one of the next-best options for volt-nuts who aren't (yet) willing to fork over the cash for a 3458A.I'll kick us off with this one:I believe I've just confirmed that you can substitute whatever 7V Vref you'd like in place of the LM399.This means an LTZ1000 upgrade should be possible.(or paralleled LM399's for lower noise)The core of the 34401A Vref is a bootstrapped LM399 (see attached).I removed R409, unplugged the LM399 (U403) (it is socketed!), and plugged an MV106 into the zener pins of the LM399 socket. I was able to move the ADC reading around by changing the dials on the MV106.Note that there is a leg of resistors hanging off of +Vz (a voltage divider which forms a 5V ref: R441 and R442), which total to about 50k, so your 7V ref will need to supply about 140uA.

author: cellularmitosis
author_rank: Unknown
timestamp: April 26, 2018, 12:50:51 am
content: (pics)

author: cellularmitosis
author_rank: Unknown
timestamp: April 26, 2018, 12:53:11 am
content: I wonder if the decision to socket the LM399 was so that they didn't throw away any pre-ageing by soldering in the LM399.

author: rhb
author_rank: Super Contributor
timestamp: April 26, 2018, 01:03:05 am
content: Quote from: cellularmitosis on April 26, 2018, 12:53:11 amIn which case, why not a crimp?

author: cellularmitosis
author_rank: Unknown
timestamp: April 26, 2018, 01:06:37 am
content: Quote from: rhb on April 26, 2018, 01:03:05 amGood question.I have noticed that the selected zeners (1N829A) in EDC geararesoldered in place, which is an interesting counter-point.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 26, 2018, 01:17:54 am
content: hello Jason,nice worknow if you could replace the MV106 with a series connected 6x NiCd cells totalling around 7.2Vand then measure 1x, 2x, 3x ... 6x cells of other NiCd cells and plotting the histogram of values obtained.and then repeat the test with LM399 / MV106 / LTZ1000 as a reference.best regards.-zia

author: cellularmitosis
author_rank: Unknown
timestamp: April 26, 2018, 01:21:22 am
content: Quote from: zhtoor on April 26, 2018, 01:17:54 amUse the NiCad cells to find the noise floor of the ADC / frontend?

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 26, 2018, 01:23:32 am
content: yes, and the potential headroom for vref noise improvement.also an interesting way to evaluate references wrt. noise.(maybe somebody does this for an HP 3458A)best regards.-zia

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 26, 2018, 01:41:11 am
content: Quote from: cellularmitosis on April 26, 2018, 12:53:11 ammaybe some kind of factory adjustment procedure by plugging in a test-pod?-zia

author: Mr. Scram
author_rank: Super Contributor
timestamp: April 26, 2018, 01:56:57 am
content: Opening the thread I thought messing with a 34401A is a terrible idea. Reading the thread it sounds like potentially a great idea. Carry on!

author: kj7e
author_rank: Frequent Contributor
timestamp: April 26, 2018, 02:14:10 am
content: Looks like you could easily make an L bracket off the side of the chassis to support an LTZ1000 ref board. Maybe even shock mounted with rubber grommets. I would even look at buffering the LTZ down to the same voltage as the LM399 it would be replacing. Since the buffer would have - gain, there should be little harm in noise or stability.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2018, 06:47:04 am
content: The ADC in the 34401 has some intrinsic limitations. The reference is not really the worst part of the 34401. So even without the reference noise, there will be considerable noise and possible INL left. Some of the weakness is an intrinsic problem of limited resolution of the µC internal ADC and the resistors to the integrator - so nothing one can do about it as a kind of hack.As just replacing the socketed LM399 is kind of reversible, it can be still a possible way. However the effect is likely limited.

author: TiN
author_rank: Super Contributor
timestamp: April 26, 2018, 08:27:40 am
content: I remember someone over 38hot.bbs did LTZ mod for 34401A.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 26, 2018, 11:11:08 am
content: Quotehello,here is a scenario:(pardon me if i sound naive, i am)1. let us make 2 reference sources for the "hacked" 34401A, call them REF-A and REF-B2. REF-A is an inaccurate voltage source using some NiCd batteries in the general ballpark of 7V (required for 34401A)3. REF-B is an accurate / calibrated reference source using a long-term stable device such as LTZ1000 / LM399 etc.4. there is a computer controlled switch which can switch between REF-A and REF-B at will.5. switch in REF-A and measure REF-A itself and REF-B and call these long term records AA and AB.6. switch in REF-B and measure REF-B itself and REF-A and call these long term records BB and BA.7. switch in REF-A and measure Vx and call this long term record AX.8. switch in REF-B and measure Vx and call this long term record BX.now the question:-what can be said about real value of Vx using AA, AB, BA, BB, AX and BX? and using what kind of analysis?how "long" is long-term?(fourier, correlation, autocorrelation, other?)the scheme can also be generalized to N+1 references being switched, one being the low-noise one and others being long term stable ones.best regards.-zia

author: cellularmitosis
author_rank: Unknown
timestamp: April 27, 2018, 06:13:38 am
content: Quote from: TiN on April 26, 2018, 08:27:40 amI think I found an LTZ mod for the 34410A:http://bbs.38hot.net/thread-69139-1-1.html

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2018, 07:30:52 am
content: Quote from: zhtoor on April 26, 2018, 11:11:08 amThere is no need for such a complicated switching scheme - if would loose quite some time for data not or hardly used.The idea of using 2 references is not that new. AFAIK some of the Keithley meters (e.g. K2001,K2010,K2182) use this : a low noise zener reference for the ADC itself and a long term stable LM399 that is measured to get a stable scale. The measurement of a reference is needed anyway to compensate for possible gain drift of the ADC. How often to measure the long term reference and how to use those data depends. The extremes are a kind of extra ACAL step that is done rather infrequent and a reference conversion after each measurement.One has to find a good compromise between not loosing to much time for the actual input measurement and added noise from infrequent reference measurements. Ideally one would have a kind of digital filtering on the reference data, so that the long time reference (e.g. LM399) would give the very low frequency part (e.g. < 0.01 Hz or even lower) only, while the low noise reference at the ADC (e.g. 1N82x, 2DW232 ?, battery ?) would be responsible for higher frequencies.However using 2 refs in a good way would need a suitable software control. So this is nothing to easily add to an existing meter.The LTZ1000 offers long time stability and low noise, so it can be used for both the ADC and long time reference. The measurement of the 7 V ref is than mainly to compensate for ADC gain drift and is less influenced by reference noise. So it is still better to have a single reference that is both low noise and low drift.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 27, 2018, 07:54:46 am
content: Quote from: Kleinstein on April 27, 2018, 07:30:52 amthanks Kleinstein.as far as adding functionality to existing meter, i am not proposing that, instead i am proposing to use a computer for this analysis.moreover, if the "character" of noise introduced by the ADC is measured by switching in the lowest-possible noise reference source (battery),then this "character" can somehow be "subtracted" from measurements to enhance accuracy and/or resolution. moreover the datasetsrepresented by AB and BA can be used to determine the base-line measurements of both reference sources.best regards.-zia

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 27, 2018, 08:47:21 am
content: The reference sections around the LM399 inside the 34401A and the 34410/411A are very similar; only difference is the value of the sourcing resistor, 750Ohm inside the 41xA, for +/-9V ADC reference, and 1k47 inside the 34401A for +/-10V.Both circuits deliver 2mA to the LM399 zener, as stated in the 38hot blog.If you think about the 34465A /34470A, these are probably copy/paste circuits from the 41xA instruments, as they have the very same multislope IV A/D converter.. so the reference topology is probably the same.If you look at the pictures of the 465A and 470A main boards, there seems to be no difference.They only replaced the LM399 by the LTZ1000A board, which is virtually identical to the 3458A reference board.Of course the supply is a bit different, but the LTZ reference output itself just plugs into the LM399, and the original 2mA supply is just sourced by the LTZ circuit, i.e. there's no need to remove the 1k47 / 750 Ohm resistors.Unfortunately, I did not succeed in fully reverse engineer the exact way KS merged the LTZ1000 circuit into the LM399 topology, but I think it would be easy to include the whole board inside the case of the instruments w/o external supply.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2018, 09:17:40 am
content: Quote from: zhtoor on April 27, 2018, 07:54:46 amThere is no way to subtract the "character" / typical noise from normal DC measurements in DC. Noise is unpredictable random with an average value of 0 and thus the best guess is to subtract 0. It might be possible in some noise / AC measurements to subtract some "offset" due to the instrument noise, but that is different and does not need a special extra reference, more like a dummy input (e.g. short).Doing the analysis afterwards it would need quite some channel switching and looses time measuring the input. It might be viable, but likely in the more simple way with a low noise at the ADC and a long time reference measure through the normal input channel.

author: rhb
author_rank: Super Contributor
timestamp: April 28, 2018, 12:13:22 am
content: Quantifying the noise behavior will give you more refined uncertainties, but you can't remove them because they are random. You *can* reduce the errors by averaging, but every 6 dB of improvement will cost a 4x increase in measurement time. TANSTAFL.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 28, 2018, 12:47:03 am
content: Quote from: rhb on April 28, 2018, 12:13:22 amare we assuming that the noise introduced by the ADC alone is random?(assuming that the reference is noiseless - batteries)-zia

author: rhb
author_rank: Super Contributor
timestamp: April 28, 2018, 02:57:24 am
content: I'm referring to the thermal and 1/f noise. My analysis to date has been limited to the aliased thermal noise of an integrating ADC. I've not looked into the ADC noise. I've also not rigorously examined the 1/f noise. But as it is random, I don't see anything one can do about it.

author: floobydust
author_rank: Super Contributor
timestamp: April 28, 2018, 10:41:32 pm
content: Looking at the main A/D, U501 CMOS gate-array contains a 24-bit counter driven by 12MHz clock.I don't see the integrator period as enough, with 30K, 10Vref, 440pF integration capacitor. It seems low for close to 6.5 digits, or is my math is wrong here.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 29, 2018, 08:20:11 am
content: Quote from: floobydust on April 28, 2018, 10:41:32 pmAFIAK the ADC uses the feedback from the current sources and in addition the µC internal 10 Bit ADC to measure residual charge. As there is no switch to separate the input, the ADC runs continuously more like the old Solartron's, but with the extra residual charge reading. So the µC internal ADC should add some extra resolution.With the rather fast modulation (needed due to the small cap) I find it difficult to get the not that fast ADC to sample at the right time to really get significant extra resolution, but it somehow seems to work.Due to the rather close timing, I consider it a good idea not to touch / modify anything here. There is essentially no chance to improve the µC internal ADC limitation anyway. From my analysis I see the 10 Bit ADC as a limiting factor for short integration times and the current noise of the OP27 in the integrator as the limiting factor for longer integration times.

author: aronake
author_rank: Frequent Contributor
timestamp: March 24, 2023, 04:24:07 pm
content: How has this developed? Anyone modified their 34401a with a LTZ1000 with or without sucess?

author: GigaJoe
author_rank: Frequent Contributor
timestamp: March 24, 2023, 09:38:01 pm
content: in theory you can, on a practical side, i doubt of any improvement, soo guess none done.i may assume original 399 are selected , and some dmm like 10-20 years old, means, do replace original 399 with same long-term stability would be a significant challenge.quite interesting set an array of few modern adr1399, but again do it right \ selection \ageing takes triple cost of used unit. I'm speculating, that out of set 10 399 about 1-2 would be acceptable for drifting

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2023, 10:15:33 pm
content: I don't consider the reference the major weak point of the 34401. A slight upgrade to an ADR1399 may be OK, but the old, well aged LM399 is likely more stable.IFAIK the 34401 does not do an ACAL to correct for dirft in the ADC gain. So the resistor array would also effect the gain. Usually the 34401 still hold up calibration very well.The ADC has quite some noise on it's own and no easy way to improve on this, at least not by much. So the main point of a better reference would be less popcorn noise.

author: iMo
author_rank: Super Contributor
timestamp: March 25, 2023, 10:18:20 am
content: Frankly, I think the most important mod for the 34401A is the installing an internal temperature sensor.You may cope with the noise with some averaging/smoothing, but without knowing the meter's internal temperature (and subsequently its TC) you cannot target any improvement, imho.

author: aronake
author_rank: Frequent Contributor
timestamp: March 26, 2023, 06:43:52 am
content: Thanks for input on LTZ1000 mod.I bought 2 very nice looking 33401a locally for 200 USD each. One early Agilent and one HP. Didn't really need them but thought I could have them for some experiments, and LTZ1000 mod came to mind. Not that I think it would make the meter much better, but more because you can thing. I may still do it, and if so will report back on outcome.I am now starting off with changing electrolytic caps as first step.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2023, 07:02:10 am
content: There is one weak point of the 34401 that one could improve on:https://www.eevblog.com/forum/repair/hp34401a-autorange-not-functioning-with-half-wave-rectifier/msg4768076/#msg4768076The meter has no way to tell if the input drives the amplifier into saturation for short times. The aut-ranging and also over range detection can only use the ADC result, but has to way to see short time overflow. Clipping at the integrator could be visible in theory, but amplifier clipping may happen first and not sure the ASIC supports it - it does no seem to help with the autoranging example.One could add a relatively simple window compartor for the amplifier output signal and show overflow with a LED, so one would at least know if there could be clipping (the threshold would have to be a little lower than actual clipping). It could still be a bit tricky to ignore overshoot from switching, so in times when it would not matter.

author: iMo
author_rank: Super Contributor
timestamp: March 26, 2023, 08:19:09 am
content: ..and an another hack would be to add an LED (turquoise one) - for when the box is powered on and the display is set off..

author: GigaJoe
author_rank: Frequent Contributor
timestamp: March 26, 2023, 06:55:06 pm
content: another thing that I noticed : Linear VR using board as a thermal sink. in a theory it unevenly warm up the board.im not sure if desolder it and make vertical with own heatsink, may improve something. it may do better runaway from cold to hot state.would be great to find one in close state to graveyard , would be no much of remorse if killed , but its kinda impossible task.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: May 17, 2023, 03:00:31 am
content: so i got one, half dead , while in process of repair, i did play with 18V sources ,basically as for developers it always no space on board to put everything they want, im guess caps was cut off as wellso 50uf poly i add as shunt to adj 317\337 ; and special 2700 low esr to outputfast 6 digit gives me around 0.5 - 0.7 uV P-P noise , while slow was 0.12 uV ish ..i'm guessin it the same numbers as non mod version.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 04:39:10 am
content: Can you look at the +/-18V rails for noise - strangely it powers the A/D but also the VFD... and there was a change to the Front Panel board grounding (ribbon cable) and some ferrite beads added in later models. It might be something that can be improved.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2023, 06:33:30 am
content: There is not much that can be done about the ADC noise of the 34401. The main noise sources are from the quantization and other noise of the 80196 µC internal ADC. So if a little more supply filtering could help it would be at the 5 V for the µC.The design of the 34401 is not made for high performance, but for low cost. There is not that much that can be changed as quite some parts rely on the ASCI and custom resistor arrays that make it nearly impossible to change much. Instead of tweaking the old design it would be likely easier to build a new one.I don't think the board space was such a constraint for the developers, more the costs and maybe not touching the working ADC design as it needs quite some testing to verify that a design actually works also with the scattering parts. The noise is more like the easy part. The linearity part is more tricky.For the voltage regulators large very low ESR capacitors at the output side are more like a problem for the regulator and may make regulation worse (increased ringing).Much larger capacitors at the input side for ripple filtering mean more load to the transformer from a reduced power factor and the more pulse like current can cause more problems with hum. So more capacitance is not always better.

author: iMo
author_rank: Super Contributor
timestamp: May 17, 2023, 08:41:32 am
content: What would be the theoretical improvement provided the 80196's ADC will be replaced by a modern 14-16bit external one?PS: the 80196' ADC has got its 5V Vref (pin13) off the 399 via a 706 opamp - see below. I wonder whether there is something to improve around the "+5REF"..

author: GigaJoe
author_rank: Frequent Contributor
timestamp: May 18, 2023, 04:19:04 am
content: soplay with 5V bus - and really nothing muchshielded display cable by sticky aluminum foil - nothing muchbut ... after everything , my AC V noise , around 0.2 microvolts, that much less then DC,i'm not certain if AC has more samples for averageing so result less noisy or finally all this actions accumulated? no idea ...but my another where wasnt my dirty hand - for ac gives around 1.4 microvolts.

author: Andreas
author_rank: Super Contributor
timestamp: May 18, 2023, 05:42:41 am
content: Hmm,the 34401A is very non-linear below 0.5 mV AChttps://www.eevblog.com/forum/metrology/low-ac-voltage-measurement-issues/msg3842051/#msg3842051with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2023, 08:22:42 am
content: Quote from: iMo on May 17, 2023, 08:41:32 amA better ADC could improve the noise quite a bit. However the useful resolution is somewhat limited as the TC of the integrator capacitor (likely C0G and thus < 30-50 ppm/K) enters in the gain there.So more than some 12 bits would be of limited use. I pefect ADC could reduce the noise about 3 fold for the 10 PLC case and more with shorter integration. It would still be quite some effort and more like a new design of the ADC with a few more changes to also help with other noise sources.From my calculation I get an RMS noise for the 10 PLC mode of about 1-1.5 µV from the µC internal ADC noise and some 0.5-0.7 µV from the 1/f current noise of the OP27 as the 2 largest noise sources. There are than 3 sources that contribute some 350 nV: The resistors, non filtered part of higher frequency reference noise, jitter (mainly the HC4053). The noise adds as squares (geometric sum) and the larger ones are thus more imports.If at all a better supply filtering could help with the 80196 to reduce the ADC noise there. Chances are it is already OK and not that much room for improvement and it could be just the intrinsic noise of the µC internal ADC.The low hanging fruits are more the OP27, the HC4053 and adding a little filtering to the reference. Still this is only a small part of the noise and tweaking an existing ciruit is tricky. At some point is may be easier to build a separate, simple voltmeter with a similar cuircuit (e.g. use a more modern µC to replace the 80196 and ASIC).The AC part with an analog RMS->DC converter does not work well at very low voltages. It is tricky to look at the noise of the AC mode. There is the rather nonlinear response at low voltages (different units may behave differenet in the details) and also a bandwidth that gets lower with low amplitude. So it is expected that the noise depends on the AC level and possibly the AC waveform.The AC part has some filtering for the result and the AC readings are this a bit slow by design. When reading slow the ADC uses multiple 10 PLC conversions and averaging for DC and likely also for AC.

author: Hydron
author_rank: Super Contributor
timestamp: May 18, 2023, 08:29:25 am
content: If anyone wants to have more of an idea of how the FW calculations and ADC work on the 34401A then there are some interesting reverse-engineering notes from alan.bain here:https://www.eevblog.com/forum/testgear/hp-agilent-34401a-hidden-menu/msg4644106/#msg4644106I guess it might be possible to graft in a better ADC to get some improvement as noted above, but I suspect the software hacking effort would be pretty heroic - just gonna enjoy mine as is (well, with the other added FW features from that thread), and use another meter when I need better performance at short integration times (or indeed the ability to use 6.5 digits at <10PLC at all without fiddling with the custom aperture setting).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2023, 09:37:38 am
content: The linked other thread has indeed a really great describtion of the software details.A somewaht tricky point with measuring the charge on the fly is the right timing on when the ADC is sampled. Dending on the exact time when the µC internal ADC is sampling a fraction of the last run-up count (pos or neg reference) may be included or not. For the software description it looks like they adjusted the timing just right to have only full cycles included and no frational part.Another interesting point is that they include a numerical correction for an U² part of the ADC nonlinearity. This part would to a large part come from the 74HC4053. So changing that chip (and possibly also just the supply of this chip) would requite a new (may be special low level ?) calibration. They could get this correction parameter from the cal steps with +10 V and -10 V so it could be part of the normal calibration.I am a bit surprized to also see a correction for a U³ part. AFAIK the normal calibration does not include an extra test point for 5 or 7 V to also measure this parameter. So to measure this correction may need a special (low level) calibration.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: May 18, 2023, 02:40:44 pm
content: im not sure about AC linearity , as it basically AC-DC RMS AD637 converter , same i see in the fluke 8840\42 ; same in Keithley 175 (4.5) 197(5.5) , and many others .

author: Ecogeek
author_rank: Contributor
timestamp: May 20, 2024, 10:54:21 am
content: Interesting discussions on the HP34401A, but maybe a couple of simple mods are still in order. The 34401A takes (mine at least) 2+ hours to internally stabilise the temperature, indicating many opportunities for some improvements.- Replace the 3x 0.01uf XR7 capacitors in the Vref circuit with COG, as any local heating causes the short-term Vref to vary considerably.- R409 1k47 +/- 100ppm resistor supplying the LM399 current could be replaced with either 1k47 +/-10ppm or 1k5 +/- (2-5) ppm, lage TC improvment- to a small extent the Vref start circuit diode leakage could be reduced by replacing the diode with a BJT diode.- The +5V reference divider resistors R441,R442 are +/-100ppm and could be easily replaced with +/-10ppm resistors while only a small change like the BJT diode it improves the TC and the +5V ref stability.- I'm not sure if anyone noticed the +5V change in ground reference from AGND to GND C449. This could be an opportunity for greater noise filtering.- ADC MUX IC U411 HC4053 seems to be less than ideal, the three switches being operated at +/-20mV )(approx) into the virtual ground -ve input, the MUX resistance and non-linearity is poor with the ohms/V being high (the ideal is (VCC-VEE)/2). Is it possible to add a -5V VEE (shift the input switch to the idea (VCC-VEE) /2 to not only lower each switch resistor but also decrease the delta R and improve the linearity? Unless this mod would increase the switching charge injection into the channel? The biggest problem or opportunity is the replace the MUX with a modern typ but the packages are not available in SOIC.Anyway somthing to think about.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 20, 2024, 01:46:17 pm
content: Given the large rectified voltage headroom before the linear regulators, I'm using a bucking transformer to reduce nominal 240Vac to 210Vac, as one way to alleviate internal heat dissipation. Another easy mod is to solder copper strips to the pcb ground grid around the linear regulators (aka heatsink fins), to enhance the thermal transfer to internal airspace, as a way of reducing pcb temp rise/gradient around the linear regs. Both those mods should alleviate the warm-up time, along with turning off the screen.I also added an LM35C and used a DB9 spare terminal to allow monitoring of the internal shield temp - more so as a way to confirm known internal temp conditions when doing cross-comparisons.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2024, 02:05:30 pm
content: The LM399 circuit does not really need that stable resistor around. The differential resistance is quite small for the reference.There is not that much that can be done for the ADC. The main limiting part is the noise of the µC internal ADC followed by the quatization noise. So a point could be additional / better decoupling at the µC.There are other weak points too, but it would be hard to see the small improvements from the smaller noise contributions.The 5 V to power the HC4053 mux at the ADC has likely an only moderate effect. There is some effect on the swich resistance, but this compensated out quite well because of the symmetry.As a small upgrade one could replace the HC4053 with an SN74LV4053 (the LV version from phillips / ONS is different). The LV4053 is available in DIP and SOIC, so no problem there. The advantages are lower R_on, slightly less capacitance and likely less jitter.Another small point could be a little filtering for the higher frequency (e.g. 100 kHz range) part of the ref. noise.One could get faster settling of the internal temperature by adding a fan - still this comes with noise.

author: Ecogeek
author_rank: Contributor
timestamp: May 22, 2024, 04:50:24 am
content: Agee would be good to regulate the incoming AC to better manage the internal temp. Keithly managed to do that within one of their DMM I seem to remember, for the same reason.

author: MK
author_rank: Regular Contributor
timestamp: May 22, 2024, 11:13:07 am
content: Quote from: GigaJoe on May 18, 2023, 02:40:44 pmWith a Gilbert cell ac-rms converter there is a trade-off between keeping some output set for 0 in to give a better low level linearity. using a calibrator in the past allowed me to see how taking away all the noise left one with worse linearity, so the best settings depend upon your particular dmm.

author: iMo
author_rank: Super Contributor
timestamp: May 24, 2024, 04:03:06 pm
content: I spent some time with filtering the 399 zener. Cut the trace and applied an RC in the front of the AD706 - 1k/10k/47k/100k / 3u3 foil. It changes the calibration, and I did not see any measurable change in stddev while measuring my 10V.Also mind the analog part of the ADC is powered by +/-18.3V, where the +/-15V is made of 3.2V zeners (3 or 4 pairs) in series with the power lines. Unbelievable, indeed..One important prerequisite with all those improvement attempts is your ability to completly re-adjust the meter.

author: iMo
author_rank: Super Contributor
timestamp: May 24, 2024, 04:09:49 pm
content: Quote from: trobbins on May 20, 2024, 01:46:17 pmWhat is the TC of your meter? Where you put the temperature sensor?

author: trobbins
author_rank: Frequent Contributor
timestamp: May 24, 2024, 11:06:36 pm
content: iMo, I followed your comments fromhttps://www.eevblog.com/forum/metrology/getting-one-more-digit-from-a-6-5-digit-meter-without-using-gpib/on location for LM35C, and noted your tempco comments. Meter tempco (whatever it is) was the main reason to take note of internal temperature as a first-order and cheap way of mitigating meter temperature difference when making comparison measurements with other meters and/or voltage references over time. I haven't as yet made a plot of internal temp versus bench ambient, so can't confirm what temp rise I get and how influenced that is by diurnal benchtop temp, or whether there is any benefit in implementing some local temp controlled heater for the local meter environment to buffer from room temperature changes, and further stabilise internal meter temperature.

author: Ecogeek
author_rank: Contributor
timestamp: May 30, 2024, 12:01:44 pm
content: Yes I did similar, soldering some copper wings on the LRs to at least attempt to lower the peak temperatures and speed up the distribution of heat by way of a greater surface area. My mains voltage is typically 240V but ranges from brief periods of 230V to 250V during the day with all our solar PV systems lifting the mains voltage. Most houses where I live have solar PVs. The other thing I did to paint the internal aluminium shield matt black, which goes from a very good IR reflector to a good absorber and radiator to speed up the thermal transfers to hopefully reach some equilibrium quicker.The Linear Regulars are dumping a lot of heat, the designers of the 34401 made sure the DMM was under regulation down to 200V or below (for 230V mains) or so.The thermal environment of the 34401 has both external ambient and internal heating to contend with.The ideal world would stabilise both the mains voltage to just above the LRs regulation point around say 210VAC and then the external ambient temperature.I did plot the temperature of the internal shield, it takes hours to stabilise, a log curve, you need at least two hours before it flattens off then with both the mains voltage variations (variable LR heat dissipation) and ambient temperature fluctuations.I did a quick check with all the covers off and used my soldering heat gun set to 100C i.e. a small point source of hot air. While watching the external Voltage plot from the DVM, applied the heat source briefly to the various components. The good news most of the instrument makes only small differences, but the three Vref 0.01uF X7R capacitors are extremely sensitive and cause significant changes to the measurement voltage.Replacing only three 0.01uF capacitors from X7R to COG around the Vref circuit would eliminate one source of TC instability affecting the warm-up period in particular. One could ask why such unstable TC components were used in the Vref circuit in the first place apart from the cost 10+ years ago.

author: iMo
author_rank: Super Contributor
timestamp: May 31, 2024, 04:49:26 am
content: So, the potential changes around the Vref..The 78/7915 are the placeholders for a low drop regulators (18.3->15V), there are just the 3V2 dropper zeners there now (and the same situation at the ADC powering too).

author: Andreas
author_rank: Super Contributor
timestamp: May 31, 2024, 05:50:46 am
content: Hello,caution. 7915 (and usually LDOs) need more than 100 nF as output capacitor.And do not forget the reverse bias diodes (anti parallel to output) to prevent latch up during start up.(especially when putting a large capacitor across the heater).with best regardsAndreas

author: babysitter
author_rank: Frequent Contributor
timestamp: July 26, 2024, 05:42:19 pm
content: So, the linear regulators love some lower mains voltage, sometimes?A trick for old radios comes to mind:Put a 24V mains transformer in a box and connect it so that the 24VAC are substracted from the mains voltage before feeding the unit.cooler device without opening.BRHendrik

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 07:41:18 am
content: One hack I tried has been with the RC filter after the HP399.Tried with 10k/47k/100k and 3.3uF wima foil.Cut the trace and carefully soldered the RC in (WARNING: any soldering around the opamp's pins WILL move the calibration by some ppms!).I did not see any "significant" change at 10V, however. It would need more elaboration..See below ADEV with 10k/3u3..I saw a significant change when I replaced the HP399 with my lowest noise LM399 - some 20-30% based on a quick stddev evaluation. It would require a complete readjustment of the meter, however. Moreover, my LM399 does shot from time to time (like 2-4x per day) making around 5uV (@10V range) shift in one direction.Another thing to eleborate is to add a small capacitor at the input of the ADC in the MCU.There is 3K16+1k resistor in front of it, so perhaps 100-300pF against chip's ground might show something..Also decoupling of the MCU might help.The ADC's comparator is in the ASIC, so decoupling of the chip there could be worth of try as well.And the replacement with a 1399 will be the best move.. It means to add the RC snubber (do not do it at the 399 pcb pads directly as that would increase the noise due to thermal flows), and replacement of the zener's cathode resistor.Needs complete readjustment.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 08:12:50 am
content: ..and a preregulator like in the Keithley 2001 might help too..V2 is the voltage generated by an LED->optocell, the LED driven by electronics on the secondary side..Keeping "stable" 220V AC at the primary.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2024, 08:21:15 am
content: The µC internal ADC has to be quite fast as the feedback waveform does not stop. Adding a capacitor there would add a delay and settling time.The comparator performance is not relevant. The comparator is only used to control the feedback and it's noise has essentially no effect unless excessive to cause saturation of the integrator. The "comparator" could even be something like a schmidt trigger input and not a real comparator.The main point with the ADC is that the quantization noise is limiting and there is nothing (except a complete redesign, like a different µC) that can be done about this part. So all improvements for the shorter time scale are small.The preregulator like in the K2001 could help a little with the thermals - lot of effort with a limited potential.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 08:29:44 am
content: Quote from: Kleinstein on October 30, 2024, 08:21:15 amIndeed, but worth of seeing the impact.. We do not know how is the exact timing of the ADC's sampling..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2024, 10:16:15 am
content: We don't know the exact timing, but there is not much room to play. The switching is with some 1 µs and 1.6 µs and the sampling time of the ADC is some 1 µs - no barely enoug time and of the long time slot is used any delay effects how much of the last feedback step is effective.One could test the impact, but i have little hope for an improvement and it can have a negative effect on the linearity.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 05:12:31 pm
content: 80C196KB has got a separate Vref input for the internal 10bit ADC, so we may try to filter there..ADC timing - see below (we run it at 12MHz).. For higher than 6MHz clock the prescaler should be on..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2024, 05:46:43 pm
content: The ADC needs to run rather fast and there is now good way to change anything there. So likely no room for the prescaler. I had just looks at a shorted DS from Intel, that shows 1 µs as min sampling time. Even that is not very fast for the given signal that is modulated to some 370 kHz and thus not stable for the 1 µs. Already the quantization noise is limiting, extra noise from the ADC would be on top. How noisy the ADC is with the relatively high clock and very likely no prescaler is another question.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 06:03:38 pm
content: Does it sample at 370kHz modulation? I can hardly imagine it can do something with that data at 12MHz clock and intel 16bit architecture (many clocks per instr.).Moreover the ADC conversion time will be something like 23us at those 12MHz.. (say max 50kHz sampling rate)..My bet would be it does sample the residual voltage at the beginning and after say 1000 runups or something like that..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2024, 06:33:13 pm
content: There is no way to stop the run-up modulation. It has to contineously run to avoid saturation. IFAIK it only samples the voltage 2 times, once for the start and once for the end of the effective integration. So only 2 µC internal ADC conversions per conversion, but still with the 375 kHz of modulation going on all the time. The effective integration time is defined by the time when the µC internal ADC reads the integrator.Already the µC internal ADC can not follow the 375 kHz - so at most the µC could sample the voltage for every 4th cycle and average a little. This could give a little improvement, but I doubt that they use this.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 06:40:22 pm
content: The ADC "conversion time" cannot be 2us. Noooo way. It is 10x time longer at least..ADC's "S/H acquisition" time could be 2us when overclocked.I highly doubt the 80C196 at 12MHz with its ADC can follow 370kHz modulation and do something with the data (or the above DS table is wrong)..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2024, 07:07:29 pm
content: In the DS that I have they claim down to 1 µs aquisation time, though already with a desiclaimer : "1. These values are expected for most parts at 25°C but are not tested or guaranteed." They would want a pretty short aquisition time, even though the relevant part is mainly from the end of this time anyway. So even if they have more only the last 500 ns or so would really matter if the signal changes.The 375 kHz feedback cycle is handled by the ASIC and the control is via the "comparator" input there and not from the µC. The µC only has to somehow synchronize the ADC conversion with that cycle and do the 2 conversions for start and stop and read the counters inside the ASIC.The somewhat tricky part not described that well in the patent is when to exactly sample the integrator and how to handle the feedback phase during the sampling. Chances are this would include 2 feedback steps that are only partially effective. Chances are they have to do some calibration measurement to see how to split the 2 steps to before an after. They may avoid the split steps when sampling just before the actual variable time, even though this includes the reference switching.

author: iMo
author_rank: Super Contributor
timestamp: October 30, 2024, 07:16:40 pm
content: They define in DS the minimal "conversion time" is 91 "state times".The state time is 2 clocks (based on the DS) that is say 166ns at 12MHz clock.The internal clock of the MCU == state time (166ns period with 12MHz external clock).That is 15us for the conversion time when overclocked (no prescaler).Add the "acquisition time" which is minimal 8 state times when overclocked (DS) that is 1.3us.So you cannot get an ADC value into the MCU's ADC register earlier than after 16.3us overclocked.Then you have to do something with the ADC value, you would need a couple of us to place it somewhere, no math, no decisions, imho..The average instruction exec time with primitives is aprox 6 state times (aprox 1us) with simple math instr. perhaps 14 or more state times (User's Guide p 20-21).

author: Korchahin
author_rank: Regular Contributor
timestamp: July 13, 2025, 11:22:25 am
content: Got to my HP 34401. I was lucky and the multimeter is in good condition. It looks like it was not used much. The screen is in excellent condition.Question about electrolytes: The capacitors have typical measurements and it looks like it is in good condition (for example, i add foto other new narmal and Low-ESR capacitor). Do they need to be replaced? I am considering quality Nichicon UHW1H102 capacitors (High Reliability 10kHours, Low Esr)

author: iMo
author_rank: Super Contributor
timestamp: July 13, 2025, 11:44:40 am
content: Mine is from 1999 and I replaced the all big elytes with those black-gold ones. The old one were all ok, so there was no need to replace them, actually. Also the tantalum one seem to work ok. The issue with tantalum caps is when they got short they will smoke the zeners (usually short them). The zeners create +/-15V off the +/-18V (a primitive series dropper) on many places and when a zener is replaced the adjusting might be needed.

author: Korchahin
author_rank: Regular Contributor
timestamp: July 13, 2025, 11:51:05 am
content: I want to do a thorough preventative maintenance and upgrade all reasonable weak points, and install the ADR1399 and send it in for recalibration after minimal aging.

author: iMo
author_rank: Super Contributor
timestamp: July 13, 2025, 12:11:39 pm
content: Then people recommend to replace all tantalum bricks with same capacitance bricks but higher max voltage (like 25V or 35V - doublecheck) than the original one, afaik.PS: with 1399 you have to change the zener's cathode resistor (such you get 3.5mA) and you have to add the RC snubber. Also I would not solder anything directly at the 1399's socket pins (from the bottom side) as it visibly increases noise (perhaps because the higher thermal flows) then.

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2025, 12:57:38 pm
content: Quote from: Korchahin on July 13, 2025, 11:51:05 amDo you really think that replacing a manufacturer selected (from many, to meet the specs) and pre-aged LM399 with a arbitrary ADR1399 with unknown drift and history is a upgrade?Before doing this I would pre-age a bunch of ADR1399 and then select them for minimal drift, 1/f noise and T.C.with best regardsAndreas

author: Hydron
author_rank: Super Contributor
timestamp: July 13, 2025, 01:09:24 pm
content: I don't know why you would want to replace an electrolytic that tests good unless it's extremely old, in a position where failure would cause damage, or there's another reason to be suspicious (e.g. another of the same type is bad, or it's a known problem part - are these known to go bad often in a 34401A?).I've got plenty of stuff from the 90s and earlier built with quality caps that work as good as new (at least where they aren't abused or in particularly stressful use).

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2025, 01:40:40 pm
content: Quote from: Hydron on July 13, 2025, 01:09:24 pmI am only aware of some Keithley production lots which used leaking electrolytes (from 2002-2004)https://www.eevblog.com/forum/testgear/keithley-2000-and-the-leaking-caps/msg458613/#msg458613with best regardsAndreas

author: Hydron
author_rank: Super Contributor
timestamp: July 13, 2025, 02:07:55 pm
content: Thankfully my Keithley gear is mostly from the brown era (and by and large has not needed any caps replaced, other than the early 80s K181 which had been abused by a prior owner). If I had a K200x I'd certainly be swapping caps, I've seen enough horror repairs documented on those!

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 13, 2025, 02:13:02 pm
content: My 34401A is one of the first instruments, I bought it in 1990.Up to now, no need at all to replace the capacitors, they do not leak, and ESR / cap value are perfect for their age.Please use an ESR meter, before uselessly changing them.I even had not to replace U203 in the Ohms current source yet.Frank

author: Korchahin
author_rank: Regular Contributor
timestamp: July 13, 2025, 02:22:55 pm
content: Found two radiators from the 220th. Bent the side fins and attached to the 317/337. I will leave the installation of AD1399 for the very end. For now, I will make a small upgrade and see if it will give any effect. I am making a list of spare parts. Tantalum capacitors, and update on 0.01 COG, I need to determine which ones. Does it make sense to replace HC4053 with SN74LV4053 as suggested in the thread earlier?Quote from: Andreas on July 13, 2025, 12:57:38 pmI have been warming up two demo boards with ADR1399 for three months now (more than 2000 hours). In the last days I have not noticed any deviations (at least if I believe that it turned out to be 6.5 dmm Siglent SDM3065x)Quote from: Dr. Frank on July 13, 2025, 02:13:02 pmI checked their parameters, they are normal. But for peace of mind I would like to install LongLife capacitors.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 13, 2025, 02:29:18 pm
content: Quote from: Korchahin on July 13, 2025, 02:22:55 pm"Peace of Mind" hat offensichtlich nichts mit Enegineering zu tun, das ist nur Voodoo Quatsch.Such Dir besser ein anderes Hobby.Frank

author: iMo
author_rank: Super Contributor
timestamp: July 13, 2025, 03:36:25 pm
content: Quote from: Korchahin on July 13, 2025, 02:22:55 pmFirst do ignore Frank's comment.. This is a forum for Enthusiasts..Replacing the critical parts in the ADC could be dangerous, as it is not known how the adjustment software actually works. There are perhaps many settings related to ADC itself (see some previous posts on the math inside) made by HP after manufacturing and when you replace the components in the ADC the changes might not be considered by the adjustment procedure.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 13, 2025, 05:13:41 pm
content: The ADC of the 34401 has intrinsic limitations, especially the noise and resolution of the ADC inside the 80196 µC. Another limitation is the resistor ratio at the integrator input that amplifies many noise source about 3 fold compared to the simpler equal resistor case.So even with replacing parts like the switches and OP-amp there is not much improvement expected. On the downside it can effect the adjustment of the µC internal ADC scale and thus can effect the linearity. The main advantage of the ADC in the 34401 was the relatively low cost and OK linearity, not so much the low noise.To get a significant improvement it would need more changes, like replacing the µC and ASIC - so not much left of the original 34401 ADC.An alternative to modifying the existing DMM with quite some fixed parts is to build your own (or an existing project) ADC / DMM from scratch.A modern µC can control a similar MS-ADC as in the 34401 without the support of the ASIC.

author: Korchahin
author_rank: Regular Contributor
timestamp: July 13, 2025, 05:24:43 pm
content: I am interested in achieving long-term stability from 34401. That is, to make maximum quality once with minimum efforts. I have another thread where I am starting the topic of building an ADC. Studying the upgrade topic more and more, I am inclined to abandon the idea of ADR1399 due to the lack of guarantees of success. But I like the idea of adding a fan that will try to control the temperature inside.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 13, 2025, 06:33:33 pm
content: Uh, I don't know. The 34401A is a fine bench meter as is. I don't think there is much to be improved on without major rework at which time you're better off starting from scratch. I'd leave it as it. If it doesn't suit your needs, get one which does and sell the 34401A to offset the cost of that better one. But then, what are you really trying to accomplish? What makes you think the 34401A is not adequate?I have a multitude of DMMs and the 34401A might on the 10VDC range be the most accurate I have. It's actually quite difficult to say with certainty.

author: iMo
author_rank: Super Contributor
timestamp: July 13, 2025, 09:52:49 pm
content: Quote from: Korchahin on July 13, 2025, 05:24:43 pmWhy to mess with fan? Simply measure the temperature inside and adjust the measurement accordingly in math.. I can imagine the fan might help to speed up the internal temperature distribution. Also the big "advantage" of the meter is it has no fan so it is silent..PS: the typical temperature inside (measured at the aluminum shield over the ADC) is Tambient+18C, fyi.And there are two free pins in the RS232 connector..

author: Korchahin
author_rank: Regular Contributor
timestamp: July 14, 2025, 11:29:03 am
content: Quote from: Dr. Frank on July 13, 2025, 02:29:18 pmFrank, your message is very informative and meaningful. And most importantly, it shows friendliness and a desire to help!- Ingenieurskunst ohne praktische Beweise ist nur Gerede!Perhaps I will listen to your advice and take up another hobby.Speaking of "peace of mind" I mean that while I will now spend the necessary time, then I will forget and not climb inside for many years. And also, if I decide to sell the device to my friend, I will not be gnawing at the worm that there are capacitors there that would be good to replace, because I do not know how much longer they will last. This is my point, formed by servicing the devices I have developed, as well as based on the experience of servicing rare equipment.While the parts arrive to me, I will put everything back together and leave it under supervision to work for some time (with the screen turned off). In the meantime, I will free up the table for other things and start making a reference to track the result of the modifications.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 14, 2025, 12:04:14 pm
content: 34401a is a fine meter the way it was designed in that time, unless some capacitors or tantalums goes bad, you don't touch anything in it, it was designed like this and should stay like thisanything added in it may affect it's performance and stability, theses meters age pretty well if well taken care ofhad 3 of them, and at least 20 to 25 years old, and pretty spot onone was calibrated (after a non needed full recap of lytics and tants) and frankly was not outside it's specsthe 2 others: one needed an new x-former and the other a new vfdthe only real upgrades at some point is some mcu fw updates, keysight offer the mcu v11 .. but you need to buy the chip too, but it was leaked on the weband what is mostly known is the front panel pcb unobtanium NEC vfd driver chip, normally look the banana plugs, all red is mostly the "old / unobtanium", and black red are the newest vfd driversbut to be sure there is a Hp Agilent document with the changes according to the serial numbers ....you can not mix an old vfd front panel board with the newest ones, or the opposite.

author: iMo
author_rank: Super Contributor
timestamp: July 16, 2025, 08:48:34 am
content: Another easy hack idea: to replace 3 or 4 pairs of 3.2V zeners (serial voltage droppers creating +/-15V off the +/-18V) with a single pair of 7815/7915 (or better regulators). Remove the smd zeners and feed the voltages via wires accordingly. Adjustment recommended.

author: Korchahin
author_rank: Regular Contributor
timestamp: August 01, 2025, 10:27:10 am
content: Completed work: replacing large electrolytic capacitors with new Nichicon HW (LongLife, standard ESR).Tantalum capacitors were replaced with new Vishay TH3 (+150 °C). Tantalum at 1uF, replaced with polypropylene, In the LM399 reference circuit, X7R were replaced with NPO. There was a contact failure of the resistive matrix for 4W. Carefully soldered and everything worked. Front/rear switch and everything is fine.I got 34470 and compared the measurements of the reference on ADR1399.As for me, 34401, released in 2000, has preserved its PPM very well))I am delighted with 34401.

author: Korchahin
author_rank: Regular Contributor
timestamp: August 28, 2025, 03:17:49 pm
content: After almost a month of using the HP34401, its readings have changed slightly and they are close to the readings of the 34470A.Self-calibration?Infected with PPMs?Or normalization of electronics after a long downtime in the warehouse?

author: coromonadalix
author_rank: Super Contributor
timestamp: August 28, 2025, 03:29:21 pm
content: could be an heat transfert of the instrument(s) on top of it, i would make 1 inch space clearance around and on top of ityou are not optimal, it's not how i have my instruments, each one has it's own tablet, no mechanical contacts with others around themmy 2 cents

author: iMo
author_rank: Super Contributor
timestamp: August 29, 2025, 05:31:22 am
content: For "precise" measurements you may need to know the internal temperature of the meter - as the meter has got some TC (you have to know that TC as well). 34401A follows the ambient temperature with some delta_T (like 17-18degC) and with some propagation delay (like 30minutes)..PS: also it may take many hours/days to get it settled after power on..

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 03, 2025, 04:03:40 am
content: out of curiosity i record a difference cold - hot for about dozen 01A10V , turn ON , wait 5min, record; in 24 hours - record ... differencethe best was 36 microvolts , the bad one 137 microvolts. 11A was 47 uVaverage tend to 60 80h ish .. somethingnewer with round button shows better drift comparing to rectangular one , except one without back bracket and front glass - 43 .. I guess due to cooling effect of holes , so it not counted ..I'm still thinking of addon , to measure internal temp, get a data by AR488 , recalculate data ( 7 digits + temp correlation + stats + graph , and throw it on some large screen ..( wild think ... large sensor screen as a add-on insert to the front as replacement to existing controls .. no need to do anything just clip on front style , But i just imagine the amount of coding thingi ..)

author: iMo
author_rank: Super Contributor
timestamp: September 03, 2025, 06:08:34 am
content: The simplest hack would be to mount a 10k NTC thermistor inside the 01A and enable the temperature measurement in the fw. There are 2 free pins in the RS232 rear connector, so you may access the thermistor via those pins (mount the thermistor isolated such you do not short inguard and outguard grounds).More complicated is to have an external MCU doing a "data fusion" - reading the first thermistor (or any temp_sensor) placed inside the dmm, the second/third outside (for ambient and a DUT temperature) as well as the serial dmm data (adding the time from the RTC), making the filtering/smoothing/calibration/temp_calibration etc. inside the MCU, thus creating a long .csv record upon each 10/100NPLC measurement, the record you may save then somewhere. I've been doing that for years - using two LM35 sensors, transmitting the fused data via bluetooth into my smartphones, where I've been logging the .csv records with help of a terminal app. My 01A display is constantly off, and I read the actual data off my smartphone.

author: mawyatt
author_rank: Super Contributor
timestamp: September 03, 2025, 01:11:43 pm
content: What is the typical TC folks are seeing with the older HP and newer AG34401As operating around room temp 25~26C?BTW like the idea of using a couple DB-9 RS232 unused pins for an internal temp sensorBest

author: iMo
author_rank: Super Contributor
timestamp: September 03, 2025, 03:16:13 pm
content: The typical TC of 01A in my case is aprox +0.5ppm/C in 10V range from 38C till 46C internal temperature (that is aprox 20-28C ambient), pretty linear fit.PS: I've been using the 2 free pins like this - one pin is +5V from the outguard's 5V regulator (for powering the stm32f103+2xADS1110+HC-10_BT+DS3231_RTC and some LEDs), the second pin is the output of the internal LM35 temp sensor (powered from the outguard 5V) glued on the shield above the 01A's ADC, read by the ADS1110.. I would recommend to use an ADS1115 instead (4 inputs), for example, but I had bunch of ADS1110s at that time handy (1 input).You may wire an LCD display to the MCU and have the fused data there (instead of using the 01A's display).I also planned to switch off the filament of the internal display (several Watts), but have not found the suitable place for the switch yet....it is 6y already..https://www.eevblog.com/forum/projects/thermostating-the-hp-34401a-meter/msg2617395/#msg2617395

author: trobbins
author_rank: Frequent Contributor
timestamp: September 04, 2025, 12:40:22 am
content: iMo, did the approx +0.5ppm/C include an 'open air' front panel 4mm plug connection to your 10V ref, or bare copper wire wedged into front panel sockets, or ..?To minimise the tempco contribution from my 34401A, from previous threads I've also implemented the LM35 and RS232 socket internal measurement capability, as well as psuedo-buffer the enclosure top/bottom/sides and rear from ambient, and modified the internal heating situation by soldering in copper tab heatsinks around the regulators and using a 240V to 210V bucked mains feed. I turn off the display (and use Patel's software), but also haven't attempted to de-energise the display filament supply. I have a make-shift foam cover for front panel 4mm connections, as I tend to use generic commercial cable interconnects. The nominal internal temp is circa 45C for 23C ambient, but I haven't tried to log a 34401A specific tempco.

author: iMo
author_rank: Super Contributor
timestamp: September 04, 2025, 07:38:22 am
content: The tempco of my 01A was measured many times, where I used banana plugs, and the front panel incl. wires and plugs was covered by several layers of thick cloth (as well as the entire meter). The internal temperature slowly rose from ambient (in winter from aprox 18C) to aprox 50C. Also I compared that TC with many measurements done at "stable" ambient temperatures between 20C and 26C. You also have to consider the TC of your external 10V reference.For very precise measurement you would need a temp chamber and wait for several hours (like 6) for the meter to settle at each temp step, however.PS: I have been using an internal thermo sleeve (aprox 3mm thick foam) and the internal temperature is aprox 18C higher than ambient. Your is delta is higher, but that is perhaps matter of the LM35 mounting place. Never considered the mains voltage (a good point to look at), but my final data are adjusted by the internal temperature and the known TC_dmm, so it does not matter much in my case.Another hack could be the mounting of an internal fan, to speed up the initial warm up. I collected small size fans from blade servers, but again, there is not much place for a switch in the rear (two switches - for the filament and the fan).The switches could be avoided when using a small 8pin mcu inside the meter, driving a relay for filament (based on some internal signal when the display gets off) and and for fan (based on the internal temperature).A nice Sunday's afternoon project, indeed..

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 04, 2025, 05:42:03 pm
content: my # really approx4 round units S\N 47 around 0.3 (at best) to 0.54 old "red" S/N 32 () - 0.5-.83 old black - 0.4 - .6it about the same but round was a bit better ... and really coarse approx ...i live in underground facility my ambient 18-21C all year round , if +35 outside , it +22 .. it doesn't count ..

author: felixd
author_rank: Regular Contributor
timestamp: September 07, 2025, 04:31:11 pm
content: Maybe that can help You a bitI have started long term monitoring of old CCCP Voltage reference using four 34401A I own.Voltage reference:https://github.com/felixs-lab/hardware-vref-iona-cccpYou will find there:- AC voltages- Environment data (room temp, humidity, pressure)- inter-instrument comparison etc.DDMs under test:*  AGILENT 34401A MY45037309*  AGILENT 34401A US36109397*  HP 34401A US36067486 - my first 34401A*  HP 34401A FLAMEIT001 (my recent buy - old HP 34401 - most probably 1998 version. Serial number is missing)You can notice stable room tamperature for last few days. I have managed to finally keep the room closedIt will stay that way for 7 more days.Grafana:https://monigra.flameit.io/public-dashboards/502ed47be1574d5aaeb2303422a8f8fb

author: iMo
author_rank: Super Contributor
timestamp: September 08, 2025, 07:14:22 am
content: Hmm, your meters show identical waveforms at the sub 1uV level, as I can see..That is weird, as with the 34401As their noise at 8V might be 4-7uVpp (100NPLC), imho..PS: could it be the heavy smoothing did the the trick?

author: TUMEMBER
author_rank: Regular Contributor
timestamp: September 08, 2025, 10:25:09 am
content: Quote from: iMo on September 08, 2025, 07:14:22 amIt's just the "mathematics of drawing a graph" that caused the graphic similarity. Details are lost because they're too fine for this scale. These aren't digital clones; they have different DNA when enlarged appropriately.

author: felixd
author_rank: Regular Contributor
timestamp: September 08, 2025, 11:06:43 am
content: Quote from: iMo on September 08, 2025, 07:14:22 amYes I have made averaging on data. Please check TOP LEFT chart labels/names. Yellow circles.Furthermore if You click on individual names on charts You can see individual data (red lines on image below). It works on every chart. If You hold CTRL and click names You can select data You are interested in.ALL DMMs are set to 100 PLC and autorange, which strangely causes oldest HP 34401A with 3-1-1 FW to autorange ffrom 10V to 100V with no reason during that run. You can see some individual, solo downspikes with 4th DMM.Last attached picture shows difference between individual DMMs (1-2 / 1-3 etc). Data in one graph is 1h averaged, one is not.EDIT: I have also updated first graph in Grafana. I have duplicated it removing averaging from one. So You have now "pure 100PLC DMM data" there.