Author Topic: Teardown : Fluke 845A/AB/AR nullmeter/HZ voltmeter tweaks and mods (and repairs)  (Read 37807 times)

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Offline TiN

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Quote
My 845A is lying mostly disassembled in a corner, looking at me, whispering "why am I not repaired yet".
Mine already gave up whispering... just looking with one remaining eye...slowly getting covered by layer of dust.  :'(
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Offline Squantor

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Mine already gave up whispering... just looking with one remaining eye...slowly getting covered by layer of dust.  :'(

One of your transistors had some broken off pins, right?
 

Offline TiN

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Yes, but after fixing that something else got broken, as after assembly it wouldn't work at all.
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Offline Squantor

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I found the Fluke 845 a pain to work with, how it is put together is annoying and reaching the various parts or replacing them requires a lot of work.

I recently repaired a hp419A, it is much easier to take apart and reach all the components. It is now my main null detector and it has a new rebuilt batterypack too. I only have the 845A version and I have noticed that to get low noise levels a battery pack is very useful.
 

Offline Conrad Hoffman

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Yeah, I've got a couple 419s and they're really a pretty decent null meter. They're on the shelf right now with various problems, and I've been using the 845 for quite a while. I think its performance has deteriorated, as the output is very noisy. Have to decide what to fix, as the 845 is such a pain. One of my 419s developed leakage between the front banana jack and the panel, probably some corrosion in the hole. Not sure what shape the batteries are in- I rebuilt the pack, but it's not been charged in a couple years. Probably dead and gone. Sometimes Fluke just has a special knack for mechanical design... not!
 

Offline Vtile

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Every poor 845s can be send to me, so I can expose them properly.  :)
 

Offline martinr33

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I looked at the posted schematic, and TiN's pictures, to understand the circuit. .

You could use the other AC winding to drive the optofets. Fluke drove their board off of the rectified DC supply that also uses this winding, so we know there is plenty of current.

The 5V regulator on the Fluke update board is the power source for the optocoupler emitters. The optocoupler photodiodes are controlled by the transistors on the board, driven off of the AC feed through an optoisolator. This design should make  the chopper really consistent, and more independent of input voltage shifts.

For a minimalist solution, you could replace the LDRs with the optocoupler transistors, and drive the emitter side from the 12V winding through a 500 ohm resistor. Wire the emitter diodes back to back, and you only need one resistor, as each diode will only conduct on its half cycle.  This approach will be well superior to neon or CdS cells, even without the regulator circuits.

The residual trimmer is surprisingly simple - it just feeds a little current into the zero side of the chopper input. I am thinking it is there to deal with coarse differences between the optoisolators.   



"Hello Martin R,

My 845A is lying mostly disassembled in a corner, looking at me, whispering "why am I not repaired yet". I wanted to replace the neon+LDR chopper for a long time but I am probably going with a different route.

I have a bunch of equipment that either uses mechanical or LDR+Neon choppers to create the autozero functionality. I am investigating a replacement for them all by using a small custom wound transformer (using a high Al pot core with shielding) and optofets to recreate the chopping function. The reason I am for a small custom wound transformer is that I want to reuse the neon drive windings but the current is too low on those. The optofets need a substantial current to really go into low resistance mode (30ma according to the datasheet of the H11F1).

Have you reverse engineered the optofet chopper PCB that was posted here? I am interested in the residual zero trimming part. So the 5V regulator is there just to provide a DC bias so the differences between the optofets is canceled?
This might help that you dont need to match the optofets so tightly."
 

Offline Conrad Hoffman

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I don't know if this trick is applicable here, but when I use LEDs for something where neon bulbs are used, I try to make them turn on in a similar manner. That is, a neon bulb only turns on and conducts suddenly when you exceed 90 volts or so. With a sine wave, the bulb may only flash near the peaks. Scaling everything down in voltage, I'll put a zener in series with the LED so it turns on later in the driving waveform. Two LEDs and zeners if I need bipolar operation like a neon. The pairs also protect each other from excess reverse voltage. I do this with strobe lights for turntables to get a shorter flash and better resolve the bars. No idea if the technique is useful for choppers, but throwing it out there if needed. If the lamps are driven from a square wave, it shouldn't be, but with a sine wave it might help.
 
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Offline Squantor

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I don't know if this trick is applicable here, but when I use LEDs for something where neon bulbs are used, I try to make them turn on in a similar manner. That is, a neon bulb only turns on and conducts suddenly when you exceed 90 volts or so. With a sine wave, the bulb may only flash near the peaks. Scaling everything down in voltage, I'll put a zener in series with the LED so it turns on later in the driving waveform. Two LEDs and zeners if I need bipolar operation like a neon. The pairs also protect each other from excess reverse voltage. I do this with strobe lights for turntables to get a shorter flash and better resolve the bars. No idea if the technique is useful for choppers, but throwing it out there if needed. If the lamps are driven from a square wave, it shouldn't be, but with a sine wave it might help.

I have actually did some tests with replacing the mechanical chopper of a GM6020 DC millivolt meter from philips. As a mechanical chopper does not switch instantaniously from signal to the ground, using a zener to introduce a similar dead time is a good idea, not that the chopper worked that well but I did not pay that much attention to matching and the meter needed some some repairs first. I always wondered/worried about possible cross conduction creating a large offset.

If you look for instance at the original neon circuit of the Fluke 845A(B) is that they use a capacitor in series. So it is no hard turn on or off, but is this to momentarly boost the voltage so the neon ignites?
 

Online Kleinstein

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The oscillator might depend on the capacitor used to couple the neon tubes. At least one can expect the neon tube part of the circuit influence the frequency and amplitude. I would expect the waveform to be something in between a sine and a square, so some dead time for the neon tubes, but not that much.

With modifying the circuit (replace the neons) it is a question on how much of the original circuit one would really like to keep. Keeping much of the circuit would be think of keeping it as an historic instrument. In this case the logic path would be to get some new neons.

I don't think that even with replacing the LDR/neons with a photo-mos or similar parts would give really good performance. It is not just the neons that are outdated. I new design based on an AZ OP can likely give better performance and could be low power battery operated - just the analog output would be a little more difficult and power hungry.
 

Online lukier

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I new design based on an AZ OP can likely give better performance and could be low power battery operated - just the analog output would be a little more difficult and power hungry.

Do you have any example schematic of a modern null-meter? I would rather DIY something instead of buying antiques. Every time I'm thinking how it would work I end up with some sort of a multimeter-like front end (ranging resistors -> op amp comparing that vs fixed reference and outputing zero if equal, all battery powered). I don't see where would the very high impedance came from - it would have the impedance like multimeters, 10M or whatever the setting of the input divider.  :-//
 

Offline Conrad Hoffman

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The 845 is nice because of the high input impedance, but remember that in a true null application it often isn't necessary. After all, with zero voltage difference, current approaches zero, even if the meter is low impedance.
 

Online lukier

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The 845 is nice because of the high input impedance, but remember that in a true null application it often isn't necessary. After all, with zero voltage difference, current approaches zero, even if the meter is low impedance.

I see. So, for example, if I want to compare 2 references and connect them back to back, then a normal DMM is fine to measure the voltage difference?
Also, Fluke 5440B manual asks for a null-meter to calibrate some of the ranges (with the help of Fluke 752A - I'll try to DIY a Hamon divider instead, thanks to your website  :-+), but again this is around zero comparison (divided reference vs calibrator) so a decent DMM should be fine? (and on these low ranges the impedance is huge anyway).

I guess that explains why null-meters are rather extinct species, so what are the modern uses where one is still required?
 

Offline Vtile

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It were known early on as poggendorff compensation method.  ...Well similar current cancellation method that is.
« Last Edit: August 29, 2017, 09:55:25 pm by Vtile »
 

Offline Dr. Frank

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The 845 is nice because of the high input impedance, but remember that in a true null application it often isn't necessary. After all, with zero voltage difference, current approaches zero, even if the meter is low impedance.

I see. So, for example, if I want to compare 2 references and connect them back to back, then a normal DMM is fine to measure the voltage difference?
Also, Fluke 5440B manual asks for a null-meter to calibrate some of the ranges (with the help of Fluke 752A - I'll try to DIY a Hamon divider instead, thanks to your website  :-+), but again this is around zero comparison (divided reference vs calibrator) so a decent DMM should be fine? (and on these low ranges the impedance is huge anyway).

I guess that explains why null-meters are rather extinct species, so what are the modern uses where one is still required?

Are you aware, that the 5440/42 are Autocal /artefact calibrators in reality?
Once calibrated by a 752A for all 4 direct volt ranges (10, 22, 250, 1000V), the gain constants will not change greatly (old instruments), and the internal gain calibration is fully sufficient.
Only the 10V basic calibration might be necessary, only a small drift is to be expected, but the divided 2.2V/ 220mV ranges need frequent re-calibration.
carefully read the addendum of the service manual..

By the way, nobody here, or in the volt-nuts mailing list has ever checked the real leakage / bias current of this famous 845A.. it's not specified in this sense, so i think it's overrated.
Frank
« Last Edit: August 29, 2017, 10:13:25 pm by Dr. Frank »
 

Online lukier

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Are you aware, that the 5440/42 are Autocal /artefact calibrators in reality?
Once calibrated by a 752A for all 4 direct volt ranges (10, 22, 250, 1000V), the gain constants will not change greatly (old instruments), and the internal gain calibration is fully sufficient.
Only the 10V basic calibration might be necessary, small drift expected,, and the divided 2.2V/ 220mV ranges need frequent re-calibration.
carefully read the addendum of the service manual..

Yes I know, but well I got my 5440B rather cheap and it had a lot of issues. The most minor one is that it is missing the Display Board PCA, so while I'm designing a replacement out of 16 segment green LED displays, shift registers (digits), small MCU and 20x2 LCD (text display), I'm using it via GPIB only.

The real problems were that the +-17 inguard supply was dead, due to the power resistors - one of the 1K balancing resistors (R22) for the +-17V. While I was repairing that I've noticed that 0.15 Ohm resistor for 5V outguard looks like it has seen the better days.

After repairing that, cleaning the relays with isopropanol and pieces of paper and replacing all the electrolytic caps except the huge Sprague ones on filter boards. I've picked quality caps, min 105 deg C (originally there was a lot of 85 deg C) - this machine thermal management is ridiculous, everything runs super hot, not only the ovens and one finds darkened spots on PCBs a lot.
Then I discovered (thanks self-tests!) the unit had more problems - around Analog DAC PCA.

The voltage reference was very low - it turned out the DAC wanted to draw ~100mA from the reference, loading it. I quickly removed the Vref board from the oven and provided 13V from a linear PSU when debugging not to stress these poor SZA263 anymore.

Long story short, it turned out Q9, Q10 and Q12 JFETs failed on short or open. Some NPN (AFAIR Q13) also failed. Unfortunately, Q9, Q10, Q29 and Q30 are rather special matched set.
I've picked AFAIR J107 JFETs and matched the parameters (thanks nifty transistor tester!) and on resistance as good I could (Fluke specifies within 1 Ohm) and soldered them. Very difficult to get JFETs these days. I've replaced also the NPNs with the same part - 2N2369A.

I bet it was the Analog DAC PCA that has taken out the +-17V supply.

Now all the self-tests pass without any problems. Therefore there are few things left for me to do:
- finish display board replacements,
- measure the huge filter boards caps, I hope these don't need replacement as they are extremely expensive,
- replace the fan with brand new EBM Papst 4800Z (already bought),
- calibrate!

For the last point I need to finish my LTZ1000 based 10V standard first and maybe someone from the calibration club thread, with a recent Volt at home, can help me with that. Once I'll bring 10V home I'll try to run only the internal calibration, but I'm prepared that after such repairs the full calibration might be necessary - therefore the need for 752A or DIY alternative. Also, for the 2.2V/220mV ranges.
 

Offline Edwin G. Pettis

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Dr. Frank:

I have to disagree with you, null meters was/are still are an important instrument, I still use them and I know others who also use them.  There is a procedure in the Fluke manual, paragraph 4-22 which tells how to measure the leakage resistance and at null, a properly operating null meter does achieve an input resistance of near infinity at balance, no DVM or auto-zero Op Amp is going to get anywhere near that plus there is more bias currents from the DVM and auto-zero op amps than a null meter at balance.  The null meter is the superior instrument for use with bridges, I use them in my lab all the time, all of my bridges use null meters, whether on board or external.  They can achieve a voltage measurement of as little as 10nV resolution, a 3458A cannot do any better than 50nV resolution and the null meter is continuous unlike the DVMs.  I've tried using DVMs in place of null meters, not near as good, I've used auto-zero op amps for null detectors, good but still lacking a bit.  I even use a vacuum tube null detector for special purposes which actually outperforms the solid state units.

I have several 845 units and two 419As, while the 419 is a bit easier to work on inside, I still prefer the 845, for a few years, ESI put the 419As in their 242 bridges (mine has one, it hasn't worked in some time), ESI dropped it (unknown reason), I'm using a Keithley 155 as an out board detector right now.  If you're wondering, I simply didn't have time to fix the 419s and 845s are all busy.

A null meter can make a world of difference when attempting to make such sensitive measurements, forget the DVMs and put the null meter back where the procedures call for them.

 
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Offline Conrad Hoffman

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OK, drifting far OT and out to sea, but it's also amazing the sensitivity of measurements made in the past using mirror galvanometers and telescope or projection methods. A true null meter, though not very high Z by todays standards.
 
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Online Kleinstein

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A mirror galvanometer can be really sensitive to low DC voltages - no problem measuring thermocouples and similar. The big advantage over an electronic solution like the Fluke  845 is that there is definitely no bias current.  Usually the input impedance is not that high - but this may not be such a big problem with a null meter. It is only a problem before getting the adjustment right. With a galvanometer noise limit and impedance are coupled - electronic amplifiers can get better than that limit, at least at not so low frequencies.

The 845 still uses a kind of chopper amplifier at the input. So one has to expect some input bias current, though not that much. At least expect something like input offset (without compensation) times input resistance. Even the LDR can create some offset / bias, due to unwanted photovoltaic effects.
A chopper stabilized OP in a non inverting amplifier configuration would have a similar (or like higher than the LDR version) high input impedance in the > 10 G Ohms range. Not many AZ OPs seem to specify input impedance. For the ADA 4522 (low noise rather high bias) a value of 100 GOhms (common mode) is given.

This would apply to low voltages like up to maybe 2 V. Only higher voltages would need an input divider and would thus be lower impedance, this also applies to the 845 !. A tricky part might be to add protection to ensure high impedance also with higher voltages. Just a series resistor like in the 845 is a compromise toward input noise.

To get a very low bias, one might want to combine a modern Null-meter with an TIA to measure pA range currents as well, so one could adjust / check the bias.
 

Offline martinr33

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On the 845 and impedance...

The 845 is an AC design. It is bizarre by modern standards, but it is what you did with awful transistors - and it works well. The input comes through a capacitor, and the output - to the meter - is through a transformer. The display meter does rectification and averaging to give the result. There is a DC section in the middle of all this, but it carries the AC signal.

The input impedance is mostly going to be the DC leakage through the two LDRs (one of them is always on). Then, there's an AC component to the input impedance through a 0.1uF capacitor that is grounded through a 9M resistor. At 86Hz, this won't be much.

Aside from the ranging resistors, there's not much in this unit that matters from a precision perspective. I'd suspect that the neons are the biggest problem, followed by noise in the front-end transistor.

It would also take a really high input voltage to do any damage. I think you could put 1kV in on the 1uV range, and not see problems. The manual warns against doing that if you disconnect the 1M resistor in the input stage, need to figure out why. When the input is at zero, there's no path for current except for LDR and capacitor leakage. Upgrade those parts, and the null impedance is enormous with a well-protected input.



Good upgrades that preserve the philosophy, and the very fine chassis, of the instrument would be:

        - Optocouplers for the chopper, with a clean drive
        - Better front-end transistor
        - Digital 86Hz generator
        - improved DC gain stage (precision op amp) (or maybe rengineer the whole chain)
         - maybe upgrade the input cap

Could also do a simple optical output, driven off of the output square wave. The on/off ratio would indicate the null point, and maybe even the percent of range.


The AC design; environmental and electrical isolation; precision divider; the isolated output; would all be preserved. 





 

Offline TiN

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Sounds like a great few year long project  :-+.
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Online Kleinstein

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The 848 has good input protection through the dioden CR101 / CR102 and the 150 K resistor (R110).

The amplifier it not that unusual, its a chopper amplifier with DC feedback. The unusual part is having the output with an extra isolated stage that uses the same chopper frequency, but starting from the DC signal at TP9. For me, the only thing odd is they way the voltage for the DC offset adjustment are generated / taken from the circuit.

Leakage through the LDRs is attenuated by the loop gain of the amplifier. So it is reduced quite a lot.

There is an AC input resistance for the AC amplifier (1.2 M + 500 pF to GND and the input current to the transistor) that would act back on DC through the chopper. Like the LDR leakage it is also attenuated from the loop. However an loop internal offset could cause some bias current. If needed one should be able to add an adjustment for the offset of Q107/Q108 and thus the input bias. I might be interesting to know the input bias - from the circuit I would not expect it to be that exceptionally low (maybe 10s of pA range).

I am not so sure that building an chopper amplifier from discrete parts would be needed to day. At least one could do better today, especially by using an JFET instead of Q101, as this could give lower noise and higher input impedance (resulting in less bias).
 

Offline martinr33

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The Fluke opto upgrade added a circuit for DC offset into the low-side optofet (well, a pot connected to 5V). So they clearly agree with your diagnosis!

Although the "output" amp is DC coupled, the AC signal carries through the whole circuit. The use of the chopper signal in the last stage is interesting. They are effectively resquaring the signal. Still amazed by that use of the meter movement to average the AC output.
 

Online Kleinstein

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I don't see the signal going all the way through as AC. As far as I understand the circuit, at TP9 the signal is DC (following the input) and the part around the transformer is just a kind of DC/DC converter or isolation amplifier with reasonable (good enough for an analog meter) accuracy. It is only a kind of convenience / saving parts that both parts use the same clock - today one would ideally use a something like 2 times the clock for the isolation stage (and the power supply), so that there would be less possible interaction.
So we can treat those two parts separately. If wanted one could replace those 2 parts separately with maybe an AZ OP and a different type of isolation amplifier. Just for the meter, I don't think one would actually need the isolation. The isolation is mainly for the recorder output.

There are two places for an offset adjustment. One is for a possible offset voltage at the input, like done with R117. However that can be a second place where an DC offset can occur: That is in the DC amplification part from Q107/Q108 on. A DC error in this area (e.g. from mismatch of Q107/Q108) would result in requiring a correction signal from the chopper stage and thus a not fully vanishing AC signal at the input when the meter reads 0 V. The DC error can be corrected via R117, however this would also cause an Bias current. So ideally one would have a second adjustment for the DC stage (e.g. make R148 or R150 adjustable), so that one could adjust the local error of Q107/Q108 here and this way have adjustment for offset voltage and bias current. I don't think the opto-upgrade added a separate trimmer for the DC stage. As far as I understood it they just replaced the adjustment via R117.
 

Offline Dr. Frank

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Dr. Frank:

I have to disagree with you, null meters was/are still are an important instrument, I still use them and I know others who also use them. 

Mr. Pettis, I did not make any statement about the importance of null meters.

I simply referred to the fragmentary specification, which lacks the bias current of this instrument.

This instrument is mystified in the metrology community, also by its 2nd hand price, like having 'infinite resistance', and that only THIS instrument can be used in the commonly known bridge arrangement, like with standard cells, or precision dividers as 752A, 720A, and also for the calibration of the 5450A.

There is a procedure in the Fluke manual, paragraph 4-22 which tells how to measure the leakage resistance and at null, a properly operating null meter does achieve an input resistance of near infinity at balance, no DVM or auto-zero Op Amp is going to get anywhere near that plus there is more bias currents from the DVM and auto-zero op amps than a null meter at balance.
 The null meter is the superior instrument for use with bridges, I use them in my lab all the time, all of my bridges use null meters, whether on board or external.

Sorry, Mr. Pettis, that is exactly this false mystification, overrating, or simply a misconception about the 845A, that I want to deflate.

I refer to the original manual of the 845AR from about 1975, as the online versions are incomplete.
The specification on page 1-1 only mentions :

"INPUT ISOLATION
Better than 10^12 ohms at less than 50% r.h. and 25°C regardless of line, chassis, or recorder grounding... with driven guard, isolation improves .. to 10^13 ohms."

This only parameter about high ohmic behavior, is further described on page 4-5, in paragraph 4-19 'Leakage Resistance Test', with Fig. 4-10.

It is evident, that this parameter and associated test describe the isolation resistance between the chassis ground, recorder ground and power ground, which are all shorted together, versus the Guard and Null- Input /  Common only.
It makes absolutely no statement about the bias current of the Null-amplifier-Input, nor its input resistance.

The null-amplifier also does not achieve 'near- infinity resistance at balance'.. that statement I've read very often, and it's wrong, or simply not appropriate.
Instead, the 845A input resistance is simply a fixed 10MOhm in the low voltage ranges, from 1mV down to 1µV.. and that's exactly so specified in the manual.

What you obviously mean instead, that in the case of perfect balance, there will not flow any current across the bridge-legs, as zero Volt divided by 10MOhm gives zero current..

Speaking about 'Infinite Resistance' is simply not the correct parameter to describe 'zero loading' of the bridge.
Instead the flowing cross-current, created by the possible bias current of the null amplifier, has to be considered.
Well, and this important parameter of all things is NOT specified.

Any other FET chopper amplifier, like in DVMs, or also in the 7650/7652A OpAmps create bias currents on the order of  several pA, up to 20..50pA.
This can be measured quite easily, also on the 845AR/AB.
Maybe, that this photo chopper principle is really superior, but this is to be proven, not to be assumed.

A 2nd remark :
The 845A draws current for the case of non-perfect balance, which is a used case in bridges, when you want to monitor differences and their drifts, like a standard cell versus a fixed 1.018V output.

If you measure 1mV difference, that would at least give 100pA, plus the possible bias current of the chopper.

In comparison, a 3458A draws 20pA at most,  for all voltages between 0V and 10V.
For this used case, of differential mode, modern DVM are superior over the 845A.



  ... They can achieve a voltage measurement of as little as 10nV resolution, a 3458A cannot do any better than 50nV resolution and the null meter is continuous unlike the DVMs.  I've tried using DVMs in place of null meters, not near as good, I've used auto-zero op amps for null detectors, good but still lacking a bit.  I even use a vacuum tube null detector for special purposes which actually outperforms the solid state units.

The 845A can't resolve 10nV, but the 3458A resolves very well 10nV, in its 1V and 100mV range.
 I think, you mix up here a different parameter, and that's noise.

Low noise nV - meter,  do not have necessarily very low input bias currents, no, in contrary!

There exists analogue NV meter, but also nV DVMs, like the HP34420A, and the Keithley 182A/2182A.
These are superior over the 3458A in aspect of noise, but they both have higher input currents.




I have several 845 units and two 419As, while the 419 is a bit easier to work on inside, I still prefer the 845, for a few years, ESI put the 419As in their 242 bridges (mine has one, it hasn't worked in some time), ESI dropped it (unknown reason), I'm using a Keithley 155 as an out board detector right now.  If you're wondering, I simply didn't have time to fix the 419s and 845s are all busy.

A null meter can make a world of difference when attempting to make such sensitive measurements, forget the DVMs and put the null meter back where the procedures call for them.


It's fine, that you own these old instruments, I also would like to have one, as these have some advantages over many DVMs, like high isolation, guarding, high common mode operation, and battery mode.

But anyhow, your appreciation for them is not appropriate.
Otherwise, these instruments would still be in production by FLUKE (and the other manufacturers), as FLUKE still sells bridge type instruments, like the 720A, 752A, and so on.

Fluke themselves instead promote their 8508A, and prove, that its performance as a null-meter in such bridge application can very well replace these old analogue instruments.

The 3458A also makes a very good null-meter.
It has a guard also, and the Isolation Resistance to case ground is also 10^12 Ohm, like the 845A.
It is specified having < 20pA bias current, which the 845A is lacking.
I determined this bias several times in my bridge applications really being less than 10pA, depending on the common mode voltage used.

It also has very low noise down to 0.01ppm @ 10V range, that is on the order of < 100nV rms. The 0.1V and 1V ranges deliver 20nV noise, only.
Please visit our DMM noise comparison test here in the forum.


These parameters are fully sufficient for the different bridges that I had to calibrate.

Maybe an even lower bias current, and the battery mode like the 845A would give an improvement.

So I would like to ask you, please really measure the bias current of your 845A, and present your results here.


Frank


« Last Edit: September 01, 2017, 12:53:28 pm by Dr. Frank »
 


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