HP 34401A hacks and upgrades

[GigaJoe]

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

[Kleinstein]

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.

[iMo]

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.

[aronake]

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.

[Kleinstein]

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/#msg4768076

The 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.

[iMo]

..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..

[GigaJoe]

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.

[GigaJoe]

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 well

so 50uf poly i add as shunt to adj 317\337  ;  and special 2700 low esr to output

fast 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.

[floobydust]

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.

[Kleinstein]

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.

[iMo]

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"..

[GigaJoe]

so 
play with 5V bus - and really nothing much
shielded display cable by sticky aluminum foil  - nothing much

but ...  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.


 

[Andreas]

Hmm,

the 34401A is very non-linear below 0.5 mV AC

https://www.eevblog.com/forum/metrology/low-ac-voltage-measurement-issues/msg3842051/#msg3842051

with best regards

Andreas

[Kleinstein]

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"..

A 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.

[Hydron]

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/#msg4644106

I 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).

[Kleinstein]

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.

[GigaJoe]

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 .

[Ecogeek]

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.

[trobbins]

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.

[Kleinstein]

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.

[Ecogeek]

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.

[MK]

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 .

With 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.

[iMo]

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.

[iMo]

..
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.

What is the TC of your meter? Where you put the temperature sensor?

[trobbins]

iMo, I followed your comments from https://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.