1uV voltmeter DIY

I developed 24bit ADC voltmeter diy project long time. Its in a personal hobby level project not professional. My goal was getting 1uV voltmeter diy by my self. I had very hard time for developing but now I got satisfactory result and here is the post. My final goal is not just uV meter. I thinking trying to develope many measuring meters like Ohmmeter, ammeter and internal resistance meter etc. with precise.
During developing I realized precise low level voltmeter is essential for every "meters" and I used 24bit ADC.

For example, to measure the 0.1milliohm cases, when applying 100ma then 0.1 x 0.001= 0.0001v (100 uv) should be measured. so need uV meter. In general, measurable about 10 uv range is needed for most precision mesurement. IMO.

At first, I tried 16 bit adc. Its easy with plenty of source code and low cost chips. But 24bit adc was differnt story. I had very hard time to implement 24bit adc to work correctly. I experienced many trial and errors and finally succeeded. 24bit adc is very sensitive and noise is most difficult problem when measuring micro level voltage. You need clean circuit design and also precise software controle. First time when I implemented circuit I found out was very unstable so I blame ADC chips so I tried many other ADC's but I finally found its not ADC problem but ground noise problem. I seperated ground and get stable result.

1. About 10bit AVR voltmeter
In the past, I implemented atmega8 adc voltmeter. its rough 10 bit and internal reference voltage was not stable and also low internal resistance. Eventually I found error was 1~2v and not result was not linear so can't use so I throw it away I think avr voltmeter was just toy.

2. 24bit adc choices
There are lots of 24 bit adc's in the market. you may confusing because many features and many different kinds of products.Within the different functions, clock, amplifier, ocillator, reference etc.. First, select Delta-sigma adc for accurate voltmeter its littlebit slow but more accurate SAR adc's. Secondly, select fully differntial mode adc's. you can seperate ground with that and can measure with minimum noise. Thirdly, choose with pre amplifier included version mostly x128 is normal with this, you can amplify low voltage and can measurely more easily and also get high imput impedance with high imput impedance characteristic of OPAMP amplifier. High imput impedence is very important for voltmeter because low imput impedance cause "loading effct" on measuring resistor followed by inaccurate result.
I used many brand chips like AD, TI, MAXIM,LT.. but my preference is TI and I finally implement with TI 24bit ADC(ads1248 ) and satisfied with the result.

3. Can measure 1uV ?
In theory, its possible with 24bit adc but in practical situation its almost impossible for amature hobbyst environment. for example you can't make exact 1uV reference with not exact resistor. and alsow several uV noise is exist all over ther place.
I barely get 10uV level accuracy. I satisfy with this levely I think 1uV measurement almost impossible for home environment.

4. 24 bit adc circuit design.
24 bit is very sensitive to noise and common way is very unstable. When you implement 24bit ADC, I recommand to refer evaluation board circuit design that manufacturer provide. Evaluation board circuit was public and well designed for maximum performance and also fully understanding of data sheet also needed for proper hardware/software design. Eventually, you should develope most driver software with throughly understanding of data sheet contents.

5. Use of relay
When implementing automatic ranging you need some kind of switching device like relay. First time I used FET relay but failed because tens of uV noise exist with FET relay. secondly I used reed relay but failed again. When I thouch the relay tens of uV generated inside of relay. I searched internet and found certain metal junction create voltage with in side the reed relay. So I finally used normal realy and noise gone. Please keep in mind when selecting relays.

6. Auto to ranging
I made auto ranging volt meter. most voltemeter use resistor devider ladder to change voltage level but I only used one devider(10M:100K) and used software amplification level adjust instead to get maximum accuracy. and you need additinal software loging avoiding ping-pong between measuring level boundaries, fast auto ranging algorism and over voltage protection.

Anyway, I successfully made voltmeter measuring about 10uV~200V with auto ranging feasures. My final goal is implementing precision ohmmeter,current mter, internal resistance meter and discharger all in one device for my own. I will try this in near future with basis of this ADC voltmeter. Thanks.then..







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