Author Topic: HP 34401A hacks and upgrades  (Read 16671 times)

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Offline cellularmitosisTopic starter

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HP 34401A hacks and upgrades
« on: April 26, 2018, 12:48:59 am »
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.   :-DMM

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! :wtf:), 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.
« Last Edit: April 26, 2018, 01:00:17 am by cellularmitosis »
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Offline cellularmitosisTopic starter

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Re: HP 34401A hacks and upgrades
« Reply #1 on: April 26, 2018, 12:50:51 am »
(pics)
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Offline cellularmitosisTopic starter

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Re: HP 34401A hacks and upgrades
« Reply #2 on: April 26, 2018, 12:53:11 am »
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.
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Offline rhb

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Re: HP 34401A hacks and upgrades
« Reply #3 on: April 26, 2018, 01:03:05 am »
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.

In which case, why not a crimp?
 

Offline cellularmitosisTopic starter

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Re: HP 34401A hacks and upgrades
« Reply #4 on: April 26, 2018, 01:06:37 am »
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.

In which case, why not a crimp?

Good question.

I have noticed that the selected zeners (1N829A) in EDC gear are soldered in place, which is an interesting counter-point.
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Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #5 on: April 26, 2018, 01:17:54 am »
hello Jason,

nice work  :-+

now if you could replace the MV106 with a series connected 6x NiCd cells totalling around 7.2V
and 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
 

Offline cellularmitosisTopic starter

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Re: HP 34401A hacks and upgrades
« Reply #6 on: April 26, 2018, 01:21:22 am »
hello Jason,

nice work  :-+

now if you could replace the MV106 with a series connected 6x NiCd cells totalling around 7.2V
and 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

Use the NiCad cells to find the noise floor of the ADC / frontend?
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Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #7 on: April 26, 2018, 01:23:32 am »
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
« Last Edit: April 26, 2018, 01:32:42 am by zhtoor »
 
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Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #8 on: April 26, 2018, 01:41:11 am »
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.

maybe some kind of factory adjustment procedure by plugging in a test-pod?

-zia
 

Offline Mr. Scram

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Re: HP 34401A hacks and upgrades
« Reply #9 on: April 26, 2018, 01:56:57 am »
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! ;D
 

Offline kj7e

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Re: HP 34401A hacks and upgrades
« Reply #10 on: April 26, 2018, 02:14:10 am »
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. 
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #11 on: April 26, 2018, 06:47:04 am »
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.
 
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Offline TiN

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Re: HP 34401A hacks and upgrades
« Reply #12 on: April 26, 2018, 08:27:40 am »
I remember someone over 38hot.bbs did LTZ mod for 34401A.
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Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #13 on: April 26, 2018, 11:11:08 am »
Quote
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.

hello,

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-B
2. 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
« Last Edit: April 27, 2018, 12:12:12 am by zhtoor »
 

Offline cellularmitosisTopic starter

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Re: HP 34401A hacks and upgrades
« Reply #14 on: April 27, 2018, 06:13:38 am »
I remember someone over 38hot.bbs did LTZ mod for 34401A.

I think I found an LTZ mod for the 34410A: http://bbs.38hot.net/thread-69139-1-1.html
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Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #15 on: April 27, 2018, 07:30:52 am »
Quote
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.

hello,

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-B
2. 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
There 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.
 
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Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #16 on: April 27, 2018, 07:54:46 am »
There 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.

thanks 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 datasets
represented by AB and BA can be used to determine the base-line measurements of both reference sources.

best regards.

-zia
 

Offline Dr. Frank

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Re: HP 34401A hacks and upgrades
« Reply #17 on: April 27, 2018, 08:47:21 am »
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
 
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Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #18 on: April 27, 2018, 09:17:40 am »
...

thanks 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 datasets
represented by AB and BA can be used to determine the base-line measurements of both reference sources.

best regards.

-zia
There 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.
 

Offline rhb

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Re: HP 34401A hacks and upgrades
« Reply #19 on: April 28, 2018, 12:13:22 am »
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.
 

Offline zhtoor

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Re: HP 34401A hacks and upgrades
« Reply #20 on: April 28, 2018, 12:47:03 am »
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.

are we assuming that the noise introduced by the ADC alone is random?
(assuming that the reference is noiseless - batteries)

-zia
 

Offline rhb

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Re: HP 34401A hacks and upgrades
« Reply #21 on: April 28, 2018, 02:57:24 am »
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.
 

Offline floobydust

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Re: HP 34401A hacks and upgrades
« Reply #22 on: April 28, 2018, 10:41:32 pm »
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.
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #23 on: April 29, 2018, 08:20:11 am »
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.

AFIAK 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.
 

Offline aronake

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Re: HP 34401A hacks and upgrades
« Reply #24 on: March 24, 2023, 04:24:07 pm »
How has this developed? Anyone modified their 34401a with a LTZ1000 with or without sucess?
 

Offline GigaJoe

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Re: HP 34401A hacks and upgrades
« Reply #25 on: March 24, 2023, 09:38:01 pm »
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
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #26 on: March 24, 2023, 10:15:33 pm »
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.
 

Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #27 on: March 25, 2023, 10:18:20 am »
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.
 

Offline aronake

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Re: HP 34401A hacks and upgrades
« Reply #28 on: March 26, 2023, 06:43:52 am »
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.
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #29 on: March 26, 2023, 07:02:10 am »
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.
 

Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #30 on: March 26, 2023, 08:19:09 am »
..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.. :)
 
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Offline GigaJoe

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Re: HP 34401A hacks and upgrades
« Reply #31 on: March 26, 2023, 06:55:06 pm »
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.
 

Offline GigaJoe

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Re: HP 34401A hacks and upgrades
« Reply #32 on: May 17, 2023, 03:00:31 am »
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.

 

Offline floobydust

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Re: HP 34401A hacks and upgrades
« Reply #33 on: May 17, 2023, 04:39:10 am »
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.
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #34 on: May 17, 2023, 06:33:30 am »
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.
 
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Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #35 on: May 17, 2023, 08:41:32 am »
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"..
« Last Edit: May 17, 2023, 03:49:31 pm by iMo »
 

Offline GigaJoe

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Re: HP 34401A hacks and upgrades
« Reply #36 on: May 18, 2023, 04:19:04 am »
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.


 
 
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Offline Andreas

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Re: HP 34401A hacks and upgrades
« Reply #37 on: May 18, 2023, 05:42:41 am »
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
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #38 on: May 18, 2023, 08:22:42 am »
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.
 
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Offline Hydron

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Re: HP 34401A hacks and upgrades
« Reply #39 on: May 18, 2023, 08:29:25 am »
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).
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #40 on: May 18, 2023, 09:37:38 am »
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.
 

Offline GigaJoe

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Re: HP 34401A hacks and upgrades
« Reply #41 on: May 18, 2023, 02:40:44 pm »
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 .
 

Offline Ecogeek

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Re: HP 34401A hacks and upgrades
« Reply #42 on: May 20, 2024, 10:54:21 am »
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.
 

Online trobbins

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Re: HP 34401A hacks and upgrades
« Reply #43 on: May 20, 2024, 01:46:17 pm »
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.
 

Offline Kleinstein

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Re: HP 34401A hacks and upgrades
« Reply #44 on: May 20, 2024, 02:05:30 pm »
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.
 

Offline Ecogeek

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Re: HP 34401A hacks and upgrades
« Reply #45 on: May 22, 2024, 04:50:24 am »
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.
 

Offline MK

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Re: HP 34401A hacks and upgrades
« Reply #46 on: May 22, 2024, 11:13:07 am »
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.
 

Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #47 on: May 24, 2024, 04:03:06 pm »
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.
 

Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #48 on: May 24, 2024, 04:09:49 pm »
..
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?
 

Online trobbins

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Re: HP 34401A hacks and upgrades
« Reply #49 on: May 24, 2024, 11:06:36 pm »
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.
« Last Edit: May 24, 2024, 11:12:49 pm by trobbins »
 

Offline Ecogeek

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Re: HP 34401A hacks and upgrades
« Reply #50 on: May 30, 2024, 12:01:44 pm »
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.
 
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Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #51 on: May 31, 2024, 04:49:26 am »
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).
« Last Edit: May 31, 2024, 04:52:04 am by iMo »
 

Offline Andreas

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Re: HP 34401A hacks and upgrades
« Reply #52 on: May 31, 2024, 05:50:46 am »
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 regards

Andreas


 


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