Nanovolt design challenge - build and show your own nV-meter in 256 days [DONE]

[TiN]

What you wrote here means that you, for instance, won't be doing ANYTHING technically related to challenge of measuring nano Volts.
Nanovolt measuring challenge is all about device/DUT interconnect, front end, guarding and physical package of all that.

Sorry, but buying and using EM amplifier does not magically resolve all nV issues by itself. You still must do proper isothermal interconnect, power delivery and physical package. Otherwise you will get very poor (but low noise, yes!) thermometer of your lab. 

Nanovolt meter should be simple headless design, a box that receives nanovolt on one side and spews RS232 messages on the other. That makes digital side an A/D converter and  a MCU, even an Atmel Mega or equivalent device will do. Simple. Maybe few housekeeping messages.

That also means that experimenter must find the host with RS232 to UART hardware, setup environment for it, program code to talk, debug whatever speed/stop bits/inversion, timings etc. instead of simply connecting equipment and reading data. I don't say RS232 cannot be done, it can. But not in this challenge.

Ethernet: nope, that means i probably have to spend more time doing Linux kernel panic hard faults checking than designing the AFE.

In 10 years doing this, never had to look into any kernel panics related to Ethernet or GPIB instruments. 

That is dead simple to implement and is well standardized and common on T&M devices

Yet still many even commercial instruments have own tricks with RS232 implementations and need messing around with settings/drivers/etc. to make work with ubiquitous Raspberry Pi or alike. In my lab I always curse instruments which only have RS232 and nothing else (so far it is only Arroyo TEC controllers and Lakeshore 331, everything else is on LAN or GPIB networks).
Let's make it personal bias against RS232 on instruments and be done with it  .

To be clear, UART, I2C or SPI are totally OK to be used between AFE isolated section and outguard host interfacing external world.

Ethernet with TCP/IP requires a full computer with protocol stack. That can be added later as an optional thing. You could use Raspberry Pi for that, or anything really.

If you insist on Ethernet, there things like X-port that convert serial UART to Ethernet. Very simple.

So is it requiring full computer, or its very simple with converter? Ethernet or GPIB, this is the way. If interface upsets someone enough not to participate, well, sorry about that, nothing can be done. 

Making it modular something like that also means that different people (that have different skillsets) develop best front end and there can be simple 7-segment display module and also a graphic screen display module if someone wants that...

That would be great. As challenge terms outline, team submissions are totally OK and very welcome. There is no requirement to have everything on same PCB/same enclosure with nV AFE, only the requirement not to have external equipment (e.g. power supplies or host computers/debuggers connected to have setup working

Instead, I'm supposed to spend something like 100EUR or more getting the REF and Vishay resistors, and not just that, but spend considerable amount of time verifying the characteristics of it

.
Hm, LM399AH $17.4 USD for reference + few PTF56 15ppm/K resistors ($few) + opamp ($few). I don't see what else to go for 100 EUR to integrate ovenized ref? Also it's not much you can mess up with LM399A design, it's already all integrated and gives good performance without spending months to verify it (which you might need to do for many of the non-ovenized bandgap zeners).

[guenthert]

     I think some of the, er, rather strong opinions come from a misunderstanding.  The challenge here is to create a more-or-less well specified device in a given time frame (but no given price range *), not a solution to the problem of how best (**) measure voltages in the nanovolt range (shouldn't it then be in the projects sections, rather than in metrology?).  Accept the challenge or move on, no need for harsh feelings.



*) hence I don't quite understand the concern that someone would commercialize it.
**) pick a meaning of best here

[2N3055]

I personally prefer Ethernet and don't like GPIB. So there are the biases. I still have GPIB adapter, because some of my devices have it. I use what I have.

But as many times as you keep repeating it is not going to make it true. UART +RS2323 electrical interface is the simplest, most standard and easiest serial interface to be implemented.
You set speed and coding (with 8 bit no parity practically being the norm for 20 years) and your good to go.
There are no "alternative" or "trick" with any RS232 devices. It simply works, and parsing of the strings is all the work.
And speaking with serial ports is so trivial from any software, that I won't even talk about that.
Saying Ethernet TCP/IP is simpler is simply not true.
I know, as I have been developing stuff on all kinds of interfaces for 30+ years.  UART is simplest to develop and to program for. It is slow but for this purpose fast enough.

No need to be snarky either. I try not to speak up if I don't know what I'm talking about. X-Port IS a Linux computer, a very minimal one.
Look it up. It is simple because if you use X-port, it simply serves as serial to TCP/IP gateway. It can expose a port so you simply establish a socket to it, and speak same strings as on serial port.  Which makes it so you can decide to implement RS232 only or drop 50€ and have TCP/IP. Or connect RS232 to Raspberry PI via UART and make software for that.

Look, it is simple.

You posted this as YOUR challenge.  You came up with rules that appear heavily slanted towards something you already have been working on and in way you think is right and what is good for you.  Me and few other here voiced opinions that it would be more beneficial to community to make something more generic and focused in such a way to spend most of the energy on important "nanovolt" part. Few of us have found some of your requirements unnecessary or not instrumental to core nanovoltmeter requirements.
I spoke up, you don't like it, that's fine. I won't be repeating or interfere anymore. It is your show.

I also won't be participating, because I would do it  either for the good of community, or would make a device that fits my specific requirement, not some other person that have different needs them myself. And I don't have time to participate in this as a pissing contest, just to show off.

So good luck to everybody and will following to see what interesting ideas will be developing here.

Best,

[bdunham7]

And I'm half sure that the ref wouldn't even be the limiting component in this AFE.

I suspect (and hope) that the winner of this contest will actually post specs that would necessitate at least a decent select LM399, although perhaps not in the lowest range.  I'm not going to be entering in the contest either, but I'll admit that the actual reasons are that I'm not clever enough and I don't have good enough equipment to test/verify such a device--not that I'm too busy or don't think the project is worthwhile.

[Kleinstein]

Testing and verifying could be hard, at least to the full performance levels. Some tests (e.g. noise) are easy, but some others (e.g. INL) are quite tricky, though testing to the 5 digit level should be a bit simpler than 7 digit level. If really needed chances are one could find a fellow to do some of the tests.

For the 5 digit level one could for sure get away with less than an LM399 and for a more lower power version this may also be a good idea. There is always the option to have a separate working ref for the ADC and the LM399 mainly for a check. The Keithley 2001 and AFAIK also the 2182 use this way. The interface is something to worry last, though for testing it help to have a interface to the computer in some way already relatively early.
Chances are many attempts end up with a partially (e.g. a bit too much noise or drift) working low frequency amplifier.

[macaba]

Thank you TiN.
I like this contest very much. Yes, it's got non-trivial requirements, but that is a part of the game.

I concur, thanks TiN. It’s your contest and you’ve fronted the first (and maybe only) prize so you set the requirements.

I’ve been simulating various front ends to get a feel for the analog wizardry required, I’m very much looking forwards to seeing what comes out in 250+ days!

[maxwell3e10]

The way the rules are currently written it will motivate one toward a gold-plated design, both figuratively and literally, especially if the goal is to win the gold-plated prize. But to me a more interesting challenge is to have the simplest design that still meets the metrics and gets maybe within 20% of the gold-plated approach at 10% of the cost.

From this point of view it maybe interesting to use an ADC evaluation board or another low-cost DAQ as the back-end of the system. That would minimize the relatively trivial design aspects and allow others to duplicate the system with minimal effort.

For example ADS125H02 seems like an interesting 24-bit ADC that can make a decent DVM by itself. It has two channels with +/-15 V maximum input range, 2 ppm INL and adjustable gain. There is an evaluation board ADS125H02EVM for $99. While it does not provide galvanic isolation, it is laid out so it can be split and isolated with addition of a few parts.

[EEVblog]

The way the rules are currently written it will motivate one toward a gold-plated design, both figuratively and literally, especially if the goal is to win the gold-plated prize. But to me a more interesting challenge is to have the simplest design that still meets the metrics and gets maybe within 20% of the gold-plated approach at 10% of the cost.

A "best bang per buck" award?

[iMo]

..
For example ADS125H02 seems like an interesting 24-bit ADC that can make a decent DVM by itself. It has two channels with +/-15 V maximum input range, 2 ppm INL and adjustable gain.
..

While reading its DS it looks to me as it is a no-brainer complete voltmeter with 6+ digits resolution. Any experience with the chip? How it compares with the DIY MS_ADCs here?

[Kleinstein]

The ADS125H looks interesting for a simple DVM. One slight difficulty may be the common mode range: with one side at ground the useful input range seems to be smaller. It may still be just enough for 10 V. 
The noise is OK for a 6 digit meter, roughtly comparable to the 34401 or Keithly 2000 (though the conparison is not so easy as the input is contineously sampled (the K2000 / 34401 spend half the time for auto zero) and depending on the filter consecutive readings are not independent.
The INL looks quite good, though it can be worse when 1 side is fixed at GND.
My DIY MS-ADC is lower noise (about a factor of 5) and hopefully better INL, but lower maximum speed. However it is more effort and a bit overkill for a nV meter. I may still be tempted to start from there. The ADS125H is also my short list for the ADC.
Most SD ADC have the advantage of sampling the input all the time and this way getting a lower bandwidth for the input and input amplifier than a classical AZ clycle with an MS ADC. With input noise as the critical points, one would not want to start with twice the effective BW (as with many older DMMs).

[iMo]

The ADS125H looks interesting for a simple DVM. One slight difficulty may be the common mode range: with one side at ground the useful input range seems to be smaller. It may still be just enough for 10 V. 
..

According to the DS the single ended input range is +/- 15.5V with +/-18V AFE power supply.
PS: with more traditional +/- 15V analog power source the single ended input range will be +/-12.5V, imho.
There is also a "PGA threshold" 2V below analog power lines (it sets a bit indicating the adc reading is off the range).

[TiN]

So far I had something like this in mind as a concept for this project. 

[Kleinstein]

I don't think a nV meter would need AC coupling, at least not for the main path.
I would expect the low noise nV preamplifier output to also go though the main amplifier for the other ranges. This way less (maybe even none) gain switching in the nV preamplifier is needed. Chances are the SD ADC would need an extra divider for the 10 V (or similar) range, unless one uses the ADS125H  .

The input protection is a slightly tricky part. At least the version I have in my mind (and use with my voltmeter) can use the protection also as part of the input switching.  So protection and the low thermal MUX may be combined.

[macaba]

I was also thinking switchable AC coupling so I like that, it allows for many more use cases. It’s probably practically part of the low thermal block, using the same relays and isothermal area.

You won’t (and IMO shouldn’t) be able to chop the 1/f noise of the AC coupling. So it’ll be something like 30nVpp for the DC coupling and some higher value for the AC coupling. (and that’s ok for the AC coupling use cases)

[bobAk]

Hi everybody. I have a couple of questions. Looking at Tin proposal, it turns out that we will not be engaged in the development of the nanovoltmeter heart? In fact, this is a lego constructor and the result of the challenge depends on whether there is an nv amplifier module on the table or not. Why does a nanovoltmeter need a 24-bit ADC? If its capabilities are redundant.

[bsw_m]

So far I had something like this in mind as a concept for this project. 

What can I say. Good luck building a nanovoltmeter using this concept. 

[TiN]

To be clear, I'm not competing in any shape or form with this. And may do own nV amplifier block instead of A23. Perhaps better idea to create separate thread in Projects about this, instead of OT this one about contest.

Kleinstein is right, realized that nV amp output should go to x5 gain instead of a\d mux directly.

[justanothername]

I would consider the front end the real challenge, the rest is more like easy.

The rest is time consuming. I'd really like to have the time to make a nice low cost and compensated front end (including ADC maybe), not to program a complete instrument.
I guess this challenge is more for people that already have a design in the drawer. So if the rules are refined towards the essentials, I'm in.

[dietert1]

Yes, can we define meaningful requirements for input protection? For low noise input protection resistors are impractical. How about a pair of JFETs for bipolar current limiting. I found some with copper leads that support +/- 40 V at a 2 mA current max. I think that would be a good start.
Will put them into a piece of aluminum tube for lowest thermal EMF. That part can also be used to replace the 2x 51K resistors in the frontend of a 3457A used for voltage reference monitoring.

Regards, Dieter

[MegaVolt]

I found some with copper leads that support +/- 40 V at a 2 mA current max.

Are these items available? Can you tell me their name?

[Kleinstein]

I would consider protection similar to the Keithley2001, 2002 and 2182: so 2 back to back MOSFETs with a photovoltaic optocoupler to drive the gates. That is a bit like doing a photomos solid state relay from discrete parts. A second optocoupler can turn off the FETs relatively fast, if the ouput side voltage gets too high.
The separate part solution has the advantage that the heat producing part (the PV opto-coupler) can be a bit away from the rest.

The good point here is that one can use the protection also as switches. They are more like switches that also act as protection.

As a simple resistor replacement with lower noise one could use depl. mode MOSFETs. They work kind of similar to the JFETs, but are avilable also for higher voltages (like 800 V). Not sure if they offer copper leads (the higher power ones may).

[ferret_guy]

I reached out to EM electronics about the pricing of the A23 and they provided the attached price list.

Some highlights:

Device	Equivalent Noise Resistance	Price
DC Picovoltmeter Model P13a	0.25Ω	£9,500
DC Nanovolt Amplifier Model A10	20Ω	£695
DC Nanovolt Amplifier Model A20a	10Ω	£4,300
DC Nanovolt Amplifier Model A22	150Ω	£295
DC Nanovolt Amplifier Model A23	25Ω	£360

[maxwell3e10]

Thanks. Does anyone know who is involved in EM electronics these days? Speaking on the phone with them a few years ago it seemed to be the old guard (very knowledgeable!). Even though I don't really need one of these modules now, I've been tempted to get one in case they fold in a few years.

[TiN]

Heh, Dave and Chris had great fun on the Amp Hour #558 discussing this contest and volt-nuttery OCD.   

[ferret_guy]

A note on the requirements, 30nV Pk-Pk to RMS corresponds to about 5nV RMS. Which over a 10Hz BW is the equivalent noise of a 151 Ohm resistor (or an EM A22).

This week I put together a quick design that I think is very promising, I cribbed from ADs Low-Noise Instrumentation Amplifier with Nanovolt Sensitivity and made a design with a 8 AD8428's parallel together and an ADS125H02 to digitize. I plan to solder them one at a time and measure the noise performance, comparing with ADs article and the theoretical values.

To alleviate the headache of input switching, I just connected the second input of the ADS125H02 right to the input as well. The AD8428s will operate just fine with their outputs saturating, and with 15V rails, I should be able to read +-10V by swapping inputs and possibly do some interesting self-calibration of the gain/offset. There is no need to use the A grade AD8428, with a 25uV offset (vs. 100uV for B). Both will require offset cal regardless.

The biggest problem is the input offset drift, 300nV/C. Hopefully, by next week I should have some results to share.