HP 34401A hacks and upgrades

[iMo]

..and the ADEV/MADEV OPA140 vs. OP27..
NSD as well with 2 mysterious tones.. (??)
All 10PLC against ADR1001#1 (10V).

[Kleinstein]

The low frequency peaks (if measured with a shorted input) could be some kind of idle tones. With a zero reading the PWM ratio is close to 1:1 and the remaining small difference gives a slow drift that than from time to time gets reset with an extra phase. This can give a rather low frequency. With zero input the AZ switching is not disturbing this, as both conversion are for a zero reading.
Ideally the ADC should not care about the integrator average voltage shifting, but there can be small nonlinear effects (e.g. DA, loading) and also the maybe not perfect scale factor for the µC internal ADC that still gives a small error.

The OP27 and OPA140 will have a slightly different offset and bias current and this could be enough to shift the frequency.

[iMo]

Above are the idle tones with 10V input.
In my replay #187 there is NSD with shorted inputs and OP27 and there is an idle tone at aprox 0.15Hz.
The tones are rather large amplitudes, btw..

[iMo]

OPA140 output..
AD711 output (after the 3.16k resistor)..

[iMo]

I installed the thin foam insulation (around the inner frame) I've been using since 2019 (not used during the above experiments in the last 3 weeks). Here is the initial experiment with it. It supresses the noisy thermal flows inside the box.

After installation of the foam the 2 idle tones above disappeared and I see only single one (smaller in amplitude) on a different freq. I repeated that 2x, so it seems it is temperature related (perhaps thermal oscillation?)..

[Kleinstein]

It does not need much to change the frequency of the beating from the idle tones.
Thermal oscillations are possible, but a frequency in the 0.5 Hz range is relatively high for this.

[Dr. Frank]

..
 
It supresses the noisy thermal flows inside the box.

After installation of the foam the 2 idle tones above disappeared and I see only single one (smaller in amplitude) on a different freq. I repeated that 2x, so it seems it is temperature related (perhaps thermal oscillation?)..

I'll have a look in your link afterwards.

I'm surprised, that there should be a thermal flow inside, w/o a fan and a completely closed case.

In the 34465A, Keysight made exactly this thermal design flaw, i.e. the fan blows exactly across the reference and the aligned circuit  , creating a lot of noise. I added a plastic box all across this reference area, problem solved.
Maybe, that's a solution for you as well. Sorry, I haven't got the link to my article right now.

Frank   

[Dr. Frank]

I've just studied your other thread.

That seems to be a different story.
I think, your solution with the inner insulation is counter productive, as it rises the inner temperature too much.
The problem with all (high) precision bench meters is their T.C., which are in the same ballpark, even for the 3458A, 8508, etc.

The 3458A has an overall T.C. in its 10V range of about 0.5ppm/°C, mine has 0.45ppm/°C, lower grade ones are more like 1..2ppm/°C, both is not great.

Other 8.5 digit meters, like the Datron 1281 has probably a slightly lower T.C. 0f 0.25ppm/°C as well in the 18..28°C temperature range (oddly specified), as a lot of Vishay BMF had been used, but its direct descendants 8508A, 8588 etc. have as well a T.C. of 0.5ppm/°C, but they are not able to adapt to varying room temperatures, like the 3458A.

Mine has about 0.15 ppm/°C from the reference and 0.3ppm/°C from the A/D circuit; latter can be corrected by ACAL.
So I could reduce its reference T.C. to zero, and would be left with the 0.3ppm/°C for the A/D for the experiment run.

Therefore, high precision drift experiments well below 1ppm are not much easier, neither my regular transfer measurements of 12 sources within about 20 minutes transfer time.

Your room is not really a lab environment, isn't it?

I have a basement lab with constant temperature (+/- 0.2°C for an experiment run over 24h, +/- 2°C over the whole year), no draughts as window and door are always closed, so I don't have any problems with either the 34401A, nor with my 3458A with air draughts and I can handle the T.C. issue quite well. Over these 20minutes, the R.T. does not change more than 0.1 .. 0.2°C (caused by the operator in front of the instrument), so the drift is currently limited to max. 0.1ppm transfer uncertainty.


Concerning these fast variations you see, when you have a breeze inside your room: Do you have the handle installed?
W/o it, that would be the only way, how an outside breeze could enter the instrument and create such short termed disturbances. Otherwise, my 34401A is completely tight (handle installed), and I could not figure out, where otherwise the outside breeze could enter.


The best and only chance for you is to have a better room with mostly constant R.T. during experiments, no air draught in the lab, always monitoring the R.T, and best the inner temperature of the 34401A, as I do it with my setup and the 3458A.


Currently, I leave my 3458A always in CAL mode. If I want to make very precise absolute measurements, I make an CAL 10 against my reference bank before the experiment. Nor the 3458A neither no other DMM is a 'standard', anyway, just an adjusted scaling device with quite bad uncertainty.

So maybe you could do it the same way, if you have a reliable and "better" external reference available.

Another idea is to do RATIO measurements against a known good external reference with the usual low T.C. of about 0.02ppm/°C, so not to compromise your calibration. The 34401A also got this feature, and that would delete all such T.C. problems, additionally its uncertainty problem (35ppm/y.) using the 34401A as a scaling instrument only.

Btw.: Please try out this great RATIO function, by ignoring its mediocre and sloppy specification by HP. This is a real hidden gem, if your DUT uses as well the same 10V range, as the reference voltage on SENSE.
In this case, not the individual 24h or 365d specification applies, but the transfer specification.
For the 34401A, even latter is as well very sloppily specified.
The 3458A has 0.1ppm transfer spec, for 10 min +/- 0.5°C.
This includes both, its INL of the A/D, i.e. 0.02ppm of range plus temperature drifts over 10min.
Therefore, the RATIO function on the 3458A should give even better results, as 2 consecutive measurements are made, within a few seconds. So, you' re even left basically with the INL error of the A/D.
Same applies to the 34401A. Its INL is specified as 2+1 ppm, but my unit is
about 0.5ppm only.
Therefore, using RATIO on the 34401A might give an additional (to the reference's) uncertainty of 0.5ppm of range and practically zero T.C.

Frank

[Kleinstein]

There can still be thermal fluctations and even thermal oscillations with a close case. I have seen this would by DIV DMM with also a close case of somwhat comparable dimensions. In my case I got some 0.1 Hz and 5 mK range amplitude. This is with low power (some 3 W) and with higher power in the 34401 it could be larger amplitude. Another point is that the amplitude can change with location, and more massive parts could not follow very fast.
Due to the often small amplitude I would not expect the 34401 to be much effected from thermal oscillations - it could be a thing for the 100 mV range and maybe the related 34420 nV meters.

Thermal isolation is not a good idea, as leakage currents tend to go up with temperature and it would cause more temperature swing from warm up and thus more warm up drift. Quite some of the older meters without a fan already ran a bit warm to start with.
To reduce thermal oscillations it would be more about adding metal shields / heat sinks that better conduct heat and this way reduce heat transport via convection flow.

The TC of DMMs can vary quite a bit between individual units. To a large part there is TC matching involved and the error there can go both ways.

[GigaJoe]

in my understanding his case a bit different , as the device has internal temp sensor, and he calculated compensated voltage taken over GPIB and temp. sensor. havin constant voltage source kinda simplistic to build a correction curve per specific range.

@iMo:
have you disassociate  voltage regulators from PCB ?   i think it also produce a temp gradient ...

[iMo]

My thermal insulation does not increase the inner temperature much, perhaps 0.5C at the aluminum pcb shield. The meter follows the ambient so the ambient changes are way much larger than the difference caused by the insulation. It is about 2mm thick foam, from one side "hairy".
It supresses the temperature fluctuation measured at the aluminum pcb shield strongly, no doubt.
My understaning is there are intense flows between the pcb and outer box without it.

I am not saying there are there thermal oscillation in the electronics, but they could exist.
The thermal shielding affects the "idle tones", indeed. I will investigate more..

DMM TC compensation - I am not doing any TC comp with 1399 and OPA140 yet, that is a difficult exercise. It will be done after finalizing the replacement exercise and after the final adjustment.

With HP399 inside I made TC comp and it worked fine. The TC was aprox +0.47ppm/K, with the 1399 it looks be lower. With the HP399 meter did +6ppm from cold, now it is +3ppm. Not sure what was the contribution of the OP27.

>>> have you disassociate  voltage regulators from PCB ?   i think it also produce a temp gradient ..

Everything inside is producing temp gradients. The pcb is hot for touch even opened. Not only around the voltage regulators. I would not allow to pass the meter into production as-is, frankly..
PS: yep, I know it was created to be cheap..

[iMo]

..
Btw.: Please try out this great RATIO function, by ignoring its mediocre and sloppy specification by HP. This is a real hidden gem..

Yes, I reported the same couple of months back last autumn here (using ADR1001#1_10V as the ref), the ratio works nice even the math is not float64. I will do it again after I close this exercise.

PS: here is a sliding STD over 10samples (40secs) at 100PLC against my Vref ADR1001#1.
You may guess when I was not sitting in my lab working with PC close to the meter and Vref.
My cable is twisted copper, copper to copper into the 34401A, but nickel to copper at the ADR1001#1 box Vref.
Also the other thermals and EMI apply.
 

[iMo]

As the leakage of the capacitor in the RC low-pass (placed at the front of the AD706 in Vref) seems to me the most critical component (its leakage) in the above exercise I replaced the 47k by the 8k2 again (after a brief chat with Kleinstein).

Below a run with the 47k and 8k2 (both with the 800n green MKT capacitor, 10PLC).
Both measurements aprox at the same ambient temperature spread profile (mean with 8k2 by 1degC higher).
On the first glance when looking on the 10PLC raw data with the 8k2 are a little bit more hairy, the mean of the sliding STD (40secs) is higher by 10-20nV. The NSD's knee of the 8k2 version at 0.1Hz is higher by some 15-20% as well.

What is different is the "longer term stability" which varied some 8uV with 47k, and only 2.5uV with 8k2. That is visible on the ADEV too. My bet that is caused by the capacitor, which could be forming itself (the 8k2 measurement was made after the 47k one), and/or its leakage is still high (and depends on temperature exponentially).

[Kleinstein]

The variations on the longer time scale can still depend on the enviromental variations. This can be the temperature, but also wind and the use of doors causing pressure variations that effect the temperature inside the case by fresh air comming in / out.
There can also be just random variations with the ADR1399 reference.
So much of the small difference between the curves may be just random and not directly from the resistor change.

[GigaJoe]

Back to ADC, op-amp replacement , would this variation be better: OPA140 → (same divider) → OPA1655

[Kleinstein]

Noise wise there should be essentially no difference between a combination of OPA140 with AD711 or OPA1655.
There might be a small difference with the linearity, but it is not clear. It may as well not be the limiting part.
For the combination OPA140 (11 MHZ)  and OPA1655 (55 MHz GBW) one would ideally also change the divider a little.
The ADC design is limited by the feedback pattern set by the ASIC and the µC internal ADC performance. It is just not made for high performace, but for low cost at it's time.
There is a point where a complete new build is easier (and can get better performance) than tweaking the old ADC with its limitations.

[GigaJoe]

DC amplifier module 3, make sense to replace ?
U106-A OP27
U103,U153  AD706
 

[Kleinstein]

There is little need to replace the amplifier parts. The amplifier is not great from the overall design (source followers and OP27 behind), but it is also not that bad. U103 and U153 are not critical - here already the AD706 is overkill.
The 104 K for the protection give likely more noise than the amplifier. For symmetry reasons the amps ranges also have a 100 K resistor in series.
One could have a look at the 100 mV or amps range to check the amplifier noise, but I would not expect a weak point here.

With the 34401 the amplifier is a bit less critical than with the Keithey 2001, as they do the AZ math better (though simpler) and are thus not as sensitive to 1/f noise.

[Hydron]

Later 34401As (including mine) seem to have a different input amp with a bootstrapped OPA130, as detailed here: https://www.eevblog.com/forum/repair/34401a-bumbling-repair-attempt/msg3739384/#msg3739384

Any comments on whether it's worth looking to improve that version? (No idea whether the newer amp is better or worse than the old JFET one)
Or the ADC architecture still makes any change futile?

[Kleinstein]

An upgrade of the OPA130 might be worth it. The obvious upgrade that is also suggest by Ti is the OPA145.
From my experiance with a bootstrapped amplifier is that it might help to limit the voltage range (e.g. a dual zener where C1 is).
I would still first test how noisy the 100 mV / 1 A range is. The improvement would be mainly with these ranges with gain, not with 10 V range.

[Hydron]

Thanks, the OPA145 recommendation lines up with my thoughts too (lower BW compared to options like OPA140 is probably a good thing here I'm assuming). I assume the dual zener suggestion is in case there's potential instability in the boostrapped amplifier (especially with an opamp change)? I'd assume that say ~15V would be about right here?

If I do mess with anything I'll certainly do some testing on the more sensitive ranges first (and try and do some comparisons with other meters first before I mess with this one, probably best to leave it well enough alone if it's still within say 1yr cal specs, as I'd rather avoid adjustment if I can).

I will be going ahead with similar changes on some other meters (e.g. my Keithley 181 with bootstrapped AD542 as an input amp, though I'm sure that's far from the worst issue in that meter given the late-70s/early-80s design!)

[GigaJoe]

here is translation:
HP34401A  A new version DC amplifier circuit

This opamp OPA130 (or TL071C) has a servo circuit added to maintain a consistent input bias
under different input voltage so it can be removed.

I maybe miss something .. but it seems incorrect .... i maybe wrong as  I didn't trace it.
but TL071C are output of "XADIN"  as inversion of of "ADIN"
where R196, R197 are same as old schematic...

voltage drift for OPA130   2-10 uV/C
same for TL071C + more noise  ...
OPA140, 145 in my view would be much more suitable ...

can be OP27 -> OPA189 ??  ( i know it chopper, but how much input switching ( 10k load) and noise in theory may affect

BTW.  changing opamp , basically changing offset + temp drift ... so your CAL data will offset in some scale of offset difference  + amplification for that region ...
again im not 100% but it seems DC only, so if you have a second similar device , you can measure something stable by second device record # for first and second,  change chips on first
set voltage exact as was recorded by the second unit, and for adjustment punch an input voltage on first unit as it recorded initially ... i think it will work ..

[Hydron]

Thanks, I didn't spot the TL071C (will need to look in my own meter) - I assume it's non-critical though as it's just biasing the comparator point for U501?

[GigaJoe]

you mean "XADIN" - for comparator , for "0" cross comparator ?

[Hydron]

Yes it looked like XADIN is just for biasing the comparator, though I only had a quick look at the service manual. I am not quite sure why they are using the inverted input for this.