Millivolt Meter


The Millivolt Meter Project

This is a DIY mil­li­volt meter that was orig­i­nal­ly designed by
Scullcom Hobby Electronics and pre­sent­ed on Youtube.
Definitely take the time to watch the videos (4 parts) as they are very well done and pre­sent­ed in a way that’s easy to fol­low and learn from.

I was in need of a meter that I could ded­i­cate to low volt­age pre­ci­sion read­ings, and could be done for a rea­son­able cost. So this project seemed like a per­fect fit. As usu­al after view­ing the video I saw many areas that the meter could be mod­i­fied and improved for my spe­cif­ic use, and quick­ly began design­ing my own ver­sion of it using Eagle CAD V7.5.

Some of the changes I made were to reduce the noise and add shield­ing to the input sec­tion. I also made the deci­sion to uti­lize a pre­ci­sion volt­age divider net­work instead of the less expen­sive indi­vid­ual resis­tors.
Another major depar­ture from the orig­i­nal design was to use a MCP23017 (16 input/output) I2C port expander board that I had designed sev­er­al years ago, for con­nect­ing the LCD dis­play and push-but­tons. This reduced the num­ber of pins used by the Arduino Pro mini, and made it eas­i­er to mount the RGB LCD dis­play. As orig­i­nal­ly designed there were no pins left for future options.
I added guard rings on the top and bot­tom of the cir­cuit board around the low lev­el input cir­cuit­ry to the op-amp, and also removed the sol­der­mask around all low lev­el traces.
Bottom of board with input sec­tion on top right.

Top of board with input sec­tion on top left.

Boards were made in USA using the OSHpark board ser­vice.
https://oshpark.com/shared_projects/qgv0fpKN

The volt­age divider resis­tor is a Caddock 1776-C6815 of which the 1K, 9K, and 90K sec­tions are used in series to make up a 100K leg of the divider along with the 900K sec­tion for the oth­er leg of the divider. The 9M sec­tion was not used.
Next to the LTC2400 24bit ADC, a set of sol­der pad jumpers were added to allow selec­tion of either the 50Hz or 60Hz notch fil­ter.
Top of board with sol­der paste and sur­face mount devices except C7 mount­ed.

Board after refow sol­der­ing using a Presto Liddle Griddle and man­u­al tem­per­a­ture con­trol uti­liz­ing an infrared non-con­tact ther­mome­ter.
An impor­tant step is to clean the traces and com­po­nents in the input sec­tion so that no con­t­a­m­i­nants such as flux, oils, sur­fac­tants, or fin­ger­prints remain.

Caddock divider net­work and Sparkfun 16 MHz 5 volt Arduino Pro Mini added.

Other com­po­nents and I2C head­er installed, ready for test­ing.

After some pre­lim­i­nary test­ing I found some insta­bil­i­ty in the out­put of the ADR4540B 4.096 volt ref­er­ence IC. I had clear­ly left out the required out­put fil­ter capac­i­tor shown in the orig­i­nal schemat­ics.
I was eas­i­ly able to bodge the required capac­i­tor from the 4.096 test pin to the ground pin of the tan­ta­lum capac­i­tor direct­ly below it. The new V 1.51 schemat­ic and board files now include it.
One thing I noticed dur­ing test­ing was that the main board and dis­play used less than 50 mA and the orig­i­nal spec­i­fied 5 volt TO220 pack­age reg­u­la­tor, along with my added heatsink was much more than is need­ed. Moving to a small­er foot­print 150 mA reg­u­la­tor would free up con­sid­er­able space. Another addi­tion to the design would be to add a 250 mA PTC resetable fuse on the 9V input.

Checking fit and clear­ances of cop­per shield.

Mounting main board with shield and input wires attached. Battery guide and hold­er also mount­ed in enclo­sure.
Battery assem­bly is com­prised of 6 AA 1.5 volt bat­ter­ies for 9 V nom­i­nal input to reg­u­la­tor.

Front pan­el with dis­play, switch­es, and jacks mount­ed.

The front pan­el was designed using “Front Panel Designer” to fit a stan­dard Hammond 1455N1601 extrud­ed box with met­al end plates 6.299″ L x 4.055″ W x 2.087 H. — Link to design file at end of page.
Design file was sent to Front Panel Express in Seattle, WA. USA and was shipped five days lat­er. The pan­el is made from “Medium bronze” anodized alu­minum and is 2 mm in thick­ness. With a rec­tan­gu­lar beveled cutout for the LCD dis­play, D‑holes for the two banana jacks, coun­ter­sunk holes for box mount­ing, and stan­dard holes for the three switch­es.

RGB LCD with I2C port expander board con­nect­ed to main board.

OpenEVSE dis­play board with a 16 IO I2C port expander and bat­tery back­up RTC (Real Time Clock) DS3231. The board also includes cur­rent lim­it­ing resis­tors for the three back­light RGB led’s, and a con­trast poten­tiome­ter. For the I2C bus there are two pullup resis­tors posi­tions pro­vid­ed along with address select sol­der pad jumpers for the port expander. Four I/O ports are bro­ken out and can be indi­vid­u­al­ly con­fig­ured as inputs or out­puts, along with a ground pin. The board is sized to match the stan­dard foot­print for many 2 X 16 LCD dis­plays.
https://oshpark.com/shared_projects/J6RW88kf

The code pro­vid­ed by Scullcom Hobby Electronics was mod­i­fied to use the I2C inter­face for the LCD and input push-but­tons. RGB back­light­ing on the LCD changes col­or depend­ing on which mode the meter is cur­rent­ly in.

During start­up the EEPROM stored Cal Level is dis­played.

Pressing the Calibrate but­ton starts the cal­i­bra­tion mode prompt­ing the user to short the input leads.

After the cal­i­bra­tion is com­plet­ed the Adjust Factor that is writ­ten to EPROM is dis­played on the LCD briefly before return­ing to mea­sure­ment mode.

Measurement mode, cur­rent­ly dis­play­ing micro volts.

With all shield­ing in place and cal­i­bra­tions per­formed the meter fluc­tu­ates
±12 uV max­i­mum with the input leads short­ed and typ­i­cal­ly ± 5 uV.

Link to EagleCAD Millivolt meter V1.51 schemat­ic and board files ZIP
Link to EagleCAD I2C port expander dis­play V4.2 schemat­ic and board files ZIP
Link to Front Panel Designer V1.1 front pan­el file ZIP

Modified code Version 3.20 December 2018 using Paul Versteeg’s fil­ter, cal­i­bra­tion, and many oth­er code enhance­ments. For I2C RGB dis­play and I/O.

UPDATED Millivolt Meter Version 2.11 blog post

MilliVoltMeter320.zip firmware

 

Meter running version 3.20 software with updated code by Paul Versteeg
Millivolt Meter run­ning ver­sion 3.20 soft­ware with updat­ed code by Paul Versteeg

 

65 Replies to “Millivolt Meter”

  1. This looks like an amaz­ing project; great work, to both you and Scullcom.

    Would you con­sid­er putting togeth­er these parts into a kit (con­tain­ing a PCB, com­po­nents, enclo­sure, pan­el)? I’d read­i­ly buy one or two.

    1. Hi Simon,
      Currently with my work com­mit­ments, I am unable to put togeth­er a kit for sale. I some­times have an extra bare board that I can sell though. I will try to look into some kits lat­er this Winter.

      Thanks,
      Barbouri

  2. Hi Barbouri,

    I already post­ed a sim­i­lar request with one of your oth­er designs with the same top­ic. Do you have a BOM to share for this project?

    Many thanks in advance,

    Paul

  3. I also tried to get to a BOM or this project, by extract­ing the part­slist from Eagle.

    I noticed a dis­crep­an­cy between the board lay­out (that has R5) and the schemat­ic (that does not), although both car­ry the cor­rect ver­sion num­ber. Maybe you can update the schemat­ic?

    Here is my attempt to cre­ate a BOM, I hope you can either cor­rect on com­ment on my pro­pos­al part­num­bers:
    Part Value Package Supplier Partnumber

    C1 1000uF/25V CPOL-RADIAL-1000UF-25V 5mm UVR1E102MPD
    C2 100uF/25V CPOL-RADIAL-100UF-25V lead spac­ing 2mm? UVK1E101MDD
    C3 0.1uF MLCC 594-K104K20X7RH5TL2
    C4 3.3uF/25V tan­ta­lum “TAP335K010SCS or TAP335K010SRW or T356A335K010AS


    C5 0.1uF MLCC 594-K104K20X7RH5TL2
    C6 0.1uF MLCC 594-K104K20X7RH5TL2
    C7 0.1uf 805-CAP SMD 08055G104ZAT2A
    C8 3.3uF/25V tan­ta­lum “TAP335K010SCS or TAP335K010SRW or T356A335K010AS


    C9 10uF/25V tan­ta­lum TAP106K025SCS
    C10 220pF 0402-CAP SMDC0402X5R1C221K020BC or C0402X7R1A221K020BC

    C11 0.1uF MLCC 594-K104K20X7RH5TL2
    C12 0.1uf MLCC 594-K104K20X7RH5TL2
    IC1 7805T (1Amp) TO220H LM7805CT or NCP7805TG
    IC1 78L05 (100mA) TO92 can be used alter­na­tive­ly LM78L05ACZ
    R3 10K 0603-RES SMD AC0603JR-0710KL
    R4 10K 0603-RES SMD AC0603JR-0710KL
    R5 10K 0603-RES SMD AC0603JR-0710KL
    RN1 1776-C6815 Caddock net­work resis­tor 1776-C6815
    U$1 ARDUINO_PRO_MINI ARDUINO_PRO_MINI Sparkfun Arduino Pro Mini 328 — 5V/16MHz
    U1 LTC2400IS8 SOIC127P600X175-8N DigiKey LTC2400IS8#PBF
    U2 AD8628 SOIC127P600X175-8N AD8628ARZ
    U3 ADR4540B SOIC127P600X175-8N ADR4540BRZ

    Many thanks in advance,
    Paul

    1. Hi Paul,
      Here is a link to my Digikey shared parts list.
      http://www.digikey.com/short/393771
      R5 was only used in the V1.5 board as an extra pull-up for the reset pin.
      In V1.51 and above it is not used as there is already a pull-up includ­ed on the pro-mini board.
      Most of the pic­tures in the post show the V1.50 pro­to­type board, and do not include the impor­tant C12 capac­i­tor.
      Thanks,
      Barbouri

  4. I for­got, sor­ry.
    There is anoth­er dis­crep­an­cy between the cir­cuit dia­gram, the silkscreen and pic­ture of your stuffed board.
    You have installed a 1000uF/25V capac­i­tor for C1, and a 100uF/25V capac­i­tor for C2.

    I also added a 78L05 TO-92 (100mA) on the BOM because that can be used instead of the 7805 TO-220 (1A), right?

    Do you have a source or infor­ma­tion (thick­ness, lay­out) for the cop­per(?) shield you built?

    Sincerely,
    Paul

    1. Hi Paul,
      I used the 1000 / 100 uF capac­i­tors (C1 / C2) because that is what I had in stock, but the designed capac­i­tor val­ues 220 / 10 uF will work great. I had rec­om­mend­ed a 150 mA reg­u­la­tor in the post, but the 100 mA one should work as long as an effi­cient dis­play is used.

      Barbouri

  5. I worked with the hot­line from DigiKey and I now have access to the list. I will work on that and see if I can coor­di­nate what I’m doing with Louis Scully, he is hope­ful­ly doing the same thing as he did with the mill-ohm meter. Maybe I can help.

    Thanks,
    Paul

  6. Hey Greg, I final­ly fin­ished my built of the volt­meters, I actu­al­ly built two. I believe that there are some inter­est­ing enhance­ments to the design, so I here is a link to my blog that details it all.

    I must say that I am very impressed with the accu­ra­cy of the meter, they are much bet­ter than I antic­i­pat­ed and a very wel­come addi­tion to my toolk­it. Thank you again for doing this all!

    Paul

    http://www.paulvdiyblogs.net/2016/09/building-6-digit-digital-milli-voltmeter.html

    1. Hi Paul, I have been fol­low­ing your blog since your ear­li­er post.
      Your meters look great. You might be inter­est­ed in the cal­i­bra­tion code used in the Programmable Voltage Reference project as it might be used as a basis for set­ting up a mul­ti-point cal­i­bra­tion scheme for the Millivolt Meter.
      Greg (Barbouri)

      1. Thanks for point­ing this out. I saw it ear­li­er when I looked at some of your oth­er projects. As a mat­ter of fact, I had been con­tem­plat­ing doing some­thing sim­i­lar for the volt­meter as soon as I saw the lin­ear­i­ty devi­a­tion.

        Compensating for lin­ear­i­ty devi­a­tions and the likes is a tech­nique I read about some time ago, but I have nev­er used it myself. I will need to do some soul and Google search­ing to get my arms around it, for now I have a few too many oth­er projects to fin­ish. One of which is the mil­li-ohm meter. The PCB is done and works great, I now need to order some more parts to fit it in the hous­ing I have in mind. When done I’ll report back on that project too.

        Thank you for your con­tri­bu­tions!
        Paul

  7. Greg,

    There is a nasty spike intro­duced to the main 5V and also the 4.096 ref­er­ence volt­age, which is caused by the switch­ing of D10, the LTC_CS sig­nal. I used a 4n7 decou­pling capac­i­tor on the Arduino PCB, but you may want to con­sid­er adding some room on the main PCB when you do anoth­er turn. Look at my blog for more details.

  8. Greg, I think we may have a poten­tial bug in the LTC code that will com­pro­mise the aver­ag­ing of the mul­ti­ple read­ings.

    I assume that Louis got his orig­i­nal code from Martin Nawrath, (Academy of Media Arts Cologne, Germany) it looks a lot like that, but Martin uses no aver­ag­ing in his code.

    In the main loop, we use a for-next loop that incre­ments the num­ber of sam­ples we get from the Spi_Read func­tion and then aver­age the result by div­ing the result with the num­ber of sam­ples.

    However, in the Spi_Read func­tion, we skip the com­plete read­ing of the ADC if it is not ready.
    Here:
    if (!(PINB & (1 « 4))) {

    }
    This means that the aver­ag­ing can be off. If I’m right, the Spi_Read func­tion should have a flag to sig­nal a cor­rect read­ing back to the main loop.

    I will put a log­ic ana­lyz­er on the pins to see if this is hap­pen­ing, and how often.

    What is your take on this?

    Regards,

    Paul

  9. I could not find instances of this hap­pen­ing, but I did fix the code. I also made some mea­sure­ments with my Logic Analyzer and put that on my blog also. (address some­where in anoth­er post)

  10. hi Paul did­n’t see my ques­tion? I think I get the answer : this is a DC mil­li­volt meter since there is a ref­er­ence volt­age ‚baut I think we can turn this device into AC mil­li­volt­meter and make it more use­full in elec­tron­ics . do you agree?

  11. Paul,
    Ik heb een bor­d­je gekocht op Ebay van een Finse aan­bieder.
    Daar heb ik na veel exper­i­menteren goede resul­tat­en bereikt.
    De ingangss­pan­nings del­er is direct op de ingang van de LTC2410 aanges­loten.
    Er is dus geen buffer­ver­sterk­er aan de ingang toegepast.
    Het bor­d­je heeft de vol­gende spec­i­fi­caties;
    (I bought a board on Ebay from a Finnish provider.
    I’ve achieved good results after a lot of exper­i­ment­ing.
    The input volt­age divider is direct­ly con­nect­ed to the input of the LTC2410.
    Thus, there is no buffer ampli­fi­er applied to the input.
    The board has the fol­low­ing specifications;)GoogleTranslate

    24bit LTC2410 ADC Module designed for Arduino or oth­er embed­ded sys­tems to acheive high res­o­lu­tion analoge to dig­i­tal con­ver­sion. The mod­ule use LTC2410 24bit ADC IC and MAX6126 2.048 volt ultra high pre­ci­sion, ultra low noise ref­er­ence.
    A p‑p noise of 3uV can be achieved when mea­sur­ing the mod­ule Voltage ref­er­ence with a volt­age divider using a USB pow­er source.
    LTC2410 24bit ful­ly dif­fer­en­tial Analog to dig­i­tal con­vert­er
    2 ppm INL, No miss­ing code
    2.5 ppm Full Scale Error
    0.1 ppm Offset
    0.16 ppm Noise
    Maxim MAX6126 ultra high pre­ci­sion, ultra low noise ref­er­ence.
    Reference volt­age out­put 2.048 volt
    ±0.02% accu­ra­cy
    Ultra low 3ppm/C max tem­per­a­ture coef­fi­cient
    Ultra Low 1.3uVp.p noise
    High sta­bil­i­ty Voltage divider for volt­ages up to MAX 32 volt.
    Vishay UXB0207 1Mohm 0.1% 5ppm
    Vishay UXB0207 33K2 0.1% 2ppm
    Measurement max­i­mum volt­age
    Voltage V1 – ½ Vref (-1.024 volt) to ½ Vref (+1.024 volt)
    Voltage V2 – Maximum 32 volt. (resis­tor divider,

    Na enige aan­passin­gen aan het bor­d­je (koel­ing van IC’s) en de soft­ware, heb ik de vol­gende resul­tat­en bereikt.
    (After some adjust­ments to the sign (cool­ing IC) and soft­ware, I have achieved the fol­low­ing results.)GoogleTranslate

    Calibratie mod­ule Gemeten LTC2410 Deviatie
    2,501144 2,501142 ‑2,0 uV
    5,00241 5,002386 ‑24,0 uV
    7,50235 7,502346 ‑4,0 uV
    10,00313 10,003146 16,0 uV

    Het enige prob­leem wat ik nog heb is dat de off­set nog wat zwab­bert. Ongeveer zo’n 15 uVolt. (Loopt langza­am op en neer — ongeveer een paar minuten).
    Bedankt voor je inbreng op het ontwerp van SCULLYCOM.
    Ik ga ook dit ontwerp bouwen en heb inmid­dels de print en onderde­len besteld.
    (The only prob­lem I still have is that the off­set some wob­bles. Approximately about 15 uVolt. (Runs slow­ly up and down — about a cou­ple of min­utes).
    Thanks for your input on the design of SCULLYCOM.
    I’m going to build this design and have already ordered the PCB and components.)GoogleTranslate

  12. I’ve been look­ing over the design for this board, and one thing trou­bles me a lit­tle. The AD8628 input op-amp does­n’t have a neg­a­tive sup­ply input, it’s only using dig­i­tal ground. This means the input can’t quite go down to true 0V, because the out­put of that chip is lim­it­ed to around 1mV above its V- sup­ply rail. Given the 10:1 divider net­work on the input, that lim­its it to an input volt­age of 10mV.

    Would this be improved if the input op-amp had a neg­a­tive sup­ply rail too — e.g. a lit­tle charge pump chip like the ICL7660 to pro­vide it some­thing below-0V to give it true to-ground head­room?

    1. Paul,
      The input of the AD8628 can go down 0V, and even down to ‑0.3 volts. It is the out­put which is lim­it­ed, even being a rail to rail op amp. The AD8628 is also tied to the ana­log ground sec­tion, not the dig­i­tal ground. So the input divider has no lim­it­ing effect on the input volt­age.
      I have done some tests with input volt­ages in the 1 to 10 mV range with good track­ing. I do see a lot more noise at these low­er mV ranges, but for me this is accept­able.
      The cir­cuit would ben­e­fit from a +/- pow­er sup­ply, but would be lim­it­ed to 6 volts total for the AD8628.

      If some­one is inter­est­ed in devel­op­ing a work­ing pro­to­type using a dual sup­ply, I would be inter­est­ed in putting togeth­er a final cir­cuit board for it.
      Greg (Barbouri)

      1. I’m intend­ing to build mine in two halves, with a total­ly dif­fer­ent dig­i­tal sec­tion on a sep­a­rate board, con­nect­ed to your board con­tain­ing the ADC via an iso­la­tor chip (I have a spare ADU1402 from a dif­fer­ent project). This dis­con­nects the dig­i­tal (dis­play, but­tons, ser­i­al port?) half from the mea­sure­ment fron­tend. An iso­lat­ed DC-DC con­vert­er will pow­er it, let­ting the mea­sure­ment half float inde­pen­dent­ly of bench pow­er — I’m not intend­ing to use bat­ter­ies in mine.

        Because of this, it occurs to me one adap­ta­tion I could make is to move the “ana­log ground”, the lev­el that’s at the end of the resis­tive divider and the “- IN” ter­mi­nal, by split­ting the sup­ply 5V via an op-amp of some sort. If I did that, then the range is extend­ed into part­ly neg­a­tive val­ues, by reduc­ing the pos­i­tive end of the range. Of course now I’d have to sub­tract the off­set from the ADC’s read­ing in soft­ware, and this would require some frag­ile cal­i­bra­tion. I don’t know how that would drift over time though. It would how­ev­er, give me a bipo­lar mea­sure­ment abil­i­ty.

          1. Hi Vincenzo,
            Currently there is not a revi­sion with the TI iso­la­tor.
            But I was just think­ing about updat­ing this project this morn­ing, and one of the updates would be the iso­la­tor.
            I most like­ly will not get to this until August, after com­ple­tion of the DC Load project.
            Thanks,
            Greg (Barbouri)

  13. i’m con­fused about the ana­log ground. how does this work when you only have a sin­gle-end­ed ADC? sure­ly if the ADC is expect­ing the ref­er­ence volt­age to be ref­er­enced to its ground, then the ADC and the volt­age ref­er­ence must be using the same ground? i would think that hav­ing a sep­a­rate ana­log ground would only make sense if you were using a dif­fer­en­tial ADC?

  14. Hi Barbouri,

    I have built this project and I have a ques­tion regard­ing the enclo­sure. Would it be pos­si­ble to post a pic­ture of the mechan­ics involved in mount­ing the LCD dis­play to the front pan­el. I did order the front pan­el and Hammond enclo­sure. The cir­cuit is work­ing fine, I just need some assis­tance in the enclo­sure mount­ing. Thanks to you and Louis for all the hard work.

    1. Hi Rob,
      In the pho­to with the cap­tion “Mounting main board with shield and input wires attached” there are four black plas­tic machine screws with nuts attached to the cor­ners of the dis­play. After cen­ter­ing the dis­play, these can either be epox­ied or attached with hot-melt glue to the back of the front pan­el. After attach­ment I used the nuts to adjust the clear­ance of the dis­play for a light con­tact fit.

  15. Anyone have any spare V1.51 PC boards that I could pur­chase ? I thought that I’d give it a try before I put in an order to board house. I’m in the US and could Paypal you.

  16. This is a great project, but I don’t like the drift of the last dec­i­mals. So I am in the process to rewrite the adc part by inte­grat­ing the results over time, pret­ty much the same prin­ci­ple as over­sam­pling. So the last dig­its get way more sta­ble and the result imho gets clos­er to my 6.5 dig­it bench meter.
    In the moment I try to devel­op an algo­rithm to fig­ure out when to start a new mesure­ment instead of over­sam­pling the old one.

    Once I am hap­py with the result I will share my code if you are inter­est­ed.

    Cheers
    Rubi

  17. Hi Borbouri,
    Nice project, i’m look­ing to build one or an off­shoot depend­ing on what I want to add on/improve. I noticed Paul Versteeg had issues with the CS pin caus­ing noise on the volt­age rail. Is this per­haps caused by no lim­it­ing resis­tors on D10, D12 and D13?

  18. Hello again Borbouri,
    Looking at mak­ing an off­shoot of the V1.5 Board with 3x 300K and 1x 100K 0.05% resis­tors in place of the Caddock. I’m also think­ing of push­ing it to 100x100mm and adding addi­tion­al con­nec­tors for more but­tons or rotary encoder and the banana jack con­nec­tions right to the PCB. Any thoughts?

    https://www.digikey.com/product-detail/en/susumu/RG2012N-304-W-T1/RG20N300KWCT-ND/600935

    https://www.digikey.com/product-detail/en/susumu/RG2012N-104-W-T1/RG20N100KWCT-ND/600924

    1. Hi Vincenzo,
      Yes, indi­rect­ly. The cop­per shield is con­nect­ed to the Digital Ground plane. The Digital Ground plane is con­nect­ed to the Analog Ground thru a sin­gle point.

      Greg (Barbouri)

    1. Hi Vincenzo,
      I used .5 mm / 24 awg hob­by cop­per sheet for the shield, but any­thing in that range will work.
      Some have even used cop­per clad cir­cuit board mate­r­i­al, sol­dered togeth­er to form a box.

      Greg (Barbouri)

    1. Hi Bstrag,
      As it is designed the Millivolt meter is a DC only sys­tem. The update rate of the LTC2400 ADC is less than 10 Hz so you would need to add some­thing like an AD8436ARQZ true RMS-to-DC con­vert­er to mea­sure an AC sig­nal.

      Greg (Barbouri)

  19. Hi Barbouri, I can see you are short­ing the leds to cal­i­brate the meter. Scullcom hob­by use a 5v ref­er­ence to cal­i­brate it. Have you changed the code, or can it be cal­i­brat­ed both ways? (:

    Rasmus.

    1. Hi Rasmus,
      I am only short­ing the leads for 0 volt cal­i­bra­tion and have one vari­able that was pro­grammed in soft­ware that gives me the best lin­ear­i­ty for the entire range.
      The soft­ware vari­able will be dif­fer­ent for each meter depend­ing on ADC, volt­age divider resis­tors, and volt­age ref­er­ence.

      Greg (Barbouri)

  20. Kudos on the board Greg. I bought six through OSHPark, and have built up four … very close to test­ing (maybe tonight). One small issue that I’ve not­ed is that pins con­nect­ed to the ground plane are very dif­fi­cult to sol­der. I know Kicad does ther­mal relief for flood fills. Is this avail­able in Eagle? Regards

    1. Hi Mike,
      Eagle does have the option of using “ther­mal iso­la­tion” and the OSH Park boards Ver. 1.51 do include ther­mal iso­la­tion for the ground planes.
      But because of the large ground planes on both sides of the board, even with ther­mal iso­la­tion a lot of heat does get pulled from the ground con­nec­tions.
      I get the best results when using a flat-blade sol­der tip, adding extra flux, and increas­ing the tem­per­a­ture by 20 deg C when sol­der­ing the ground con­nec­tions.

      Currently I am work­ing on an updat­ed Millivolt Meter board that uses a Coto 9002–05-11 relay for auto­mat­i­cal­ly select­ing the low­est range (0 — 4.6 volts) with the same max range of 46 volts as the orig­i­nal V 1.51 board.
      Looking for 1 to 10 uV usable res­o­lu­tion. Hopefully I will com­plete the design lat­er this sum­mer.

      1. Thanks Greg. I look for­ward to see­ing your new design.
        On a side note, I have been read­ing through the LT2400 data sheet, ad note that the input con­ver­sion range is ‑12.5% Vref to 112.5% Vref, and that there is an out­put sign bit. I have yet to think through the impli­ca­tions for the input stage, but maybe you have already done so! How far ‑ve do you think the design can safe­ly go? I’m think­ing there might be some scope to use the mil­li­volt­meter not only for absolute DC mea­sure­ment of a pre­ci­sion volt­age ref­er­ence, but also as a null meter. (I should also men­tion that I am in the ear­ly stages of lay­ing out Conrad Hoffman’s MML null meter design in KiCAD.) Regards

  21. So, I have just had a quick look at the AD8628 datasheet (front end oper­a­tional ampli­fi­er) used in the mil­li­volt­meter. The input spec­i­fi­ca­tion is strict­ly zero to 5V, although there is some wrig­gle room giv­en the absolute rat­ing is ‑0.3V (diode clamp). Who knows what hap­pens for a neg­a­tive input 🙂 !
    I’ve also had a cur­so­ry look at the three ver­sions of soft­ware that seem to be com­mon­ly avail­able. Louis’s V33 from his site, an ear­li­er sim­pler ver­sion V7 from github, and PaulV’s ver­sion from his web­site (http://www.paulvdiyblogs.net/2016/09/building-6-digit-digital-milli-voltmeter.html). None rec­og­nize the LTC2400 neg­a­tive bit. I think V7 cal­i­brates at 0V, V33 at 5V, and PaulV’s at zero *and* a num­ber of ref­er­ence volt­ages.
    With a ground­ed input, any noise enter­ing the sys­tem after the AD8626 will make a pos­i­tive con­tri­bu­tion (even if neg­a­tive). There is also a 1uV off­set volt­age for this device.
    In short, the cal­i­bra­tion looks like it can be improved. I have my first board up and run­ning, and using V33, I get a 2 mil­li­volt read­ing with input ground­ed. No cop­per shield yet, and the board was cleaned, but before the Caddock was sol­dered in.
    Greg, when you revise the cir­cuit :-), that small neg­a­tive input range for the LTC2400 (actu­al­ly ‑0.3V because of the diode clamp) sure looks entic­ing. If it could be car­ried through the front end, it would allow a true zero cal.

    1. Hi Mike,
      I will look at some options for replac­ing the AD8628 with an op-amp that sup­ports a small neg­a­tive input volt­age.

      Greg (Barbouri)

  22. Greg, you are a absolute cham­pi­on!
    I have been think­ing a lot about the mil­li­volt­meter, and the gen­er­al “hob­by­ist” through volt-nut appli­ca­tion of the LTC2400. I’m sure you’re aware of the var­i­ous threads on EEVBlog et al. Notably, no-one has yet pro­duced a com­pa­ra­ble easy to imple­ment project for the hob­by­ist with rea­son­able skills, or made any use­ful soft­ware pub­lic.
    Although I seem to be devel­op­ing volt-nut aspi­ra­tions, I real­ly don’t want to own an Agilant (HP) 3458, nor do I want to be respon­si­ble for keep­ing one in cal­i­bra­tion. I would how­ev­er, like to have a pre­ci­sion low volt­age DC mea­sur­ing tool, and the Scullcom/Barbouri design ticks a lot of box­es for me.
    I was already work­ing on the Chinese knock off of the Pro-Mini plat­form, and espe­cial­ly the low pow­er angle. Using the RocketScream library and remov­ing the reg­u­la­tor and pow­er LED from the board allows the pro-mini to sleep and use only 6uA. Outstanding, giv­en it is an Arduino for a few bucks. It was an *excel­lent* choice for the mil­li­volt­meter.
    The LTC2400 also uses a miser­ly amount of pow­er when not sam­pling.
    What wor­ries me most about the design (as I’ve men­tioned before) is the accu­ra­cy of the cal­i­bra­tion. The exist­ing soft­ware does­n’t impress me.
    So, here is where I am propos­ing to go. (It’s most­ly soft­ware … you might like to think about it in terms of the hard­ware design you are con­tem­plat­ing) :
    — a 4 cell LiIon bat­tery sup­ply, charged with a bal­ance charg­er. (Hence low noise.) LT1763‑5 LDO reg­u­la­tor for the 5V sup­ply. For the most part, the instru­ment is always pow­ered on, with the inter­nal volt­age ref­er­ence get­ting more sta­ble with age. The pro-mini, LCD, and LTC2400 in pow­er down when not active­ly tak­ing a mea­sure­ment.
    — cal­i­bra­tion with a ground­ed input, and using an exter­nal pre­ci­sion volt­age ref­er­ence to cor­rect for off­set, and gain errors (I’m pret­ty naive in my under­stand­ing, which at present comes from the Atmel appli­ca­tion note AVR120 “Calibration and Characterization of the ADC on the AVR”). Silly me, I got trapped into think­ing I would need a pre­ci­sion 5.0000 ref­er­ence to cal­i­brate. I was pur­su­ing the AD586LQ which is a pret­ty fine 5V buried zen­er ref­er­ence, but now am think­ing that 5V is not rel­e­vant at all. I have a friend of a neigh­bour with an HP34401 in cal, so I can send a portable ref­er­ence (always pow­ered on) to him, and it comes back say­ing say 6.950432V. Given the caveats of tem­per­a­ture coef­fi­cient, and long term sta­bil­i­ty, I can take that fig­ure, and hard code it into my Arduino soft­ware, then cal­i­brate to that pre­cise ref­er­ence val­ue. The instru­ment should then be very sta­ble until the tem­per­a­ture changes, at which point … recal. (I have in mind to soon hack togeth­er a pro­gram to see if I’m on the right track).
    — Don’t lose the neg­a­tive sign bit from the LTC2400! Reject those val­ues on the zero cal, because they’re not help­ing. Flag over­volt­age on the dis­play, although I don’t pro­pose to get any­where near 50V.
    — put the pro-mini to sleep when the LTC2400 is acquir­ing a sam­ple, and wake from inter­rupt on SPI, hence again, reduced noise.
    — Here is the most impor­tant part. I can’t com­pete with Agilant/HP/Keithley with Kalman fil­ter­ing that responds imme­di­ate­ly to changes to the input. So I need to change the phi­los­o­phy around the way that the instru­ment oper­ates. I tell the instru­ment when I’m ready to take a mea­sure­ment by push­ing a “go” but­ton, let it go off and take say 100 sam­ples, then 16.5 sec­onds lat­er, it presents an aver­age volt­age read­ing, AND the stan­dard devi­a­tion. Isn’t 3sd a 99% con­fi­dence inter­val, and basi­cal­ly a mea­sure­ment of the noise fig­ure of the mea­sure­ment? Again, the 16 x 2 LCD hav­ing two lines was a great choice to present the infor­ma­tion on. Now, I don’t know how many sam­ples gives 3sd approach­ing the noise floor of the hard­ware. Some smar­ty­pants prob­a­bly does, but say it was 1000 sam­ples. Can I wait for 165 sec­onds? Sure. Also, hav­ing I2C allows me to eas­i­ly con­nect a real time clock, EEPROM for stor­ing mea­sure­ments, temperature/humidity sens­ing etc.

    So, lots of work to do. Should be back with some results in six months or so :-).

    I hope you don’t mind me putting these thoughts up, tacked onto your board design. (Again, con­grat­u­la­tions on the board. It’s tak­en the whole con­cept streaks ahead). Say the word, and I’ll stop :-).
    Regards

  23. Quick update. I’m wrong. I have test­ed some code look­ing for a neg­a­tive input to the LTC2400. I nev­er see a neg­a­tive flag, so I guess the op amp has a slight pos­i­tive off­set?

  24. Hi again Greg,
    Firstly, let me apol­o­gize for mak­ing this so mes­sage so long. I have attached 200 lines of code below for the Millivoltmeter which I think you will find valu­able. May I sug­gest that you back in your dis­play and but­ton changes from the basic ScullcomV33 ver­sion of the code (so that it runs on your hard­ware) have a play, and then post the code with maybe a review as a new blog entry?
    So, as promised, I have imple­ment­ed off­set, and gain cor­rec­tion, with stan­dard devi­a­tion cal­cu­lat­ed for mea­sure­ments. With my build of the hard­ware (in an alu­mini­um box, pow­ered via USB, board not prop­er­ly cleaned, and no cop­per shield yet) and tak­ing 30 sam­ples I am get­ting stan­dard devi­a­tion of around 40uV. So effec­tive­ly I think I have a 5 1/2 dig­it instru­ment.
    From here, apart from work­ing on the pow­er sup­ply and shield­ing, I’d like to try putting the Arduino into low pow­er sleep while the ADC is con­vert­ing. I hope that will improve the noise lev­els.
    Regards,

    —– cut —–
    //
    // mvm.ino — Six Digit Millivoltmeter Software
    //
    // © Mike, 12 July 2018
    // Released under GPL2 or lat­er.
    //
    // Derived from Scullcom V33/Barbouri code
    //
    // This ver­sion uses the “new” SPI library inter­face, refer https://www.arduino.cc/en/Reference/SPI, accessed June 2018.
    // Standard devi­a­tion pro­vid­ed by Rob Tillarts “Statistic” library.
    //
    // Major inno­va­tion in this soft­ware over pre­vi­ous ver­sions comes from the Atmel appli­ca­tion note “AVR120:
    // Characterization and Calibration of the ADC on the AVR”. In par­tic­u­lar, see Section 2.1 “Fixed-Point Arithmetic for
    // Offset and Gain Error Compensation”. Note that we use a scal­ing fac­tor of 2^24 (16777216).
    //
    // This ver­sion is con­trolled through three but­tons: Left, Centre, and Right.
    //
    // Shorting the input, then press­ing the left but­ton sets the “off­set” (zeroes the instru­ment). Do this first.
    // Enter a val­ue for your most accu­rate ref­er­ence volt­age below using the EXTERNAL_VOLTAGE_REFERENCE define. Connect
    // your exter­nal ref­er­ence, and press the right but­ton. This sets the instru­ment “gain”. Do this sec­ond.
    // The cen­tre but­ton takes a num­ber of sam­ples (num_samples), and dis­plays the aver­age, and stan­dard devi­a­tion. The
    // stan­dard devi­a­tion should be a noise mea­sure for your set­up.
    // Offset cal­i­bra­tion clears the gain. The cal­i­bra­tion order is impor­tant.

    #include // Serial Peripheral Interface Library used to com­mu­ni­cate with LTC2400 (MISO, MOSI and SCK)
    #include
    #include
    #include

    //
    // Hardware Definition
    //

    //LTC2400 with CS on pin 10, SDO on pin 12, and SCK on pin 13.
    #define LTC2400_CS 10

    #define BUTTON_Left 4
    #define BUTTON_Center 3
    #define BUTTON_Right 2

    #define INTERNAL_VOLTAGE_REFERENCE 4.096 // Nominal ADR4540 ref­er­ence volt­age for LTC2400
    #define EXTERNAL_VOLTAGE_REFERENCE 4.99870 // External ref­er­ence volt­age used for cal­i­bra­tion

    // 16 x 2 back­lit LCD mod­ule with I2C back­pack
    // Address is gen­er­al­ly 0x27, or 0x3F. Change if the LCD does not respond.
    LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

    //
    // Software Parameters
    //

    // To avoid loss of pre­ci­sion in the care­ful­ly craft­ed inte­ger arith­metic, Statistic is only used to keep the
    // stan­dard devi­a­tion. A side project is to rewrite the pack­age using int64_t.
    Statistic stats;

    con­st uint8_t num_samples = 30;

    int64_t ADC_offset = 0LL;
    uint8_t num_bad_reads = 0;

    con­st int64_t scaled_half_LSB = 0x7FFFFFLL; // 0.5
    int64_t scaled_gain_factor = 0xFFFFFFLL; // ini­tial­ly 1.0

    int64_t ScaledReadADC(void) {

    bool error_flag;

    SPI.beginTransaction (SPISettings (1000000, MSBFIRST, SPI_MODE0)); // 1 MHz clock, MSB first, mode 0
    digitalWrite(LTC2400_CS, LOW); // LTC2400 chip select pin tak­en low enables ADC con­ver­sion
    delayMicroseconds(10); // tim­ing delay but may not be required

    while ((PINB & (1 « 4))) { } //check to see if ADC is ready by test­ing EOC — wait while con­ver­sion com­plet­ed

    int64_t sam­ple = 0;
    for (int i = 0; i < 4; ++i) { // Read 4 bytes (32 bits) from the ADC
    sam­ple <>= 4; // Discard 4 LSBs (noise)

    digitalWrite(LTC2400_CS, HIGH); // LTC2400 chip select pin tak­en high dis­ables ADC out­put
    SPI.endTransaction(); // SPI trans­ac­tion com­plet­ed

    int64_t scaled_result = sam­ple * scaled_gain_factor + scaled_half_LSB — ADC_offset * scaled_gain_factor;

    return(scaled_result);
    }

    int64_t ScaledReadMultipleADC(void){
    stats.clear();

    // Throw away first sam­ple to clear ADC.
    // Note that on my hard­ware, when the input volt­age is changed from ground to +5V (and the reverse), the first
    // sam­ple read seems to be in error. Could be that the 220pF capac­i­tor at the Op Amp input is not
    // cor­rect­ly sol­dered on the board (it is tiny!). (Or some­thing else?) Check lat­er by com­par­i­son
    // between hard­ware builds.
    ScaledReadADC();

    int64_t sum_samples = 0LL;
    for (int i = 0; i > 24; // ADC off­set is not scaled

    lcd.setCursor(0, 1);
    ShowReading(ConvertToVoltage(ADC_offset <> 24);
    return(descaled_ADC_sample * 10.0 * INTERNAL_VOLTAGE_REFERENCE / 16777216.0);
    }

    void ShowReading(float x) {

    uint8_t decimal_places = 6;

    char pre­fix = 0;
    if (x < 0.001) {
    x *= 1000000;
    pre­fix = ‘u’;
    decimal_places = 0;
    } else if (x < 1) {
    x *= 1000;
    pre­fix = ‘m’;
    decimal_places = 3;
    }

    lcd.print(x, decimal_places); // Print volt­age as float­ing num­ber with the right num­ber of dec­i­mal places
    lcd.print(“ ”); // Add one blank space after volt­age read­ing

    if (pre­fix)
    lcd.print(prefix);
    lcd.print(“V ”); // Extra spaces to clean up when volt­ages go from large to small (8 spaces)
    }

    void DisplayMeasurement(float aver­age, float standard_deviation, uint8_t bad_reads) {
    lcd.setCursor(0, 0);
    ShowReading(average);

    lcd.setCursor(15, 0);
    lcd.print(bad_reads);

    lcd.setCursor(0, 1);
    ShowReading(standard_deviation);
    }

    // end

    1. Hi Mike,
      Thanks for the code. It will be sev­er­al weeks before I have some time to work with the Millivolt meter.
      I am cur­rent­ly trav­el­ling, which is the rea­son for the delay in respond­ing to you.

      Thanks again.
      Greg

  25. This might help!

    //
    // mvm.ino -- Six Digit Millivoltmeter Software
    //
    // (C) Mike, 12 July 2018
    // Released under GPL2 or later.
    //
    // Derived from Scullcom V33/Barbouri code
    //
    // This version uses the "new" SPI library interface, refer https://www.arduino.cc/en/Reference/SPI, accessed June 2018.
    // Standard deviation provided by Rob Tillarts "Statistic" library.
    //
    // Major innovation in this software over previous versions comes from the Atmel application note "AVR120:
    // Characterization and Calibration of the ADC on the AVR". In particular, see Section 2.1 "Fixed-Point Arithmetic for
    // Offset and Gain Error Compensation". Note that we use a scaling factor of 2^24 (16777216).
    //
    // This version is controlled through three buttons: Left, Centre, and Right.
    //
    // Shorting the input, then pressing the left button sets the "offset" (zeroes the instrument). Do this first.
    // Enter a value for your most accurate reference voltage below using the EXTERNAL_VOLTAGE_REFERENCE define. Connect
    // your external reference, and press the right button. This sets the instrument "gain". Do this second.
    // The centre button takes a number of samples (num_samples), and displays the average, and standard deviation. The
    // standard deviation should be a noise measure for your setup.
    // Offset calibration clears the gain. The calibration order is important.

    #include // Serial Peripheral Interface Library used to com­mu­ni­cate with LTC2400 (MISO, MOSI and SCK)
    #include
    #include
    #include

    //
    // Hardware Definition
    //

    //LTC2400 with CS on pin 10, SDO on pin 12, and SCK on pin 13.
    #define LTC2400_CS 10

    #define BUTTON_Left 4
    #define BUTTON_Center 3
    #define BUTTON_Right 2

    #define INTERNAL_VOLTAGE_REFERENCE 4.096 // Nominal ADR4540 ref­er­ence volt­age for LTC2400
    #define EXTERNAL_VOLTAGE_REFERENCE 4.99870 // External ref­er­ence volt­age used for cal­i­bra­tion

    // 16 x 2 back­lit LCD mod­ule with I2C back­pack
    // Address is gen­er­al­ly 0x27, or 0x3F. Change if the LCD does not respond.
    LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

    //
    // Software Parameters
    //

    // To avoid loss of pre­ci­sion in the care­ful­ly craft­ed inte­ger arith­metic, Statistic is only used to keep the
    // stan­dard devi­a­tion. A side project is to rewrite the pack­age using int64_t.
    Statistic stats;

    con­st uint8_t num_samples = 30;

    int64_t ADC_offset = 0LL;
    uint8_t num_bad_reads = 0;

    con­st int64_t scaled_half_LSB = 0x7FFFFFLL; // 0.5
    int64_t scaled_gain_factor = 0xFFFFFFLL; // ini­tial­ly 1.0

    int64_t ScaledReadADC(void) {

    bool error_flag;

    SPI.beginTransaction (SPISettings (1000000, MSBFIRST, SPI_MODE0)); // 1 MHz clock, MSB first, mode 0
    digitalWrite(LTC2400_CS, LOW); // LTC2400 chip select pin tak­en low enables ADC con­ver­sion
    delayMicroseconds(10); // tim­ing delay but may not be required

    while ((PINB & (1 « 4))) { } //check to see if ADC is ready by test­ing EOC — wait while con­ver­sion com­plet­ed

    int64_t sam­ple = 0;
    for (int i = 0; i < 4; ++i) { // Read 4 bytes (32 bits) from the ADC
    sam­ple <>= 4; // Discard 4 LSBs (noise)

    digitalWrite(LTC2400_CS, HIGH); // LTC2400 chip select pin tak­en high dis­ables ADC out­put
    SPI.endTransaction(); // SPI trans­ac­tion com­plet­ed

    int64_t scaled_result = sam­ple * scaled_gain_factor + scaled_half_LSB — ADC_offset * scaled_gain_factor;

    return(scaled_result);
    }

    int64_t ScaledReadMultipleADC(void){
    stats.clear();

    // Throw away first sam­ple to clear ADC.
    // Note that on my hard­ware, when the input volt­age is changed from ground to +5V (and the reverse), the first
    // sam­ple read seems to be in error. Could be that the 220pF capac­i­tor at the Op Amp input is not
    // cor­rect­ly sol­dered on the board (it is tiny!). (Or some­thing else?) Check lat­er by com­par­i­son
    // between hard­ware builds.
    ScaledReadADC();

    int64_t sum_samples = 0LL;
    for (int i = 0; i > 24; // ADC off­set is not scaled

    lcd.setCursor(0, 1);
    ShowReading(ConvertToVoltage(ADC_offset <> 24);
    return(descaled_ADC_sample * 10.0 * INTERNAL_VOLTAGE_REFERENCE / 16777216.0);
    }

    void ShowReading(float x) {

    uint8_t decimal_places = 6;

    char pre­fix = 0;
    if (x < 0.001) {
    x *= 1000000;
    pre­fix = ‘u’;
    decimal_places = 0;
    } else if (x < 1) {
    x *= 1000;
    pre­fix = ‘m’;
    decimal_places = 3;
    }

    lcd.print(x, decimal_places); // Print volt­age as float­ing num­ber with the right num­ber of dec­i­mal places
    lcd.print(“ ”); // Add one blank space after volt­age read­ing

    if (pre­fix)
    lcd.print(prefix);
    lcd.print(“V ”); // Extra spaces to clean up when volt­ages go from large to small (8 spaces)
    }

    void DisplayMeasurement(float aver­age, float standard_deviation, uint8_t bad_reads) {
    lcd.setCursor(0, 0);
    ShowReading(average);

    lcd.setCursor(15, 0);
    lcd.print(bad_reads);

    lcd.setCursor(0, 1);
    ShowReading(standard_deviation);
    }

    // end

  26. Nope. Sorry. Ok, looks like you do have some seri­ous edit­ing to do.
    If you can’t recov­er the code (at least the read ADC func­tion is miss­ing bits as well as the includes), PM me, and I’ll send you a zipped file.

  27. Hi thought i would give this a go! I can sol­der!! But do you have a parts list for the two types of board I ordered? with digi key or sim­i­lar

  28. Hi Greg,

    How accu­rate your Millivoltmeter please? I can see on the low­er end, for exam­ple, mea­sur­ing 1mV is giv­ing me around 500uV, which is 50% error. I think lin­ear­i­ty error is high­er clos­er to a LTC2400 lim­its. Accuracy improve clos­er to 10mV (Millivoltmeter is read­ing 9.5mV). I have prop­er shield­ing in place. Also using dif­fer­ent volt­age stan­dards (I tried 2.5V, 5V and 10V) to cal­i­brate the meter is giv­ing me slight­ly dif­fer­ent results, but as a rule of thumb if I cal­i­brate it at 10V accu­ra­cy is the best at 10V. If I cal­i­brate it at 2.5V, around 2.5V accu­ra­cy is the best. High res­o­lu­tion is good, but how do I achieve a good accu­ra­cy on the 0–10V scale please?

    1. Hi Alex,
      My V1.5 Millivolt meter is with­in 25 uV at 700 mV, and with­in 50 uV at 100 mV using a 5 volt cal.
      Something that I am work­ing on with the new pro­to­types, is the abil­i­ty to cal­i­brate at mul­ti­ple points and have the soft­ware use the most appro­pri­ate cal for the volt­age mea­sured.
      I have also imple­ment­ed a dual input range of 0 — 4V and 0 — 40V on the most recent pro­to­type which should improve the low end res­o­lu­tion and accu­ra­cy below 4V.

      Greg (Barbouri)

  29. Thanks Greg,

    That’s pret­ty much my prob­lem. Either it is more accu­rate around 5V using 5V cal or around 10V using 10V cal. Like in Ian Johnston’s vref (or in fact in your vari­able vref), there has to be a mul­ti­ple points cal­i­bra­tion.
    Looking for­ward to your new pro­to­type.

    Regards
    Alex

  30. Hi Greg,

    This looks like a great build and design. I was plan­ning on build­ing this (and the MiliOhm project too). Looking through the com­ments I see that a while back you were work­ing on a dual range ver­sion with a Coto relay. Was there any fur­ther progress in this — I am sure I would be pret­ty pleased if I can get the same sort of res­o­lu­tion as you have achieved but would be good to try an updat­ed ver­sion of you had made fur­ther mod­i­fi­ca­tions .

    1. Hi John,
      Next blog post will be on the Millivolt Meter ver­sion 2.11
      The blog post is most­ly done, just fin­ish­ing up all the doc­u­men­ta­tion.

      Thanks,
      Greg (Barbouri)

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