HP 34401A hacks and upgrades

[Kleinstein]

The XADin signal is not critical. So the TL071 is good enough there (even the new 071H would do).

For the actual amplifier the OP27 should be good enough - it is well tested (the 34401 has a relatively good reputation when it comes to INL).
With the 34401 the 1/f noise is not as critical as with the Keithley meters and the impedance is not too high, though 10 K are on the high side for an OP27.

The extra zener idea is just in case if there are issue (like stuck at one rail with not enough supply to the OPA145 left after power up) - it would be 13-14 V types, 15 V with the extra forward diode would already be rather high. The transistor circuit may well not be susceptible to this issue.

The OPA130 specs for the noise are still only 50 nV/sqrt(Hz). This is not great, but not that bad with already 100 K from the prodection that give a fixed 41 nV/sqrt(Hz). So lower noise would help, but not a huge difference. How noisy the amplifier actually is can still vary - not all units are the same, especially in the LF range.

[GigaJoe]

any advice on replacement MC34081AD , it running on 37V ..   TLE2141AIDR - ?

[iMo]

..read the TLE2141's revision history (Chapter 8 ) first.. 

[GigaJoe]

oh .. another ne5532 ...
so what would be an option to replace MC34081 in the input amplifier ..  +-Vs exceed 36V

im battle with sporadic noise of one 01A , it not gaussian at at all  , random\sporadic, can go to 20-30 LDS on 10V ( count it you see +100 microvots, P-P  300uV , but may show exact # for a couple iterations) , amplitude related to input voltage lowest at short highest at 10V input.
my highly suspicion it custom HP chips internal problem, but having some effort to cross all stuff around it ..

so far im curious about  MC34081  ;  settling time spec probably most important for that particular application .
and N-channel dual JFETs - consider it SST404

[Kleinstein]

The MC34081 in the Dc input amplifier should not be that critical. Chances are one can get away with a TL071H. It is a bit noisy, but should be OK with the high supply. I don't see a real need for very high speed.

[GigaJoe]

well it doing this ...  (upper level cut )
so I'm thinking need something with good settle time ....   ( my second thought to pimp it  -up , so I'm playing with component on not a good one , to see how they fit )


so far mad scientist list of change:

all tantalum to polymer 600uF
any 18 V shorted by electrolytes ( 4-5 i think ).  (actually positive, especially one ceramic that shorted+ and - 18, 36V total,  i add 2 polymers 100uf )

ADC:  ADA4523-1 + OPA1655  -  compare as much a i can against 34411A , consider wild noise , so far seem linearity not destroyed, no oscillation. 
DC gain - ADA4522-1
AD706 - ADA4075-2  , 5 in total (  dont like 3uV \ temcp , but for a test ok )

left:
74HC4053D
MC34081
SST404  ( ..5 ,  ..6 ) will try - LSK489 ( what would be as a test replacement for SST404 one , even 2 discrete N Jfet , any at 50-60 V)

so again, assumption HP chip behave , or some PCB vias - not likely, as i apply forces there and there - it indifferent

[Kleinstein]

All those changes to the 34401 roughly bring down the 10 PLC noise from some 1.6 µV ? for the original to some 900 nV and hardly below 850 nV with even more changes.  The one small change that may still make some sense would be changing the 74HC4053 to an SN74LV4053  (Ti version). This would reduce the jitter. In the spread sheet I get a noise drop from some 890 nV to 860 nV. This is not much, but may still be visible. The design just reaches its limits.

The ADA4523 is not a good choice at the integrator, because of the current noise. In addition the switching spikes (from the AZ OP and reference switching) can cause upsets and interference in both directions. With the resistors for input protection there in limited use of a lower noise amplifier. Similar the ADA4075 would fail due to too much current noise for the relatively large resistors in the reference amplification. An ADA4077-2 or OPA2205 would be the better upgrade (but still only a rather minor effect).

When you consider changing so much, not much of the original is left and one could as well build a new DMM or DVM. This would allow to also change to parts that are really limiting:  limited ADC resolution at 1 PLC, resistor ratio at the ADC that causes a high noise gain and the related run-up pattern with relatively little variable part, the rather high feedback frequency that makes jitter and non ideal reference switching more of a problem.

As a complete new design my DIY DVM gets a noise of some 500 nV for 1 PLC. I have not tested 10 PLC and averaging gives the sqrt(10) advantage from oversampling or some 160 nV. The main improvement is from less noise gain at the integrator and more resolution  already at 1 PLC (from an extra rundown step before reading the residual charge). The core parts, like LV4053 switches, OPA141 for the integrator and ORN resistor arrays are not that special or expensive, though still good performance.

[iMo]

FYI - I've been playing with 100k/800n RC low pass and the ADR1399#1 in Vref. The results are similar to the 47k.
The TEMA is after a TC compensation, showing some negative TC caused by ADR1001#1's TC.
The ADR1399#3 is in burn-in, in several weeks I will try with it too..

I doubt you may get something better off the 34401A after all the changes I made.

[GigaJoe]

I'm agree .. consider the noise level almost nothing much possible done ...  an option would be to look at TC level. to drop it down a bit...

for this particular unit 10NPLC in the voltage range 100mV i have short - 1.2uV noise P-P ,  with 10V range 10V apply - roughly  250 uV P-P . I would probably put LM358 everywhere and hardly see a difference. For that particular case .

I look at ADA4523  , seems current noise it about the same as installed OP27 at 1Khz  ; but in overall flat spectrum .. for switching i did look as well:  330Khz switching ,  and this paragraph:  "The ADA4523-1 uses circuitry to suppress these spurious artifacts  to below the offset voltage. The typical ripple magnitude at the  330 kHz chopping frequency is less than 1 µV rms."  Is my assumption in this application for chopping 1/f noise are incorrect ?

[Kleinstein]

The noise with 10 V applied does include reference noise. Still 250 µV_pp looks too high. So more like a defect / poor part to fix. The easier test for comparison is with a shorted input. This removes much of the reference noise (internal / expternal) from the test.
1.2 µV pp would be 200 nV RMS noise and this also a bit high for the 100 mV range. Part of this could still be the higher than normal ADC noise.
So this unit would be first more about a repair and not so much an upgrade.
A first step would be getting measurments to see how bad the noise actually is. I would use a short and look at 1 and 10 PLC mode (AZ enabled) for both the 100 mV and 10 V range.

They try to make the residual chopping spikes small, but when looking at best performace it is not a good idea to have an AZ amplifier with 330 kHz range chopper in an ADC with 375 kHz feedback modulation. One may well get some intermodulation effects - even of nothing happens there is the uncertainty of maybe an extra linearity error of glitches.
With no 1/f part the current noise would not be as bad as the OP27, but still 1 pA * 15 kohm give a contribution like 15 nV/sqrt(Hz) of voltage noise.

If one wants to tweak the ADC gain TC, one could adjust the 42 K resistor to ground at the integrator input. A larger resistor should give less effect of the switch resistance TC and thus a change towards a positive gain TC. I would roughly expect some +1 ppm/K with 50 K. Increasing the resistance would have even a slight positive effect on the noise, a smaller resistor however also a bit more noise.

[GigaJoe]

well , there is no option to change PLC , in 01A  Fast\slow 6 digit , i believe corresponding to 10 100, and same for 5 digits.

so ...

SET SLOW 6 DIGITS
here is integrator output , input shorted, means Vinput == Voffset to simplify it:


zoom to it slow 6 digits, 33kHz


FAST 6 DIGITS


zoom to fast 6 digits , about the same


and same zoom to FAST 5 DIGITS, same frequency


so integrator working with same frequency  generating the same rate of dataflow , regardless of PLC, and then some software calculation ....
I'm correct ?

so in hardware point of view, if it working on the same frequency, producing same digital dataflow , where is difference to change PLC and draw conclusion from it , where is my mistake ?

I'm sorry, "375 kHz feedback modulation"  I'm not quite understood about feedback modulation ...

[Kleinstein]

The integrator works just the same, all the time. With a shorted input there is also no difference visible between a shorted input or an internal zero. So the integrator and feedback from the reference would be the same all the time and one would not see difference between data rates. The only difference is how often and when the µC internal ADC is read and used to calculate results. There should be one reading, than switching the input path and with maybe 0.5 ms range delay a 2nd µC internal ADC reading. The first reading is the end of one conversion and the 2nd reading the start of the next.

The frequency at the integrator still looks a bit odd. The 33 kHz is still a mistake from the scope: from the screen picture it looks like a little more than 1 unit of 5 µs. AFAIK the frequency should be a little higher. I don't know for sure, but there might be slightly different versions.
The pattern also looks odd, there should normally be a mix of short and longer sections.

[GigaJoe]

174 kHz ..  ( honestly i didn't count , just look at freq. meter ... )
so your reference to 375 kHz feedback modulation related to this frequency ?  now i get it ... as i assume 30KHz and whatever switching spectrum would be far away ....  ok .. need to think ..

[Kleinstein]

The 174 kHz would be roughly half of what I remember. There was also a different waveform with extra wiggles. To get 3 µs range feedback steps this would need the the up and down sections to be separate feedback steps. So the Frequency is not that much off - I mainly miss the extra wiggles to make sure to a fixed number of switching events.

One possiblity could the use of a different feedback pattern, following the US5200752 patent. The waveform looks a bit like this. At least with my ADC implementation it did not work out and I got INL issues. I don't understand the exact reason and with different hardware it may not be all that bad.

[iMo]

The signal at the OPA140's (OP27) output is a square aprox 2.66us low and 2.66us high or aprox. 4us high and 1.3us low. See my scope pictures..

https://www.eevblog.com/forum/metrology/hp-34401a-hacks-and-upgrades/msg6266784/#msg6266784

[GigaJoe]

damn ...   that it ....

Good unit:  input 10V:
from DC ampl perfectly square 10V [ADIN] going to HP chip Pin15  ; output Pin21 around 100mV   [ I would really happy if someone explain to silly man, why output not  match with schematic ]
R440:  https://i.postimg.cc/Y01vjwWM/SDS00032.png
and integrator's OP27 output as iMo posted 600 mV .. ish

noise unit:
DC ok 10V square [ADIN]  in P15;  output Pin21 - 30mV [bang!]
R440:  https://i.postimg.cc/x1F1fd4H/SDS00035.png
and integrator's OP27 output around 30mV  [bang!]

so it roughly running at much lower input , making more noise ... and all my mumbo-jumbo definitely drop some noise , as i see some improvement but didn't cure.  bandages doesn't heal .. 

now:  as i suspected HP chip defect ... schematic lie and 100K resistor really not  between P15 - P21 ...  and now what : Bandage some few hundred K resistor P15 - P21 to bump voltage ?  silly but may work ..

PS: and funny thing , it able to pass test, and accept calibration, and even shows acceptable result , consider noise level, and skewed adc voltage input ... wow ...