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Friday, September 30, 2016

Building a Milli-Ohm Meter

I already mentioned the SCULLYCOM Youtube series from Louis Scully, and a while ago I prototyped this design to see how well it would fit my needs.

Build a Milliohm Meter
Update

This is my circuit of the prototype I built, based on the first video.

Initially, since I used my DMM as a display, I did not use the x10 multiplier. I was so happy with the accuracy that I decided to build the real thing, so I ordered and added the INA106, because it makes the read-out a bit easier without having to do the math.

The unit worked very well, and even as a prototype, using my DMM as the display, it has been very useful. When I found out that Greg (the same one from the Milli-Voltmeter PCB), also created a PCB for this project, I was sold and decided to built one on his PCB. This is detailed in the update.

This project is also a work in progress for me, because I'm still building my BOM to be able to order the required parts when I'm back home.

Here is Greg's version of the circuit diagram, that I will use as the reference for the BOM.


The BOM for this project, using mostly UK suppliers, is now available here:  BOM
And also on Greg's site here : BOM2

I use Mouser and/or DigiKey myself, because Farnell and the likes do not want to sell to hobbyists in other countries. Many alternative suppliers do not have some of these parts available. You can use several of the Farnell part numbers that Louis provided to look up parts with Mouser, and otherwise the description will help. There are some price differences with Mouser and DigiKey parts, most are less expensive, some significantly so, especially the .1% 25PPM resistors.

The PCB itself can be ordered here : PCB

Note that there have been a number of revisions, currently at 1.5, and there may be more.

Here is some information about the above parts:

C1..4 (22uF/25V) have some physical restraints to fit on the PCB. The dimensions are diameter 5mm, height 7mm (not so critical), lead space 2mm.

C5 (can be 150, 220, 470 or 1000uF/35V) and also has some restraints. The dimensions are diameter 10mm, height 12,5mm(not so critical), lead space 5mm.

IC2 and IC5 are from Linear Technology and Mouser does not carry that brand. In that case, I usually mix my order between Mouser and DigiKey and keep an eye on the free shipping limit of 50 Euro's or more. The IST version of IC2, the LT3092, is the preferred version, because it has better specifications.

Resistors R1 through R9 determine the constant current of 100mA. A precise and clean current is the Achilles heel for the design of this meter. There are four options for R2, 3, 4 and 5.  Louis has the Welwyn RC55Y series listed, but they cost a whopping 2.56 Euro's a piece. He also has the Holsworthy H856 series listed and they cost 1.72 Euro's. Mouser has the Neohm types as an alternative available. They have exactly the same specifications in term of precision (0.1%) and are also 15 PPM/C grade, however, the other two may have other better specs in general. There is a price difference though. The NEOHM version for R1 is YR1B63R4CC, and for R2..R5 is YR1B56R24CC, and they cost about 0.20 to 0.25 Euro each, depending on the value. Greg, the maker of the PCB, uses Vishay resistors from the MRS25000 series. They only have 1% precision and a TC of 50PPM/K. They cost 0.27 Euro's each. To get the required specification, he ordered a large number and selected the best ones.
Up to you to decide if your budget allows it and if you really need that little bit extra.

R12 and R13, the adjustment trimmers are quite special physically, so make sure you order the right type: 3296P-1-104F and 3296-1-101LF. There is also a 3366 version available that looks the same but is a little smaller and also seems to fit the PCB layout. (on the revision 1.5, Greg used both types as you can see from the pictures)

I have not decided myself yet if I will use the LCD meter as a display, or continue with using one of my other DMM's to display the value. The 6.5 digit Milli-Volt Meter, also designed by Louis that I'm building as well would be perfect for this job. Because I will only occasionally have a need for the milli-Ohm meter, a dedicated and rather pricey panel meter seems like a waist to me and also makes the enclosure larger. Besides, I have had some bad experiences of the LCD driver switching noise coming from these displays getting injected to my critical signals.

This decision will determine what enclosure I will use, but that will have to wait until I have the PCB and tested some things. The added benefit of using another DMM for display is that the panel meter  Louis selected only goes to 2V, and that limits the maximum value of the resistor you can measure to a maximum of 2 Ohm.

The next update will be towards the middle of November, when I have all the parts, so stay tuned for more.





Thursday, September 29, 2016

Building an 8 Digit Micro-Volt Meter

This started out as a 6-digit milli-Volt meter project, but with the added software capabilities, the meter is now a true and accurate 8 digit micro-Volt meter with logging capabilities.

There are a large number of updates in the software, scroll down to see them.


As I mentioned in one of my other posts about the Kelvin-Varley Divider, I wanted to have a higher resolution voltmeter than I currently have. I happened to stumble on a nice set of Youtube videos from Louis Scully from Scullcom Electronics. He has described a set of very nice instruments, one of them is a 6.5 digit milli-Volt meter.

One usual caveat! The links I provide may at times no longer work, so my apologies in advance if you land in the 404 land of no returns.

Here are the links to the videos, now up to 4 in total.

Part 1 of the 6.5 digit Voltmeter
Part 2
Part 3
Part 4
Mk2

The design is quite simple and should be able to be built if you have a bit more than average skills.
The good news is that a follower of this design, Greg Barbouri, has provided a PCB through OSH Park that greatly enhances the input section of the DVM, which is the Achilles heel of such a project.

Here is the website that details the implementation using that PCB:
Barbouri Millivolt-Meter Project

I will use that PCB, but I have a few changes in mind for the version I am going to built and I'll go through these elements here.

Here is the schematic Greg made for the meter:

The front end of the design is the most critical. I'll implement that by following the PCB design. For the processor part, I originally wanted to use the same Arduino Nano that Louis Scully is using, but since the PCB layout is for an Arduino Pro Mini I'll use that. However, I have no need to drive a multi-color back light LCD, and I also do not foresee any other enhancements that will eat up the Arduino ports, so I see no need to use the Display42 PCB from Greg with the I2C MCP23017-E/SP chip.

To cut down on the amount of wires going from the Arduino the the LCD, there are 6, I am using a module that is available on eBay for a very small price : LCD Interface

The LCD that I'll end up using will be this one :
16 x 2 White on Blue LCD











My design will also power the unit from a battery, to avoid grounding issues and provide a clean supply to begin with. However, rather than using normal batteries, I will use a  rechargeable cell, and I want to provide a way to charge them while using the instrument, and also when not in use. To keep taps on the voltage level or discharge level while not on mains, I need a monitor that will warn me when the voltage is getting too low.

Here is the circuit that I used to implement the button debounce, and the power section.




One word of caution before I dive in. The DC chassis part is not quite like the Eagle symbol I used in this diagram. There is no short between the plus and minus when there is no plug inserted! The minus is however isolated from the chassis when there is no plug inserted, keeping everything isolated from the chassis. If the DC plug is inserted, you could potentially create an earth ground connection to the DUT. There is an indication on the LCD by the charging symbol, so you won't forget.

You'll notice that I deviated from the design Louis used for the two buttons. I have quite some experience now interfacing buttons if you have followed my Raspberry Pi posts. The processor of the Pi runs at 900MHz or more (yes, no typo), and interfacing with something as slow as a button has its challenges. Especially for inexpensive buttons. You'll be amazed how noisy they can be.

In any case, the filtering that I use to get rid of most switch bounce noise is by using an R/C filter on both edges (closing and opening). I always prefer to use active high buttons or switches because they avoid all sorts of power-on problems. When the switch/button is open, the capacitor is at ground level. Closing the contact will charge the capacitor through the 10K series resistor, creating a nice and clean rising edge (R/C) towards the input of the Arduino. Releasing the switch will cause the capacitor to discharge through the 10K series resistor plus the 1K to ground, again creating a nice R/C slope that will filter the high speed bounce noise. In software we can use a little delay to get rid of the slower bounce transitions, and together this will create clean signals to the Arduino program without having to resort to Schmidt-trigger gates or Flip-Flops.

One word of caution. Don't make the debounce capacitor much larger than 10nF. If you don't have 10nF, you can go as low as 1nF. The reason for this is that the slower the R/C slope, the more time the signal stays in the undefined area between digital "high's" and "low's" and that can cause glitches for the processor again.  If I peaked your button interest, have a look here : Debouncing buttons There is a lot more to buttons than you may think.

The power section
The power design is quite simple, and I've used that before. The two Schottkey (low drop) diodes D2 (this diode can actually be a 1N400X type) and D3 will decide which supply is feeding the Voltmeter. If the mains is connected (providing the 15-30V DC), D2 will have the higher voltage so it will win. If there is no mains connection, the battery supplies the voltage. The resistor (R1) in parallel of D3 determines the (re)charge current of the Ni-CAD cell. The charging current is about 0.1 x C for a 250mAh cell. Depending on the capacity of the cell(s) you're using you may need to change the value of R1 so it is within the (re)charging specification of the cell(s).

In order to keep an eye on the charge level of the cell, I have added a few parts to allow the Arduino to measure the voltage level. You don't want to run into a situation where the voltage is too low, and you will introduce errors in the measurements. Besides, you don't want to be caught with an empty battery when you're in the middle of something. R2 and R4 create a 3:1 voltage divider with easy to find resistors. You can create 20K with 2 x 10K in series. (don't use less than 10K for R4, or it will negatively influence the ADC conversion) C4 is a small filter to get rid of noise and the output goes to one of the Arduino ADC inputs. The rest is done in software and I have also designed some battery level symbols to make it look nice.

The complete multi-meter draws less than 60 mA. About 26mA of that is used by the LCD display. It is safe to use the L part for the 12V regulator, and even for the 5V regulator on the PCB.

If you are already an Arduino user, you may have the Mini Pro, and you also may have the required programming cable. If not, here is a source that provides both as a bundle :
Arduino Pro Mini with interface

The display and interface to the Arduino
Here is a picture of the small interface board that will turn the LCD module into an i2c capable interface, reducing the number of wires to 4, while staying compatible with the PCB.



You need to install a new LCD library to get the i2c driver, and I selected the library from here :
i2c / LCD library

This library is tailored for a particular interface board, the FaBo #212 LCD I2C Brick, but the only difference is the i2c address with the board I have.

First of all, you need to know the i2c address of your board.
I used a little sketch to do that:
i2c address scanner

My address turned out to be 0x27, while the FaBo brick uses 0x20.
After you have installed the new i2c-LCD library on your system, you need to edit the  FaBoLCD_PCF8574.h file that is in the library source section, and change this line :

#define PCF8574_SLAVE_ADDRESS 0x27 ///< PCF8574 Default I2C Slave Address = 20


The finished hardware
Here is a picture of the finished project. I actually build two units, because one can never have enough voltmeters. My design and the changes I made allows me to position these meters very close to my prototypes, and without and power wires attached. I can also make floating measurements, because nothing is connected to the housing. (the DC socket inputs are isolated from the chassis if no power plug is inserted)



Below is a link to the copy of the Arduino sketch. There are many changes to the original code, so have a good look at what has been changed in case you use different hardware.

I have been playing with the two units, to see what the accuracy is and what I could change in the user interface.

I must say that I am very impressed with the accuracy! I have two calibrated voltage reference units, and also a new/freshly factory calibrated 4.5 digit bench multi-meter. The accuracy and precision of this design is astounding for such a simple and inexpensive tool.


Tweaking the hardware
I was a little apprehensive about feeding the LCD display with the same 5V supply as the rest of the logic. These displays are notorious for introducing spikes and noise, so I was on guard for trouble.

When I got my scope attached, I was not surprised by the noise I found, so I started with decoupling the 5V power where it enters the display module. I used a Tantalum 3.3uF together with a 100nF to begin with, because the i2c and the LCD do not have any decoupling.
Here is how that looks:

Unfortunately, this did not reduce the nasty spikes on the reference voltage and the main 5V by much. Looking into it some more, I found that the switching of the LTC_CS line to start/stop the AD conversion cycle turned out to be the culprit. Here is a screenshot:


The top trace (A) is the LTC_CS signal coming from D10 at the Arduino PCB. The bottom trace (B) is the 4.096V reference, AC coupled. The spikes are clearly caused by the switching at the digital port. They are a few nSec in width, so I selected a 4n7F capacitor that slowed the edge down enough to not cause a spike anymore. I mounted that capacitor on the PCB of the Arduino, with one leg soldered to D10 and the other end to the unused GND mounting hole just next to it.:


And this is the result:


I noticed a potential bug in the code, related to the averaging of the results. The Spi_Read function discards a reading of the ADC if it's not ready, but the main loop code counts it as a valid sample, which could result in wrong measurements. I have fixed the code, but I could not find instances of this error when I looked for it with a Logic Analyzer.

While I had that out, I looked at the timing in more detail, to see if there were any potential conflicts.

First of all, this is a picture of the ADC sampling window:


Here you can see that the CS is going down to start the cycle and the MISO is ready 1.25uS later, virtually at the same time. There is 1.5uS between that event and the first clock going high. This is after I already eliminated the little delay in the original Spi_Read code. It is not needed. You can see here the four data bytes getting read, and the actual data presented on the MISO. Note that at the third clock, we are reading the third status bit (SIG), and that indicates a V-in signal > 0. The data towards the end is the "real" 28 bit data, of which the last 4 are the "extra" sub LSB's and they are discarded in the main loop after the averaging. (look at the datasheet for details)

I selected 8 samples to be averaged in my code, and then prepare the result to go to the LCD. As I mentioned earlier, these LCD's are very noisy. In our case, this has no real influence, because the LTC2400 is put to sleep after we have read the data, and made the CS pin high again, as you can see above.

Here is a screen shot that shows the end of the data (channel 5) going through the  I2C bus to the LCD, and the beginning of another acquisition cycle:

You can see that we have a "quiet" period after the LCD got updated, and the start of a new ADC acquisition, which is 0.129 mSec (T1-T2). The total loop time, from LCD update to LCD update is 1.5 Seconds in my case. Sending the results to the LCD only takes about 36 mSec.

Here is a picture of a complete loop:

The "dead" time, averaging the results and sending it to the LCD is only 0.22 Seconds.


After playing with the meter, I got more and more dissatisfied with the flopping around of the last 3 digits, even when I have a stable voltage reference connected to it.


Averaging, Smoothing & Filtering

At first I played around with the averaging, but that is really no solution for a system that has 24 bit resolution. The reason is the inherent amount of noise when you are down to the micro-Volt level. Below is a sample of my 2.5V reference, using my non-calibrated voltmeter (the reference is calibrated as having 2.49993V)


Averaging (using 8 values) does not have as much influence as you may think. There is still quite a bit of noise.



So, still not good enough. I then looked at smoothing, see below, but that was not good enough either, so I turned to filtering.


I tried a few approaches, and then really investigated an Infinite Input Response (IIR) filter design. And that showed a lot of promise:
Blue is the original input, red the filtering effect with a factor of 4.

And here with a factor of 48.


This filter is based on the "weighing" of the new samples, based on a division. The divisor is fixed and above I used a factor of 48. This means that a new sample only contributes to 1/48th of the value to the averaged total. This is great when you have a stable signal, but what if the input voltage changes?

Although filtering is an order of magnitude better then averaging, I was not satisfied yet. I then looked at resetting the averaged result if the sample was significantly different enough from the averaged result. I used a 5 sample input filter to avoid spikes resetting the filter, and that worked very well. If a new voltage was applied, it only took 5 cycles of 0.165 Sec. to switch to the new input.




I have just about zero experience with filters, and this extra code I designed myself, but I was convinced that there are better methods available. I eventually found the Kalman filter, which is used a lot, and it seemed perfect for my application.

Knowing nothing about it, I searched and found a very nice tutorial on Youtube that explains the Kalman filter very very well, even for total Dummies like myself. (look for Michel van Biezen - Special Topics - The Kalman Filter) I wrote a simplistic version of the filter based on his explanation but was not satisfied with the result. I also worked with another example I found, but that had the same problem. Neither of them worked with relatively quick input changes, as an example when switching from 2.5V to 10.0V. They both took several seconds to show the new value. Bummer!


So Kalman looked great on paper and in simulation (using Excel), but in reality, using my Volt Meter, it was a lot worse than the IIR filter I already used. However, I stole a concept from the Kalman filter, namely the Gain calculation. This is a dynamically calculated weighing factor, so I wrote some code around my IIR filter that accomplished what I wanted. Details are in the code. The result is fantastic, I think!

If I now connect the meter to a really stable voltage, like from a reference, 5 decimal digits are rock solid with only the 6th flopping around due to the noise. When I switch from one reference voltage to another one, within a few cycles, the voltage is updated and within a second or so the 5 digits are rock solid again.

When I tested the ability to set a voltage manually with a power supply (one of those I built in the other Forum posts), I was amazed how well the response to my tuning and the accuracy was, but also I saw how noisy my older version power supplies turned out to be. (luckily, not the one I designed myself) That's what you get when you are using a 24bit ADC with micro-voltage resolution. Oops!

Additional Calibration Levels
In any case, with this filter in place, I also added a separate calibration function, to calibrate the Volt Meter to my Voltage Standard. I already use the (Zero) Null Volt calibration to null the input level, but I now also can tune the meter to my Voltage Reference.

The accuracy is now much better as well.

With all this done, I no longer need to reduce the number of decimal digits, so that code went into the bit-bucket. In the process of going through the filter designs, I also saw a way to get to a more stable and precise acquisition delay for the LTC2400. The delay is now dynamically calculated.

Because I needed to store the calibration factor for the voltage reference, I needed to store a floating point number in the EEPROM. Turns out, the library we already used has that feature, so I could clean-up the code, and send two more functions from the original code into the bit-bucket.

My dual-button press now drives the zero calibration with a short press, and a long press enters the reference calibration.

With all these changes and loop tuning, the main loop time is now about 165mS, so the display is very responsive.

V3.11 update:
I found a bug in the filter calculation, based on a rounding error. This is caused by diving a long by a float, and the result going into a long again. The solution was to use a float as the result. The compounded rounding error caused the filter result to be a little bit below the raw averaged input level.

The difference between the filter result after 1000 samples, and the calculated median value in Excel is now very, very small.

I also added the filter weight exponent multiplier to the display.

V3.12 update:
Because the linearity of the meter was not as good as I had hoped, I created a way to measure the value of the ADR4540B chip, and updated that factor in the code. To measure that voltage with my still un-tweaked meter, I calibrated the meter with my 5V0 reference, to get as close as possible. In order to do that, I created a special cal function for all my reference voltages. Now you can select any of them, just update the cal factors.

To do this calibration, I let my 2 meters and the reference warm-up. Then I did a fresh null calibration, and a 5V0 calibration. I then measured the ADR reference voltage for a few minutes to get the best possible filtered voltage. I had already added a pin on my PCB to do that easily. This voltage was then used to update the constant in the code.  I did the same for my other meter. After updating the V-ref constant, I did the measurement again to verify and tweak if required. Finally, I did the ref voltage calibration again. After trying all 4 reference voltages, I found that I had the best linearity result by using the 10V reference.

After this calibration the overall linearity is even better.
Not surprisingly, the 10V is spot on :

Reference Voltage     Measured Voltage     Delta                   %
2.49993 V                  2.49953 V                  -400uV           -0.016%
5.00181 V                  5.001515 V                -295uV          +0.059%
7.50547 V                  7.50534 V                 -130uV           +0.0017%
10.00673 V                10.00672 V                -10uV            +0.00001%

That is good enough for me. If I have a need to really measure in the sub-volt range, I can still do the 2.5V cal.

The LTC chip can actually measure an input range of +/- 12.5% of the reference voltage, so what you can do with this feature is to null the meter with a voltage you want to monitor, say a 2.5V reference, and you can then measure the drift with uV resolution over time. (this includes the drift of the meter itself as well of course, but still)

Here is the latest version of my code: https://github.com/paulvee/6-digit-milli-voltmeter :

I am sure you will enjoy this tool!

Have fun!


[Update: June 2020]

Adding a logging Facility
For another project I'm working on, I needed a method to log the measured values of the meter, and later display them in a graph. I have added a new version of the firmware (V3.13) on the Github site that already has some changes to the firmware to start to do that. With the new version, the measured values are output in microvolts every second at the serial out pins of the Arduino, and can be seen and logged when you use the Arduino IDE and the Serial monitor. To add timestamps to the measurements, you can invoke the "Show timestamp" feature of the Arduino IDE Serial Monitor. A simple copy and paste of this data to an Excel file allows you to analyze the data and create a graph from it.

Here is a graph of measuring the voltage of an AA 1.5V battery cell. With 6 decimal digits, the meter displays down to 1 micro-Volt. At this level of sensitivity, the slightest changes in temperature or even air movement will have a noticeable effect. Keep the lid on the meter and don't wave your arms in excitement!


This graph is the output of my Siglent SPD3303 power supply set at 5.000V.  This shows a drift of about 150 micro Volt over a period of 1 hour.



This is a graph of one of my references, on the 5V setting. Note the micro Volt sensitivity again. The number of decimal digits can bet set in the firmware, or dropped from the collected values by an Excel formula.


I made the same measurement (not at the same time) using my 80.000 count 4-1/2 digit DMM, a Vichi VC8145 using the logging software from Dick Grier. link to software
He and I worked together to make it a little better. Note that the calibration of the meter after 4 years is a little off.


To me, this clearly shows the additional value the extra digits of the milli-Volt meter adds, and why I wanted to build the milli-Volt meter in the first place.

So how much noise and drift is coming from the meter itself? I nulled the meter, and while the shorting lead between the input terminals stayed connected, I measured the resulting input drift and noise. Over a period of about 30 minutes, I measured between +4 and -2 micro-Volt of noise, but no drift.



These graphs show how good this DIY meter is. This is a testament to the parts used, the design and the board layout of this meter. You will be hard pressed to get this level of sensitivity and accuracy in commercial products, unless you are prepared to pay hundreds of Euro's.


WARNING
This logging software addition is the bare minimum because to make it really usable and safe, I need to add an optical separation of the meter and the PC or whatever device you use that collects the data. The reason is that by connecting a serial to USB cable from the meter (the Arduino) to the other device you could possibly connect the DUT to earth ground, and that defeats the battery operated meter function. It also no longer allows you to make floating measurements when the meter is on battery power. Having no optical separation could be unsafe too! Know what you're doing and be aware...

Optical Isolation
I have finished the hardware for the optical isolation section. The simple circuit is as follows:

I have tried this circuit with 9600 baud and it works very well with these components. If you decide to use other optical isolators, you may have to tweak the emitter resistor values. Keep them as low as possible to keep the rise times in check.



If that still poses a speed problem, there is a nice circuit addition that Bob Pease designed (look in his book Troubleshooting analog circuits) that uses one additional transistor to speed-up the rise times. I tried that, just for fun, and even without the positive feedback resistor R8 which would improve the fall-time even further. The circuit works very well, but is an overkill for this application at 9600 baud.





Below is the isolation circuit I added to the DMM.
The connections on the left side go straight to the pins on the Arduino Pro Mini. The connections on the right side go to a serial to USB convertor board. There are several versions of these boards around, and they only cost a few Euro's. You need them to program the Arduino Pro Mini anyway. The particular one I used has a feature to let it work with 3V3 and 5V systems, selected by a jumper.



I have de-soldered the USB connector from the serial to USB board, and added header pins to the 4 connections. From these 4 header pins, I go to a USB cable that ends in an isolated USB micro chassis part that is mounted on the back panel.
The cable can be ordered here : panelmount  There are versions with stripped wires and with connectors, which is what I ordered. The cable shield is available to connect to ground.

Caveat

I implemented a fully bi-directional optical interface with the intend to also program the Arduino with this setup. Unfortunately, I can't get it to work. This means that you could eliminate the receiver circuit around VO1 and only let the Arduino send data.
I don't have the DTR connection on my setups, so there is no automatic restart of the Arduino when the downloads starts. I always have to press the reset button as soon as the script starts to download. That cannot be the problem though.

I have tried to figure out why it does not work, also looked all over the web, but I was unsuccessful. If any of you know what to do to make it work, please chime in.

Final test run

To test the logging feature, I used one of my voltage references again. The reference was set at 2.5V because that is also the intended voltage level on a device I need to test, a GPSDO. The calibration of the 2.5V was written on the reference to be 2.49993V at the time, now a few years ago. I did not let the units warm-up, after switching them on I started logging right away. Here are the results:


The logging of the voltage was every second and started at 8:43 in the morning. At 2.125 seconds (around 9:18), there was a rather "large" glitch of a few 100 micro-Volts that lasted about 200 seconds. On the right is a more detailed graph. Unfortunately, I have no idea what caused it. Was it the reference, or was it the DMM? I was not running on batteries, so it could have been the mains voltage. Who knows...

In any case, you can see the voltage very slowly moving up until about 20.000 seconds, when there was a wriggle of a few micro-Volts, and after that it stabilized. I started with a voltage of 2.499515 and stopped 8 hours later with a voltage of 2.499635. The room temperature increased because the sun was out. This may have been the cause of the slow increase. I did not keep track of the temperature when I recorded this. With these sensitivities, you really need to.

Conclusion

The setup of the voltmeter and the reference shows no significant drift, the measured voltage moved up by a mere 120 micro-Volt over a period of 8 hours and is therefore very stable over a very long period of time. This test proved and showed exactly what I needed to know before I started to apply it to my intended application. I just hope the glitch never happens again, ruining a long measurement.


Enjoy!

BTW, I sold one of my two units, and I still have one bare PCB available for sale (5 Euros + S&H). Contact me if you're interested.



Building a Kelvin-Varley Divider

I do not have very precise (expensive) equipment, and really don't want to spend the money to obtain it. I'd rather spend my limited hobby funds on parts to tinker or build things with. However, every now and then I need something with a precision or resolution that I can't deal with. Typically, I mean precise voltages and precise resistances. With these two, one can also obtain precise currents by applying Ohm's law.

I recently spend a relatively large sum to obtain a bench multi-meter with a much higher precision than the DMM's I already have. They are all 3.5 digit, and I wanted to go one step above that.

After a long search and discounting used (they need calibration typically costing 100 Euros) instruments, I decided to take the plunge on a VICI VC8145 Bench DMM. It has a 4-7/8 digits display with 80,000 counts. The precision in the DCV meter is 0,03% and 0.3% in most other measurements. I purchased one for about 150 Euro's from eBay.

New-VICHY-VC8145-DMM-Digital-Bench-Top-Multimeter-Meter-PC

Every now and then, even this is not enough when your trying to deal with a reference that needs to be 10x or better to calibrate things. As an example, I purchased a few inexpensive but precise voltage standards, the best one is the KKMOON variant with the AD584.

Google for "KKMOON AD584 Reference"
There is also a blog with a discussion about this unit here: https://www.eevblog.com/forum/beginners/t17704/


This reference is calibrated and you get the calibration details for the 2.5V, 5V, 7.5V and the 10V.


(I modified my reference with a switch and adjustment capability for the 2.5 reference voltage level, per the data sheet of the AD584) Wit this I can tune the voltage reference without disturbing the originally calibrated values. I also replaced the banana sockets)

Note: the switch is not pushing the PCB up, it's not even touching it. There was just enough room between the PCB and the Li-Ion battery (;-))

With this calibration information, you can check your equipment and see how accurate it is against this reference.

My unit listed the 10V output as 10.00673 which translates to an accuracy of +0.0673%. Now, in some cases, I have a need for a precise 10.00000V to make calculations easier, or to be used with an ADC or DAC reference or input. I can't deal with that with my new DMM, it's not accurate enough, and I need more digits.

Same with precise resistor values, how do you make sure that a resistor is 0,01% or better if your instrument is "only" 0,3%. It needs to be 10x better, so 0,001% or better.

Well, the answer I hope to have found is using an instrument that was designed 100+ years ago, the Kelvin-Varley Resistance Divider. Google it if you have never heard from it.

I studied the little information there is about this concept, and really looked in detail to the best standard still, the Fluke 720A. Obviously, even used, this is beyond my reach and most others.
However, I also found an article that was written 20 years ago, that detailed a DIY instrument, and I decided to give that a try.

There are actually two instruments described on this site. One is the KV and one is the Null Detector. I build both.
The articles can be found here: http://conradhoffman.com/mini_metro_lab.html
It provided the missing link for me.

Null Detector
First of all, I built the Null Detector following the description in the article, to be able to do the very precise resistor matching required for the K-V:






Since I already had my new DMM that goes down to the micro Volt level, I could and did use the VC8145 as the null detector most of the time. If all you have is a 3.5 digit DMM, you should really look into the Null Detector that Conrad designed.


Kevin-Varley Resistance Divider
Back to the Kevin-Varley Resistance Divider. I hope that by now you will have read the article, and I also suggest you have a look at the Fluke 720A documentation to get a feel for the complexity in dealing with 1ppm precision. Have a look here :   Fluke 720A Manual

All the way in the back are the circuit diagrams. In essence, this is the simplified circuit:
Image result for kelvin-varley-schaltung

The Fluke deviates from the original Kevin-Varley specification (look that up) and so does Conrad's design. The original specification was meant to be for resistor chains without a shunt. The lower resistance of the last decades also meant that the output impedance was as low as possible, because the only device -at the time- connected to it was a galvanic meter as a null detection meter.

We now have DMM's with 20GOhm loading so we can increase the values of the lower decades somewhat. Conrad used easy to obtain resistors, and I followed his design as close as possible with one exception, I could not get 412 Ohm resistors with 1% and 0.5W, so I used 422 Ohm. This meant that the shunt had to be redesigned but that's quite simple to do.

Here is my version of the circuit diagram :
There are a few more changes/additions that were mine. First of all, I added jumpers to "break" the shunt accross the decade resistors. This will allow you to measure and compare each individual resistor in addition to the shunt. Second, I added a buffer circuit to avoid loading the output of the divider. I can switch this in and use lower impedance meters at the output. I also added test points for my DMM clips next to the 4mm banana sockets.

Lastly, I did not use Conrad's connectors, but the more easily available connector strips.

Here are pictures of my first proof-of-concept build:



With this setup, I could start to play with this instrument and see if it indeed could deliver on my expectations. I had never used (not even heard of) a KVD before, so I had a lot to catch up with.

Resistor selection and matching
The biggest and most laborious job is the selection and matching of the resistors. It's not difficult, it just takes time and patience. What I found was that you should not skimp on the quality of the resistors.  Initially I did not want to use expensive low PPM/C super precision resistors for my proof-of-concept prototype. However, my supplier of choice send me a set of resistors in the 10K and 100 Ohm range that obviously came for a cheap source you can guess where from. The values were all over the map. As an example, from the 200 10K resistors, exactly half of them were outside the 1% specification. One resistor was even 10K3 Ohm. Not only that, there was no way I could get a matched set within 0,003% with this lot. They were awful.

Here is a picture of the Gaussian distribution :

The vertical arrows on the left and right stickers denote the 1% borders. Anything to the left or right is out of specification, and there are about 50 resistors that fell of the board, literally.
Luckily, my supplier was quick to acknowledge my issue and send me a new set. What they recommended was to get 0.5 or 0.6W versions, as they are more precise (actually, from another factory) The 100 Ohm resistors were equally bad, but I didn't need the precision. This new lot did contain a good enough set of matched resistors.

This is a picture of what a typical Gausean distribution really should look like:



Wheatstone Bridge
Per Conrad's instructions, I used a Wheatstone bridge to select and match the resistors as precisely as I could.

After some matching using his method, I decided to used another bridge concept to be able to more easily switch from resistor values, and simply increase the resolution of the bridge.


Here are two pictures to save some words:




The shunt connections are made with jumpers, and you move the jumpers as a pair to shunt the potentiometer with decreasing resistor values. After I selected a group of resistors for a KVD decade that were in the same ball-park, I then matched them among that set for the closest values. To calibrate the bridge, one resistor of the set was used as the Reference resistor, and one of the others as Rx. I then nulled the bridge with the lowest resistor shunt possible. After that I used the other resistors in the set and noted the deviation of the null (+ and -) to select the closest set.

Note that when the KVD is finished, you can use it instead of this potmeter contraption.


The K-VD resistor sets
The 10K set needs the best matching and the spec says 0.0037%. I was able to get close to 0.001%, however, the actual values of the resistor set were around 9,950 Ohms. This is within the 1% specification (0.5%), but not close enough to 10K.

During my first tests, I didn't get the expected accuracy from the lower decades (they were not multiples of 10x), and so I went back and forth calibrating and trying to figure out what caused it. To try to fix it I started with the first 10K decade and used 49.9 Ohm 1% resistors in series to match the resisters closer to 10K (actually within 1 Ohm of 10K). Although the calibration of the decades starts with the lower ones and then go up, I figured that the relationship may have been distorted so I started from the first decade down. And low and behold that seemed to have fixed my resolution problem.

Proof of concept
There is a lot more to test, but I'd like to give you a quick answer to the challenge I posted in the beginning. If you have a 10.00673V calibrated reference, can you use the K-VD to calibrate another reference to a precise 10.00000V without using a 6 or 8 digit DMM?

The short answer is, yes you can. At least that is what I was able to do (I think).

Following are the steps I took.
I connected the output of the 10.00673 reference to the input of the K-VD. The output of the K-VD was connected to the input of my 20GOhm input bench DMM. (virtually no loading, so I could cheat and not using a bridge) The KVD decades were set at minimum subtraction or division with 9-9-9-9-9-10. My DMM showed 10,007V at the output which is the best it can do.

A quick-and-dirty procedure (thank you Unknown for pointing this out) is to calculate the setting of the K-VD to create a precise 10V output as follows:
10.00000 / 10.00673 = 0.999327  K-VD setting: 9-9-9-3-2-7
This sets the divider ratio to 0.999327 and the output voltage would therefore be:

10.00673 * 0.999327 = 9.99999547071, which is close enough.


My expectation is that the output went from 10.00673 to 10.00000V, but there is currently no way for me to verify that. I need a much higher resolution voltmeter to do that. So the next project was born, a high resolution DMM.
Look here for that follow-on project : building-6-digit-digital-milli-voltmeter

Note that the proper procedure should be as follows:
You would need to use a Wheatstone bridge between the calibrated reference and the new reference, and use the K-VD as one arm of the bridge. Set the K-VD to the right value, and null the bridge by adjusting the output of the new reference, which should result in the desired output of 10.00000V.  This method is preferred because it avoids currents flowing through the bridge which will eliminate a number of issues.

So in my own terminology, we are now able to "transfer" a calibrated voltage from one reference to another secondary one without using expensive precision equipment. If you build the Null Meter, you could potentially do this with your 3.5 digit DMM.

There are a few other applications for the K-VD that I may try to cover in more details later, but don't hold your breath. It may take a long time for me to get around it with so many other projects and interesting things to discover.

In the meantime, I started the process of building a DIY simple but fairly precise 6 digit Digital Voltmeter, and that project can be found in another post.

Enjoy!