DIY nanovoltmeters and ultra low frequency noise preamplifiers, who did it?

[e61_phil]

The measurement will be noise limited.  I couldn't find noise figures for the 8842A, but it's 24h accuracy is specified as 50ppm of reading + 20 counts in the 20mV range, which one can take as a hint.  Nanovoltmeters fare better (the old Keithley 181 can be expected to be an order of magnitude better there).  But given the noise of the sources, there might be little to gain.

50ppm of µVs is in the pV range. So no one cares about the accuracy here. More important might by the floor spec of 2µV. But that should also include linearity and so on.

I don't have a 8842A, but a HP3478A and will give it a try.

[Echo88]

Very interesting project Mickle T! 
Can you give some more details about it? Like:
What was the used Yokogawa Preamp?
Is the used transformer a standard signal transformer?
When i looked at the signal shaper i thought that ive seen that somewhere ( https://www.eevblog.com/forum/metrology/em-electronics-model-n11-dc-nanovoltmeter/msg975473/#msg975473 )and indeed, there are the documents on the first picture. 

Back to topic:

https://arxiv.org/pdf/1708.06311.pdf Temperature controlled IF3602-based amplifier. It shows the limitations of a very low noise, non-chopped, JFET-amp.

[Gerhard_dk4xp]

Who has interesting links to papers and articles? Someone DIY'ed something similar already, probably with a fixed input range?   -branadic-

I have made a quick&dirty chopper stage based on EPC2038 GaN FETs.
They feature really small capacitance and small channel resistance.
That was more to see if I could solder them at all b4 I make a larger
time investment. They are nekkid chips  0.9mm * 0.9mm with 4 solder bumps
below.
Soldering was easier than expected with a little bit of hot air. A little little bit,
or one might not find them again.
I have not yet done any characterisation, I'm currently more interested into
microwaves.

I can bring the board to Stuttgart tomorow.

[dietert1]

This thread starts with a statement like "Keithley 2182A is desirable yet expensive". My experience is a little different: The 2182A resolution is 1 nV in its lowest measurement range (Ch1 +/- 10 mV). Compare this to a Keithley 148 with it's lowest full range of +/- 10 nV.
This is a severe design problem in the 2182A. The 1 nV is seven decades below full range and ADC stability is only good for six decades. With a lack of gain ADC noise or thermal variations do matter.
Of course the 2182A has other advantages, but null stability isn't the main strength. An ambient temperature transient of 1 °C can give you a 50 nV deviation. Even when running a 2182A inside a thermal chamber at +/- 0.1 °C for 10 days, it was walking about +/- 10 nV.

Regards, Dieter

[3roomlab]

the K 148 svc manual have a nice section "circuit description"
i reprinted that section. the original pdf is >4mb
it can be found inside ko4bb archives

i was quite curious how LISA/LIGO did their voltage reference, they have some measurement reports done which span down to 100uHz, but i dont know what they use to measure.

[Gerhard_dk4xp]

> a human being is like a 80w lamp heat source walking around a precision lab,
> sheds about 20g of water an hour on all the precision electronics around him/her.
> If a precision instrument has a voice, it would say get away from me you #@$%#$^ .

A human is about 100K, 1/4W 
(40y ago in the newspaper Elektor, under the title Electorture)

Cheers, Gerhard

[2N3055]

> a human being is like a 80w lamp heat source walking around a precision lab,
> sheds about 20g of water an hour on all the precision electronics around him/her.
> If a precision instrument has a voice, it would say get away from me you #@$%#$^ .

A human is about 100K, 1/4W 
(40y ago in the newspaper Elektor, under the title Electorture)

Cheers, Gerhard

For the purposes of heating/cooling calculations for the buildings a rough thermal output figure of 80-100W per person is estimated AFAIK.

[branadic]

This thread starts with a statement like "Keithley 2182A is desirable yet expensive". My experience is a little different: The 2182A resolution is 1 nV in its lowest measurement range (Ch1 +/- 10 mV). Compare this to a Keithley 148 with it's lowest full range of +/- 10 nV.

Wasn't it you calling a Prema 2080 scanner a piece for a museum recently? Now you draw comparisons between K2182 and K148? Really?

-branadic-

[dietert1]

branadic, you don't understand once more.
I am not reporting a bad design of a K148. Nor did i propose using a K148. We have one and although it works well, i would never try to run it more than some hours as its mechanical chopper is made from unobtainium. And it's a bit slow for noise studies.
I was reporting a design problem of the K2182A. The lowest range of the K2182A should not have been +/- 10 mV but +/- 1 mV. Should be fairly easy to mod this and repeat the tests. Also i will try and replace the black plastic covers by metal ones.

Regards, Dieter

[Kleinstein]

The Keithley 2182 service manual shows 2 amplifier stages, both capable of doing a x 100 gain. So in principle the HW should be able to have a 1 mV range.
The ADC in the 2182 is supposedly nearly identical to the K2010 7 digit meter. It is still similar to the K2000 but quite a bit better performance. The ADC gain stability should be a smaller problem if only a small fraction of the range is used. So I am not convinced that the ADC part is actually limiting. Chances are that already in the 10 mV range the amplifier part is limiting and not the ADC. Otherwise they would have likely enabled the 1 mV range already there in the HW.

Besides the ADC and amplifier there could also be thermal EMF caused noise, e.g. from the protection part.
The input protection with MOSFETs is somewhat sensitive to thermal effects, likely more than the ADC. The effect of the thermal transient is likely not from the ADC part, but from the amplifier (e.g. thermal EMF at the resistors or the switches) or the protection. The parts are in the extra box, but may still see same gradients. For my voltmeter I use a similar protection and it looks like 1 input shows quite some thermal effect (~ 500 nV for the effected channel, < 50 nV for the other).

The Keithley 148 is more like a dedicated nV meter for lower source impedance. The 2182 is still relatively high impedance and less speciallized.

A point to check with the K2182 may be if it also suffers from the slightly odd "bump" in the noise vs PLC curve like many other Keithley meters (e.g. DMM7510, DMM6500, K2002 ). This or may not be the case.

[dietert1]

The input stage should get metal shields. The existing plastic covers serve to prevent air drafts, but don't keep away electrical noise, e.g. from the nearby mains transformer and the VFD display. That design doesn't appear right. If you ever pulled one of the boards from a HP spectrum analyzer, you know what i mean. I already replaced the original IRF610 mosfets of the protection circuit by smaller 800V parts i recently bought. The smaller parts pick up less noise, it showed up in the numbers.

For a first test i added a 5R1 resistor parallel to R641 and R717 to increase the gain of the Ch1 input stage by a factor 10 - from 100x to 1000x. That modded 2182A started without complaints and appears to work well. Polarity reversal (FAZ=1) works but signals don't look nice, that needs some work. Most likely ACAL won't work now. For logging the display will be off anyways, with no wrong numbers visible..

The input stage JFET quad exhibits offset between 0 and 50 uV (FAZ=0, without covers). With all covers on, after some minutes it reads a pretty stable 20 uV. So these selected JFET quads are pretty special, There was a thread about 2182A JFET selection here: https://www.eevblog.com/forum/metrology/fet-selection-for-precision-circuits/msg1495213/#msg1495213.

Regards, Dieter

[Kleinstein]

An offset of some 50 µV looks rather low for matched JFETs.  Is it possible that this includes already a digitally subtracted constant part ?
AFAIK there is only 1 pair of amplifier jfets (SK170 AFAIK) for the input stage. The other 2 pairs with heat shrink are for switching / polarity reversal.
Looking at the pictures to get an idea about the circuit, it looks like there is some switching at R700 + U641 not shown in the block diagram, that may actually be some kind of coarse digital offset adjustment.

With a much higher gain one may have to do a better offset adjustment for the input JFETs. For the FET offset I would expect some 1-10 mV, which may be still acceptable with a gain of 100, but too much for a gain of 1000.
The offset translates to an AC background at the amplifier output (TP651 ?) and less amplitude would make the settling time less critical.

Instead of tweaking the existing amplifier without a full schematics, it may be more useful to build a separate one with a similar structure. The parts used are not that exotic. It looks like the LM394 transistor pairs are used for a 2nd amplification stage after the JFETs. Just the FETs as shown in the simplified block-diagram would be too little gain for the chopper part. With the initial gain of the FETs in front it should be good enough to have more normal transistors for the 2nd stage. So chances are the LM394 is overkill here. The point would be more some offset adjustment (trimmer or DAC) to ideally keep the ripple small.
One point I don't like very much with the K2182 amplifier is the odd mixed type chopper:  CMOS (2N7000 ?) for the input and JFETs (Q622+Q623) for the feedback. To me this makes not much sense. The compensation in the gate charge would likely be better with 4 switches of 1 type (e.g. 4 x MOSFETs, 4 x JFETs or a CMOS switch chip).

[dietert1]

I do have full schematics reengineered by Tin. It was incomplete but showed a lot more details than the service manual. It appeared in his youtube video "2182A trouble shooting". We can see that the LM394 is used as a cascode for the input JFET quad, so the FETs operate at constant Uds and at constant current. Ugs is about 0.3 V. The 5 mA current source is based on a TL431, an opamp and another JFET with a 499R source resistor for current measurement.
U614=MC34081 follows after the JFET quad, so there is enough gain to run the input stage at 1000x.
I wrote the JFET quads are very special and they are, once more unobtainium. Their combined offsets are really much smaller than 1 mV - a requirement for using FAZ polarity reversal at +/- 1 mV full range without losing dynamic range and it is fulfilled. I saw the 20 uV times 1000 on the scope, monitoring TP651. At G=1000 polarity reversal gives 40 mV there.
One can recognize that the JFET quads were combined from pairs already available as parts of the 2182 non A predecessor. Basically they should have used two constant current sources to drive each pair separately, but this was omitted. One can use small resistors like 5 to 12 Ohm below the common source of one pair to balance currents (2.5 mA each pair).
They made a circuit with nine interconnected discrete transistors in linear mode, of course prone for RF instability. There should have been some 33R resistors, e.g. to separate the JFETs from their cascodes and to spearate the JFET pairs from each other.

Regards, Dieter

[Vtile]

Please, send all those poor and unsatisfying HPs & Keithleys to me for proper disposal. 

Ps. As spoken, quad JFET I would suppose it does refer to quad JFET transistor at one die / TO-package.

[Kleinstein]

In the 2181/2182A they use 2 or 4 TO92 package JFETs as a matched set. So not even dual JFETs in one case, but measured and sorted parts. These are than coupled with some heat shrink.

[dietert1]

Yes, and the coupling is once more pairwise and less than perfect. Maybe they put the heat shrink after soldering the parts or the heatshrink softens during soldering, so there are air gaps. I remember remarks in a DIY low noise amplifier description that 2SK170 should be mounted on a heatsink to improve low noise operation at 10 or 100 Hz. SMD parts like the "new" TI JFE2140 should behave better in that respect.

I think the 2182A can be used down to 1 nV when operated inside a thermal chamber, with stable and noise free mains supply and if the zero gets recalibrated regularly or continuously. 2182A ACAL does not recalibrate the zero.
When used with a low thermal multiplexer, the MUX can provide either a relay for polarity reversal or a low thermal short on one channel. With continuous recalibration one can probably maintain the zero to 1 nV rms. Noise is about 1.3 nVrms for 5 sec readings (averaging 40x 2 PLC with FAZ=1).

Regards, Dieter

[Vtile]

Thank you for clarification, I was scratching my head as I was wondering if I have seen a such quad package in my recent newbie journeys to some 1970's instrumentation amplifier papers or not. Propably not.

My dilletant newbie mind is now asking if there is any sense to use as high as possible operating voltage Vce/Vds for transistor or differential pair as possible  (eg. 30V instead of 5V) as I faintly do remember that it is better for overall performance of amplifier stage. Most probably the answer is that it depends... I think I really need to read my book(s) again.

Anyhow, thank you again for interesting discussion.

[David Hess]

Tektronix was fond of using two metal can JFETs mounted in a block of aluminum where the highest precision was required.  A metal clip in the shape of an S used to be available for doing the same with plastic or metal parts.  Tektronix matched JFETs down to 5 millivolts and 10 microvolts per degree C in this way.

My dilletant newbie mind is now asking if there is any sense to use as high as possible operating voltage Vce/Vds for transistor or differential pair as possible  (eg. 30V instead of 5V) as I faintly do remember that it is better for overall performance of amplifier stage. Most probably the answer is that it depends... I think I really need to read my book(s) again.

There are a couple of reasons but you might be confusing the reason with controlling the emitter/source current.

Their is an advantage to using as high a voltage biasing the emitter/source current as possible so that the emitter/source biasing resistance is as high as possible, to improve alpha of an emitter/source follower or common mode rejection of a differential pair, but replacing the emitter biasing resistance with a current source gives the same advantage at lower voltage but higher cost.

Higher Vds and Vce keeps junction capacitance low, so where this matters, a higher voltage will be used, however in JFETs, higher Vds increases gate current through impact ionization (?).  Robert Pease mentioned rediscovering this at one point, and JFET circuits optimized for low gate current deliberately use low and controlled Vds.

[dietert1]

As this thread is about DIY: Meanwhile i "coated" the two covers of the K2182A input stage with 0.5 mm copper sheet. Need to do another two.

Regards, DIeter

[mawyatt]

Tektronix was fond of using two metal can JFETs mounted in a block of aluminum where the highest precision was required.  A metal clip in the shape of an S used to be available for doing the same with plastic or metal parts.  Tektronix matched JFETs down to 5 millivolts and 10 microvolts per degree C in this way.

Robert Pease mentioned rediscovering this at one point, and JFET circuits optimized for low gate current deliberately use low and controlled Vds.

Way back had seen some of the Tek Aluminum blocks (think they used them in the low noise diff amp plug-in, 7A22) and many used the "S" type clips which held a TO92 or TO18 can.

Bob Pease was a master at finding unique circuit/device features & niches. Remember Bob Widlar trying to beat JFET bias currents with his brilliant bipolar designs, fun times back then

Best,

[Kleinstein]

Today JFETs in a metal can are relatively expensive and even TO92 is getting rare. The main case you get is SOT23 and similar SMD ones.
The small size case has also some advantage getting the fet pair close together.  However measuring the FETs before soldering is a bit fiddely. For a well matched pair one would need to measure quite few (like 20).  I don't think the unmatched duals are a great help, more like using teh 2 in one case in parallel to get a larger area jfet.

[dietert1]

I thought about measuring JFE2140 pairs. They come in SO8, so i ordered a test socket ("programming adapter"). Their offsets are pretty low already from factory. Maybe one can combine two pairs with opposite offsets and glue one on top of the other to use them as one balanced pair.

By the way the 34420A has a FET pair labeled SNJ3600X05. Probably an Interfet N3600l geometry that has a "typical" spec of about 0.5 nV/sqrt(Hz) down to 0.01 Hz. The chip is 1.838 x 1.838 mm. Another part number was
Interfet IF3602 - without pair matching, in TO78 can.

Regards, Dieter

Edit: The noise spec is at 0.01 KHz, not 0.01 Hz. My Error. And the datasheet has a pair spec of 100 mV, that is near useless, "differential gate source voltage".

[Gerhard_dk4xp]

8 pcs. of IF3602
Wonder why it pops up everywhere this week.
There must be "some" matching. It could be even worse.

>  "typical" spec of about 0.5 nV/sqrt(Hz) down to 0.01 Hz.

Sorry to burst that bubble.   0.01 KHz.
And the noise does not follow 1/f , there is an unexplained plateau.

Gerhard

[Vtile]

The small size case has also some advantage getting the fet pair close together.  However measuring the FETs before soldering is a bit fiddely. For a well matched pair one would need to measure quite few (like 20). 

Well as a some small middle project I created a small tool for testing sot-23 transistors with ease. As shown in pictures it continues the 'metrology' tinfoil traditions. Contact surface is build with manhattan technique on cloths pin jaw, it seems to work flawlessly. It can be also used for other surface mount devices like resistor, which was my first DUT, with two different un-calibrated technologies so the non-correlation to standards can be guaranteed.

I should attend to 'Nanoamps and below like ninja' -thread with that galvanometer in Leeds & Northrup test set .. it's just crazy. Something verified with Keithley 197A Microvolt meter




Next test component was with MMBF4117 and 5$ component tester, unfortunately that JFET is just too much to tester, I need to ask about it at $5 tester thread...


I swapped to BC847C and that is in operation limits of this atmega wonder.


Last test was conducted with Meratester FET multitester, the resistance between measuring pads is over 10Gohms.


Construction is simple and by ordering gold plated PCB one can get rid of oxidation problems. That is the reason I tinned the pads, so I can remelt them easily with flux. The caps between pads are filled with cyanoacrylate (cheapest variety, wonderfull material), then swept with small wooden spatula to flush (another half of the clip). When quick glue were driedthe surface was cleaned with exacto-knife. If one needs to make temperature variation one could ie. glue a big thin SMD resistor at the another jaw and make some form of heat controller to it (I think I will do that at some point .. maybe). I can see also a future of making myself a clamp on heater for resistor selection with same type.



Hopefully this post gives some diy ideas.

 

edit. pictures attached with PC.

[David Hess]

Today JFETs in a metal can are relatively expensive and even TO92 is getting rare. The main case you get is SOT23 and similar SMD ones.

The small size case has also some advantage getting the fet pair close together.  However measuring the FETs before soldering is a bit fiddely. For a well matched pair one would need to measure quite few (like 20).  I don't think the unmatched duals are a great help, more like using teh 2 in one case in parallel to get a larger area jfet.

I was thinking close proximity of the SOT23 packages and then attaching an aluminum block to their tops with thermal epoxy would work.  Or maybe use 4 transistors in a discrete thermally coupled cross quad?

Precision monolithic JFET pairs are in theory still available from Linear Systems, and some others, like the LS843 with 1 millivolt offset and 5 microvolt per degree drift.