HP 34401A hacks and upgrades

[cellularmitosis]

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.

[cellularmitosis]

(pics)

[cellularmitosis]

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.

[rhb]

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?

[cellularmitosis]

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.

[zhtoor]

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

[cellularmitosis]

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?

[zhtoor]

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

[zhtoor]

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

[Mr. Scram]

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!

[kj7e]

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. 

[Kleinstein]

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.

[TiN]

I remember someone over 38hot.bbs did LTZ mod for 34401A.

[zhtoor]

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

[cellularmitosis]

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

[Kleinstein]

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.

[zhtoor]

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

[Dr. Frank]

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

[Kleinstein]

...

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.

[rhb]

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.

[zhtoor]

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

[rhb]

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.

[floobydust]

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.

[Kleinstein]

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.

[aronake]

How has this developed? Anyone modified their 34401a with a LTZ1000 with or without sucess?