Teardown : Fluke 845A/AB/AR nullmeter/HZ voltmeter tweaks and mods (and repairs)

[SZA263]

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.

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

Frank

The input resistance of the '845' made after 02/1979 is 1M not 10M. Please see Fluke Change/Errata information issue No: 2 2/79 #6282.
Is easy to measure the bias current of the 845: with open  input, the voltage developed across the input resistance (R104-1M) is due to the bias current flowing in it.
Connect a polystirene 1nF cap (with very clean legs!) across input posts, common linked to guard, to reduce switching artifact.
Accurately zero the instrument on 1 µV range, and wait few minutes to thermal stabilization.
Switch to "OPR" and allow a little time for stabilization.
Now you can read the voltage on the meter: is a little hard to read the result but all my 3 '845' (2 AR & 1 AB) agrees that this is well under 100 nV.
So the input bias + leakage current should be well below 100 fA (yes, femtoampere!)

[Dr. Frank]

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.

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

Frank

The input resistance of the '845' made after 02/1979 is 1M not 10M. Please see Fluke Change/Errata information issue No: 2 2/79 #6282.
Is easy to measure the bias current of the 845: with open  input, the voltage developed across the input resistance (R104-1M) is due to the bias current flowing in it.
Connect a polystirene 1nF cap (with very clean legs!) across input posts, common linked to guard, to reduce switching artifact.
Accurately zero the instrument on 1 µV range, and wait few minutes to thermal stabilization.
Switch to "OPR" and allow a little time for stabilization.
Now you can read the voltage on the meter: is a little hard to read the result but all my 3 '845' (2 AR & 1 AB) agrees that this is well under 100 nV.
So the input bias + leakage current should be well below 100 fA (yes, femtoampere!)

Great, thank you for these measurements..
Do you have the old neon choppers inside, or already the latest Opto-Fets?

How do you measure these 'switching artifacts', you obviously mean the switching spikes / glitches, and how big are these?

Frank

[SZA263]

Hi,
 I have a old neon, but I'm working to switch one unit to optofet, with a simpler circuit than the original Fluke (why use optocouplers on control side when they share the same input/output Ground connection?)
Yes I mean spikes. I have measured them with a Tek 7A22 amplifier, 10 µV/div, filtered DC to 1 kHz.
With inputs connected with a low capacitance cable, they have a peak value of about 10 µV (+-) with a fast risetime and an exponentially decay falling edge. The duty cycle is about 1-2%. The frequency is clearly that of the internal oscillator.
During measurement the two input impedance are in parallel so the result is 500 kohm//~100pF. After the transient the input return to 0 V, without any visible offset/histeresis (at least visible on a scope).

Adriano

[MisterDiodes]

Dr Frank:
You need to know that Fluke still does maintain 845's and even supply them refurbished-as-new when required when they have a very lucrative support contract - In a Fortune 100 cal room working on the latest laser diodes and quantum devices we use analog null meters for precise, quiet measurements all the time.  This is an upgraded 845 and not the old neon-bulb stuff on eBay, and that's why you don't find these for sale.  Keithley's will be maintained to new condition also when the price is right, it just depends on the customer.  The fact is that no DMM can replace a null meter in every case. 

When time is money:  With analog null meter you can get a valid measure in the some 10nV range or lower several times while you're waiting for a 3458a to Autocal, and no squiggly charts and spreadsheets required.  And the fact is 10nV on a 3458a is down in the bottom layer of mud anyway - not exactly high confidence or accuracy.  Many times we have to resort to an analog null meter to remove the noisy DMM from the measure - that's when a quiet null meter is the far superior tool.  Especially when you're trying to measure a few photons at a time on some exotic diode detector.  We do that all the time.

For a complete, accurate measurement, you use ALL the tools and techniques at your disposal to lower uncertainty and raise confidence.  We have 3458a's, null meters, 732's, 752s, 720a, RV722, automated switch gear all sorts of other precision tools - new and old equipment working side-by-side every day.  Use what is required to get the best possible measure.

When comparing null meter function:  yes a 3458a will work as a null meter, but a real analog null meter works best as a real analog null meter, and gets you an accurate, quieter result much faster if you know how to use one. 

IN essence:  A null meter is essentially a virtual near perfect two-terminal device at null, and at null most of the interior circuitry is balanced out, current offsets should be balanced out to zero, no energy is consumed driving a balanced analog meter needle at zero, its DC amp section should be doing absolutely nothing at zero, and at most there is a slight amount of energy coming from the input chopper switches gate capacitance (maybe a few pF driven by a fairly slow rise time chopper in the case of '155 - but that is mostly shunted in the input filter caps).  There shouldn't be many "meter" electrons wanting to flow anywhere at null, and virtually zero emitted EMI from the meter...unlike a relative VERY noisy DVM with running CPU's, oscillators,  input amps with bias current even at zero differential, noisy display drivers, power supplies and all that rot.  It's not the 3458a's fault, it's just always a consumer of energy and emitter of noise whenever it is powered up - null condition or not.

By the way - We can upgrade those analog null meter input chopper P-fets for even less leakage when they switch off - down to a few pA with newer parts from Linear Systems.  You really can get some great performance out of these rigs and they really hold their own against any DMM, even today.  It just takes a few upgrades.

For certain: Analog null meters in some shape or form are an absolute essential piece of gear in any serious cal room, and for some measure tasks where you need absolute quiet: For some applications they run circles around any DMM, but we love them all!

[Kleinstein]

The 845 circuit, both in the old neon and the newer photo-fet version does not look like it is low noise or super low bias. For the noise, already the 150 K resistor in series with the input gives a considerable unavoidable noise. Another likely significant noise source is the input transistor (BJTs don't get a very low noise figure). From these 2 sources alone the input noise is expected to be at least in the 50 nV/sqrt(Hz) range. So not a really low noise.

Due to the chopper operation there is some AC superimposed to the input - seems to be visible despite of filtering.

If the later stages of a chopper amplifier and the charge injection are not well adjusted, effects like an offset of the DC stage will cause an bias current - due to the not so high impedance AC amplifier this could be significant. At least in the circuit shown further up in this thread I see no adjustment for the input bias current. So far I have some doubt the 845 could compete in noise * bias current with modern Az OPs.

A null meter can have its justification. However the 845 circuit does not at all looks like a really good solution, definitely not noise/drift wise and likely also not with bias. Depending on the application low bias current or low offset / noise would be more important. So one can not expect a single meter type to fit all. I am not so sure an analog reading is better than a well made digital display. Analog filtering tends to be slow in settling - digital low pass filtering could be of FIR type and thus superior in settling. Modern µCs with everything inside the chip should make it relatively easy to avoid excessive EMI. The 845's chopper is also not that quite. Really quiet would be the old days mirror galvanometer - lower impedance, but no bias by design.

[doktor pyta]

Kleinstein,

I can make some measurements using 845A if You describe the setup, I have two in the lab.

P.S. the input transistor of 845A have really high hFE (selected part). I recall values like 500 or so.

[MisterDiodes]

So just for fun I took a Keithley '155, zero'ed it out and let it sit on zero check all night on 1uV scale...and noted a slow gentle  drift of around 50nV or so around zero, but just about no overall drift overnight.   The spec on this meter is a conservative 500nV per 24 hrs, but we've never seen one that extreme.

TEST:
So just now I took a metal film 1Meg 1/4W resistor and laid that on the '155 inputs, zero'd out the meter then switched to Operate - and once it settled down noted an apparent bias on the needle of maybe 600 nV, mostly resistor noise.  This particular meter could also use a touch up adjust on internal current cancel pot but let's just say it's in an "everyday" condition.  I think if we're conservative and called it 1pA bias current, the meter would -definitely- be below that.

Take the same resistor and attach directly to the inputs of an in-cal 3458a, DCV mode, Auto Zero ON, 100 NLPC and noted about 10uV apparent offset, very repeatable. So around 10pA apparent bias if my math is correct.  In other words the 3458a is throwing in an order of magnitude increased bias current at around zero volts input differential voltage.

Remind me again why a DMM is supposedly so much better as a null meter with less bias current & noise than a working analog unit? I'm looking at the two side by side as I write this.

Agreed - 3458a's make very good VOLT meters, but for detecting a near zero-volt condition, an analog null meter is hard to beat, especially if you need quiet and low bias current at null.  I'm partial to the '155's for being a bit more quiet in general, but some of the refurb 845's in the cal room are nothing to sneeze at.

[Echo88]

I also own a K155 and would like to ask which amount of noise is visible on yours MisterDiodes? On mine i see about 100nVpp in the 1µV-range after being on for about 2h. Ive never really cared about it, since im not really into bridge measurement techniques (yet). But since you guys mention visible 10nV resolution ive got the impression that my K155 is maybe faulty regarding the noise?

[MisterDiodes]

My unit B will usually stay right around +-50nV of zero, as a slow drift back and forth - generally very symmetrical around zero.  Leave it on for a while before zeroing out - at least a few hours to stabilize, like with any other piece of precision equipment.  Unit A will drift slow maybe around 60~75nV overnight but again no real overall drift.  You learn what the needle looks like as the terminals reach thermal equilibrium.

Applies to most Null Meters, not just '155:

Sounds like yours is somewhat in the ballpark...Make sure your batteries are good and you might go thru the cal procedure in the manual.  If your batteries have leaked at some point you want to get any gunk off the PCB.  Also replace the electrolytic caps, although these don't see much ripple current they will tend to get tired anyway.  Keep the PCB very clean at all times, and remember that some of these are light sensitive - so keep the lid on in between cal adjustments.

Keep your lab temp constant, and keep all air drafts away from the front terminals. Use the guard circuit.  Learn how to use that mirror scale, it is there for a reason!  Always make sure you have your eye positioned so the needle is lined up exactly over the mirror (the needle and its mirror image are as one), or if you have a slight offset the true needle position is halfway between the apparent needle position and the -image- of the needle in the mirror.  That mirror gets rid of the parallax error of needle position.  After a while you'll do that automatically whenever you see a mirrored analog meter.

You can also use your DMM to read the output of the null meter, if you like going digital - but usually we're after the really quiet aspect of these meters - one of the reasons we're going with a battery powered null meter is to avoid the noise a DMM introduces to the test.

Bear in mind on the 1uV scale just bringing your hand near will shift it.  If you leave these null meters "open circuit" on the 1 or 3 uV scale realize you may get an oscillation  and I don't like banging that needle at the hard stops - leave the power switch in Zero Check until your circuit is hooked up, then go to ON mode only at a higher range, and work your way down to smaller ranges while you maintain balance on your setup and get everything dialed in.  Don't just jump to 3uV or 1uV scale until you know you're within 3uV or 1uV of null balance.

Sometimes your circuit is too noisy to get to 3uV or lower, so you just stay on the scale that suits your measurement the best.  You won't be on the 3uV or 1uV scale unless you're looking at a very quiet circuit, so just make small adjustments and pull your hand back and be patient.  After a while you can detect where the center of the needle is, even if its slowly shifting slightly up and down - you have a good idea where the center of motion is.  That's your measure, and ideally that is right around zero or pretty close to it.

Null meters are best at reading a "null" or zero volts across a bridge with very good accuracy and precision.  If you're trying to read a non-zero voltage then your DMM is better suited.

Once you think you're at Null you swap the leads around on your test setup and check again.  If your making adjustments on a KVD + 732 to match an unknown voltage, you make a change on the KVD dials that is one-half the total difference you read on the null meter when you swapped leads around, and that should get you nailed right on the dot after about the second try.   

It takes a little practice but after a while it's easy and quick. 

[Echo88]

Interesting, thanks! Yes, those instruments really teaches you a thing or two about patience and nanovolt-sensitivity. There are other nV/Nullmeters out there, like the Keithley 147/148 or the Tegam AVM-2000, but i guess the Fluke 845 or K155 is the best compromise (availability, low input bias)? Maybe one of you guys own a AVM-2000 and can give further info/teardown (Tegam-Website isnt available at the moment)? 

[MisterDiodes]

Another thought:  When you're calibrating your 752 Hamon divider, if possible you want a real null meter (your '845 or '155 or similar) sitting on those "Null Detector" terminals, -not- a DMM.  As shown in the 752 manual.  Some pA or 10's of pA bias current @ zero volts differential you get with any DMM's input amp can sneak in and shift that apparent cal pot adjust point very slightly.  That may or may not make a difference to you. 

You can use a DMM (depending on how much bias current you've got you may have to find a slightly better -average- cal position when you change the + and - switch), but it's much easier with a proper null meter with much lower bias current (at null) if you're after top performance.

The same goes for resistance bridges, as Mr. Pettis has discussed above - in that case there is really no substitute for a real null meter for accurate resistance bridge measures.






 

[lukier]

Another thought:  When you're calibrating your 752 Hamon divider, if possible you want a real null meter (your '845 or '155 or similar) sitting on those "Null Detector" terminals, -not- a DMM.  As shown in the 752 manual.  Some pA or 10's of pA bias current @ zero volts differential you get with any DMM's input amp can sneak in and shift that apparent cal pot adjust point very slightly.  That may or may not make a difference to you. 

What about making a buffer from the null terminals to the DMM out of ADA4530-1 opamp? It has only 20fA of input bias and integrated guard.

[Edwin G. Pettis]

While the 20fA input current (check the specs to make sure that current is valid under a null bridge condition) solves the input bias current question (adds a bit more noise too), it still does nothing for the incremental LSD limitation of a DVM and its noise, you still can't 'see' absolute zero on a DVM any closer than ±1 digit or a half digit if it displays all zeros it isn't necessarily really zero.  A null meter (analog) is continuous and when adjusted correctly displays a true zero or how close you actually are to zero, so no, a DVM is no substitute when a null meter is called for no matter what you do on the front end.

[MisterDiodes]

Another thought:  When you're calibrating your 752 Hamon divider, if possible you want a real null meter (your '845 or '155 or similar) sitting on those "Null Detector" terminals, -not- a DMM.  As shown in the 752 manual.  Some pA or 10's of pA bias current @ zero volts differential you get with any DMM's input amp can sneak in and shift that apparent cal pot adjust point very slightly.  That may or may not make a difference to you. 

What about making a buffer from the null terminals to the DMM out of ADA4530-1 opamp? It has only 20fA of input bias and integrated guard.

As Edwin noted - you've got 40uV / rt Hz on that part, so it would be a challenge to get a quiet reading.  You want something in the nV/ rt Hz or much lower range for a useful sensitive null meter.  There is always a trade-off on op-amp features.

Again:  There is no substitute for a good working null meter when you need to detect the calm center of the "Eye of the Storm" at zero volts differential. 

This is an excellent example of where sometimes discrete transistors (or matched dual packages) are a good way to go for lowest noise and low input bias - Art of Electronics 3rd edition has a good read on designing discrete transistor amps for very low noise - this is an application where an IC amp might not be the best way when larger area and quieter, less stress discrete BJT's and Fets can do a very good job.  Analog methods are nice when you really want to look for the slightest twitch of the needle, and know you aren't blasting the test setup with noise...if you're working at low ppm DC volts or ohms measure.  As Edwin said - if you're looking at the very end of that digital display and hoping that right-most digit is going to help you find true zero - that might not work out very well.

That isn't to say people have been building various op-amp null detectors for decades with IC op-amps (Conrad here has a nice DIY article that was a great read!) - but the purpose made tools are the workhorses in the lab, and in the end they save time and money money if you don't have to re-invent a very good wheel.

[Kleinstein]

Those electrometer OPs are very good with bias, poor with low frequency noise.

Separate transistors / FETs have the advantage of better isolation between them. However the 845 circuit looks rather old in using a BJT for the demodulation and also for the AC amplifiers input. Both jobs are better done with FETs.
 
However, judging from the circuit, the Fluke 845 is also not really low noise. The resistors at the input alone give a noise level of about 70 nV/Sqrt(Hz) and the BJT based amplifier will likely about add as much noise. So it is not exceptionally low noise - the input stage on good DMMs can be lower in noise.

The high input impedance on the 845 is also limited to low input voltages. Once the voltage exceeds something like 0.5 V the protection diodes kick in and input impedance is down to 150 KOhms. The filter caps at the input can also lower the input impedance, once the signal is significant out of range - so when connecting the meter, there can be a considerable input current to charge the input caps. So the DC pA level in only reached after some time - it can start in the upper nA range.

A conventional digital display might not be that good for judging a zero. However, suitable scaled and with suitable filtering (e.g. running average) and additional statistical info it can work well, even better than the old style analog. This is especially true with a instrument that is slow anyway, like the 845. The slow analog filtering used here can be rather misleading. Averaging by eye only works well for a limited time.

[Edwin G. Pettis]

You apparently have no (or very little) hands on experience with null meters, the 845 is indeed quiet, noise is under 200nV on the 1uV scale, 250nV 1mV to 300mV and 300nV above that, granted that is with a shorted input but that is a standard method of noise measurement.  The Keithley 155 has very similar specs to the Fluke.  While it is possible that FETs may produce lower noise, that is not always the case, bipolars can produce very low noise levels as well.  What digital instrument that could possibly have a 1uV full scale range compete with a null meter for noise or accuracy with <200nV noise? 

Best practice is to use the lowest possible noise levels in instrumentation to begin with, it is poor practice to use higher noise levels and then attempt to mask them by filtering and math which has their own problems, you should know this hard and fast rule and if you don't, shame on you, if you do and don't practice it, shame on you, digital is not the end all fix or solution, never has been and never will be.  I have used null meters for decades, I'm quite familiar with their ins and outs, until you have some actual experience, don't knock them, they do their jobs exceedingly well.

The input diodes are for clamping overloads, they are inside the feedback network so no the input impedance does not fall to 150K, the lowest input impedance under 'normal' input conditions is either 10 Meg (older units) or 1 Meg (newer units), similar conditions exist in the 155 units.  Who cares, at null (after all this is primarily a null meter) there no current (or very little ) flowing in the input so your argument is again incorrect.  So an input overload causes a few seconds overload recovery, what happens in a ADC when its input is overloaded and needs to change scales?  That doesn't happen instantaneously now does it?  Correct use of a null meter is to start out on a higher voltage scale (when the input voltage isn't known closely) and adjust the scale downward so that a null can be achieved.  If you are silly enough to put the meter on a very low scale with significantly higher input voltage, guess what, the limiting diodes cuts in and shunts the current away from the input circuits, just what it is supposed to do.  Virtually all of your arguments start out with the misuse of the meter, what do you think happens in any other instrument where its input circuits are overloaded like you are describing?  The same recovery scenario takes place.

With all respect due, as to your "suitable scaled and suitable filtering and statistical info", baloney, you're already adding more noise, error and uncertainty to the mix, again you start out with the lowest possible noise, no amount of filtering and statistical dithering is going to improve on that, you claim a null meter is slow, I don't think all of your add ons are going to be all that fast nor as accurate, every addition to the signal chain introduces more noise, error and uncertainty, that is a fact of electronics .  Your assumptions are obviously not from actual use or practice, your arguments are mostly specious, and averaging by eye, I do it when needed, works just fine and is quite accurate, your digital is still incremental and has inherent limits, it is not best practice, just ask the labs who still use both old and new technology, there are some huge companies out there that rely on both technologies and know when to use the correct instruments.  I know primary cal labs that still use null meters, are they all possibly wrong in their conclusions?

Yes I have used DVMs as null detectors sometimes, their limits are quite obvious compared to an null meter in actual practice, within certain limits, a DVM can be used without problems but when you're in the hunt for PPMs or lower, a good null meter is exceedingly hard to beat.  Just check out the manuals for instruments that measure PPM or lower, their calibration setups require null meters, not DVMs.

Get a good null meter and learn how to use it properly then come back and talk about them, experience and use count for a lot when talking about an instrument, just looking at schematic and making observations is not best practice.  Try talking to others who have used null meters for years and get their opinions, it isn't just myself or MisterDiodes who share these opinions.

[MisterDiodes]

Just another thought on '845 / '155 / 419 etc. and why these are sometimes the very best tool for the job. 

For example you will see the Tegam avm-2000 advertised as a replacement for the 845 / '155 / 419, if you've got an extra $6k to spend:

https://www.tegam.com/shop/voltage-resistance/avm-2000/avm-2000-null-detectornanovoltmeter/

These can be OK if you don't have access to a real analog null meter - and this one does have a feature to trim down the bias current -  but some of the problems we found during eval a few years ago (might be upgraded in the last few years) made it not seem to be worth the price tag:

1.  A minor issue but still disappointing is that for $6k you get somewhat hobbyist grade Pomona binding posts.  They are "low thermal" but a step down in quality from the older beefier styles used on older meters.  The binding posts on old meters tend to have a much thicker gold flash and larger support insulator and seem to stand up to abuse better.  The newer style does have a slightly better thermal time lag, but we've seen the bases crack if they get bumped.

2.  The meter movement on these units isn't as well made as the older meters - the meter on the new unit has a plastic face that is a prone to storing static charge - see their app note on special procedures to overcome that problem.  If you actually test the taut-band meter movement itself on an older 1-3 scale Fluke / Keithley (usually made by API / LFE) or HP meter, you'll see that 2% mechanical accuracy spec is very conservative...we normally see more like 0.5% or 0.2% FS error on the movement itself (if that), very good linearity (<.05% error mid scale typical) and virtually no backlash or hysteresis. The newer meter movement doesn't live up to that manufacturing standard, at least not the one we looked at.

3.  The main problem is that on one test we've got a spec on the test jig setup that equipment in the local vicinity have virtually no detectable EMI over 10kHz  (That's why we also switch to battery powered halogen or LED lights during a finicky test).  And that's where this Tegam meter has its main problem:  They put in a noisy CPU where the quality PWW pot should go - and that splatters noise around the local area which might affect your DUT.

This is exactly the same type of issue that you run into when trying to run a DMM or digital anything as a null meter, regardless of relatively high input bias current on a DMM.  And the same reason why you have to keep an eye on any  AZ amp (like a '2057) very well shielded and bypassed around sensitive circuits.

This type of Tegam nullmeter itself (or DMM) could work out fine for you if your not worried about possible EMI in close proximity to your DUT.  A DMM can certainly work if you're not worried about pA or some 10's pA current flow.

So when you look at meter specs or design you're own, a pure analog (or very close to analog only even if it has a fairly slow chopper) design is really very useful.  You have to look at not only the noise of the meter's acquired signal, but you need to look closely at the meter's own clock EMI noise injected into the test system itself, and watch out for that.  Better to not have a clock at all beyond whatever the chopper is doing.

And that's where the 845 / 155 / 419a type meters really shine for the tasks we need to do. Yes they have choppers but the freq is low enough to not have a huge impact on what we're measuring.

ANOTHER TEST:  I did have a tech at the cal room quick-check some other 845's and '155s just for fun for bias current, like SZA263 suggested - and I can confirm '845 numbers around 150~250fA (slightly less with the FET switches but the Neon-driven LDR switches work well) or less bias current at null - one pimped out '155 with newer Fets in the chopper and first stage amp was running around ~75fA or less bias current at null.  So these meters are obviously -not quite- perfect, but are certainly -not- pumping out a large bias current at null if they are working and in proper adjustment.  At least an order of magnitude (or more) less bias current than a 3458a, and these older meters have none of the relatively high clock noise you get with CPU / FPGA emitters.

Could you build an even better 845 / 155 / 419 style meter with upgraded matched Fets arrays and BJT differential amps - and maybe an IC amp follower?  Probably. 

That's why in our cal rooms we have the older equipment working right along side the newer designs.  Use what is best for your application and test requirement.

[martinr33]

On the noise topic...

It would be straightforward to build a precision 86Hz oscillator using an 8-pin low-power microcontroller. SMT would keep the current loops smaller, and it would have good ground planes. An external crystal would be best, but the device can hit 2% with its internal oscillator. The trick with the 845 is that this modulator goes in the outguard section, driving the isolation transformer. The clocking can be recovered from the inguard isolation supply as it is today.

Any thoughts on whether the extra stability would affect the emitted noise enough to be a problem? The internal oscillator has a much wider bandwidth than a crystal, so the peak noise is much lower.

[MisterDiodes]

I wouldn't use an MCU if possible for our fussy application - even one single chip that is has many sync'd transistors switching fast on a clock is enough to mess up the test unless it is very well shielded - but you are on the right track about keeping the bandwidth spread out a bit.  A simpler BJT or FET oscillator works and keep currents very low.  The final freq around 85Hz (for '845) or 220Hz (for '155) is not super critical - those freqs are chosen so the 50/60 Mains power interference doesn't beat in the chopper section and make you new mystery noise.

For the '155 chopper fets and on newer / modified '845's: The chopper signal in this case doesn't want or need super-fast edges.   Slightly softer corners and not over-driving those chopper fets helps keep the system quiet. If you keep the switch gate drive rise / fall times well controlled on those switches then you're injecting less gate-charge noise onto the signal. 

There are filter caps on the inputs to deal with the leftovers but we try to keep all switching signals to an absolute minimum and as quiet as possible.  The '155 doesn't need the isolation transformer since it always used low voltage Fet choppers, and that's why we kind of lean towards that unit.  The '845 even with neons is pretty good though for most needs and the version with Fets is even quieter.  They will all get the job done with fairly low emitted HF switching noise compared to a DMM, and at a much lower bias current at null.

[zhtoor]

I wouldn't use an MCU if possible for our fussy application - even one single chip that is has many sync'd transistors switching fast on a clock is enough to mess up the test unless it is very well shielded - but you are on the right track about keeping the bandwidth spread out a bit.  A simpler BJT or FET oscillator works and keep currents very low.  The final freq around 85Hz (for '845) or 220Hz (for '155) is not super critical - those freqs are chosen so the 50/60 Mains power interference doesn't beat in the chopper section and make you new mystery noise.

hello,

what would be the effect of "dithering" the 85Hz chopper frequency from say 80Hz to 90Hz using some kind of a physical random noise source on
the performance of the null detector?

regards.

-zia

[SZA263]

hello,

what would be the effect of "dithering" the 85Hz chopper frequency from say 80Hz to 90Hz using some kind of a physical random noise source on
the performance of the null detector?

regards.

-zia

A complete disaster 

[martinr33]

That's an interesting question, because the whole design is a bit tricky.

Although the time window changes, the rise and fall times on the opto choppers do not. Therefore, as the frequency changes, the output waveform - and hence the final meter reading - will change out of proportion to the input voltage. So I would say less variation eliminates another source of uncertainty.

The whole design is a well-crafted balancing act, probably done by someone who previously had to use vacuum tubes. The photoelectric chopper was probably a huge deal.

[MisterDiodes]

"A Huge Disaster"... Yes maybe.

The problem with dithering the frequency is then becomes "how fast is the dither" and will the chopper amp front modulator and back end  demod get out of sync.  I would just enjoy the meter as it is, and don't change that unless you need to.

We have had on occasion on some test where say the '155 was showing a slight sine on the output (you learn the difference between sine error vs random noise), and so you can either tweak that multivibe freq a little, or find out what was left turned on that's beating in with the chopper, or switch meters to an '845 or vice versa.  Or find out who left their wrist watch /  cell phone / tablet /  PC / DMM / Switching power supply / Noisy LED light / USB port or whatever else left on in the lab, etc.  We've seen all of those things happen.

That's why it's handy to have a few different nullmeters and extra u-Metal foil, etc. in the lab for this situation - you can't have too many tools at your disposal.  Some tests require both a nullmeter and some DMM / PC / wafer prober in the same area, so you just have to work it out for the best setup. 

Really, these meters are nicely and thoughtfully designed - and for 1965-68 it is amazing tech that really stands up to this day.  If you take the time to really understand the schematics you'll learn a lot. Especially the clever technique of wiring the resistors around the range switch decks and how that works with the 1 - 3 scale meter to adjust input attenuators and amp gains. Neat stuff.  If you study that you can learn some tricks to make your modern circuit design better also.

By the way: Those nice PWW adjust pots and range switches ==> The original "Non Volatile" calibration memory, no noisy CPU required.  With a little care these meters are some of your best friends if you're working with high accuracy measures with 752s / 732's / KVD's / Resistance bridges, etc.

[Vtile]

uA741 did pop out 1968 about the same year as these "magical light isolated" chopping nullmeters (845). How leaky and noisy the uA741 were? 

[Kleinstein]

The circuit is really old style. The AC amp part is interesting to learn from. If available they could have used the µA741 for the DC amplification part (Q108-Q112).

The µA741 was not that good when it comes to noise leakage and similar. It was great because it was easy to use with internal compensation.

Noise wise the 741 is not that bad - 23 nV/Sqrt(Hz). The Fluke 845 should be somewhere at 100 nV/sqrt(Hz) - of this 70 nV/sqrt(Hz) are due to the resistors at the input alone.